Generic Unpacking for APK

JEB 5.9 ships with a new component for Android app (APK) reverse engineering: the Generic Unpacker.

The generic unpacker will attempt to emulate the APK to collect Dex files that would be generated dynamically, at runtime (i.e. not classes[N].dex). Many APK protectors, legitimate or otherwise – used for malicious purposes -, employ such techniques to make the Dalvik bytecode more difficult to access and analyze.

How to use the APK unpacker

First, open the target APK in JEB. In some cases, the unpacker module will let you know that there is a high-probability that the APK was packed:

In many cases, that heuristic won’t be triggered and no specific hint issued. Either way, you may start the unpacker via the Android menu, Generic Unpacking…

Start the Generic Unpacker via the Android menu

An options dialog will be displayed. At the time of writing (JEB 5.9), the only available option is the maximum duration after which the unpacking process should be aborted (the default is set to 3 minutes, although in most cases, unpacking will stop well before that time-out).

Options dialog for the unpacker

Press “Start” and let the unpacker attempt to recover hidden dex files.

After it’s done, a frame dialog will list the unpacker results, consisting of dexdec MESSAGE notifications indicating which dex files were recovered, and where. The logger will display similar information.

For each recovered dex, a corresponding dex unit will be created under a sub-folder named “unpacked” (highlighted in green, located under the APK unit).

The unpacker has completed and is displaying its results (one dex file was recovered)

Analyzing the collected Dex

At this point, you may decide to analyze the recovered dex files(s) separately. In this case, simply open up the dex unit(s) under “unpacked”, and proceed as normal (another bytecode hierarchy, disassembly view, etc. will be opened).

Alternatively, you may want to integrate the recovered dex with the already existing bytecode. To do this, follow these steps:

  • Right-click on the recovered dex unit, select Extract to… and save the dex to a location of your choice
  • Navigate to the primary dex unit (generally named “Bytecode”), to which you want to integrate that saved dex to, and open it with a double-click
  • Go to the Android menu, select Add/Merge additional Dex files… and select the file previously saved
  • The collected dex will be integrated with the existing bytecode unit, and the bytecode hierarchy will reflect that update

Limitations

The unpacker will not be able to handle all cases. Feel free to report any problem or bug you are encountering, we will see if anything can be done to support most cases.

In upcoming updates, the unpacker will also provide a small API to allow users to write plugins in the form of emulator hooks to do whatever is needed to perform an unpacking task that the built-in unpacker would fail at.

Until next time! (The next blog shall be part 3 of “How to use JEB”, to analyze more complicated/obfuscated native code. Stay tuned.)

Nicolas

Recovering JNI registered natives, recovering protected string constants

This is part 2 of the blog that introduced the major addition that shipped with JEB Pro 4.29: the ability for the dex decompiler to call into the native analysis pipeline, the generic decompiler and native code emulator.

Today, we demo how to use two plugins shipping with JEB 4.30, making use of the emulators to recover information protected by a native code library found in several APKs, libpairipcore.so.

Recovering statically registered native routines

The first plugin can be used to discover native routines registered via JNI’s RegisterNatives. As a reminder, when calling a native method from Java, the JNI will see if exported routines with specific names derived from the Java method signature exist in the process. Alternatively, bindings between a Java native method and its actual body can be done with RegisterNatives. Typically, this is achieved in JNI_OnLoad, the primary entry-point. However, it does not need to; other techniques exist to further obfuscate the target call site of a Java native method, such as unregistration/re-registration, the obfuscation of JNI_OnLoad, etc. More information can be found here.

In its current state, the plugin will attempt to emulate a SO library’s JNI_OnLoad on its own, without the context of the app process it would normally run on. The advantage is that the plugin is useable on libraries recovered without their container app (APK or else). The drawback is that it may fail in complex cases, since the full app context is not available to this plugin. (Note that the second plugin does not suffer this limitation).

Open an APK or Elf SO file(s), run the “Recover statically-registered natives (Android)” plugin.
Set optional name filters or architecture filters as needed.
The results will be visible in the log. In this case, it looks like the aarch64 library libpairipcore.so registered one method for com.pairip.VMRunner.executeVM, and mapped it to a routine at 0x5F180.

Recovering constants removed from the Dex

The second plugin makes use of an IEmulatedAndroid object to simulate an execution environment and execute code that may be restoring static string constants removed from the Dex by code protection systems.

We can imagine that the code protection pass works as such:

String constants are being removed during a protection pass.

The implementation details of restore() are not relevant to this blog entry. In the case of that particular app, it involves calling into a highly obfuscated native library called libpairipcore.so.

The plugin requires a full APK. It will emulate a static method selected by the user and let them know about the constants that were restored.

The plugin workflow is as follows:

After loading an APK, the plugin may let the user know that the code was protected.
Execute the “Recover removed Dex constants” plugin.
The user will be asked to input the no-arg static method that should be simulated. If a suitable one is found, it may be pre-populated by the plugin.
The execution can be lengthy, from several seconds to several minutes. Recovered strings are registered as fields comments as well as decompiler events in the relevant dexdec unit of your project.

Conclusion

That’s it for today. Make sure to update to JEB Pro 4.30 if you want to use those plugins.

I would encourage power-users to explore the JEB’s API, in particular IDState, EState/EEmulator and IEmulatedAndroid, if they want to experiment or work on code that requires specific hooks (dex hooks, jvm sandbox hooks, native emu hooks, native memory hooks – refer to the registerXxxHooks methods in IDState) for the emulators to operate properly.

Until next time — Nicolas.

Android JNI and Native Code Emulation

JEB 4.29 finally bridges the gap between the dex analysis modules in charge of code emulation (dexdec‘s IDState and co.) and their counterparts in the native code analysis pipeline (gendec‘s EEmulator, EState and co.).

The emulation of JNI routines from dexdec unlocks use-cases that are now becoming commonplace, such as:

  • Object consumption relying on native code calls to make reverse-engineering harder. The typical case is the retrieval of encrypted strings where part of the decryption code is bytecode, part is native code.
  • General app tweaking done on the native side, such as field setting, field reading, method invocation, object creation, etc.

Example

Here is an example of what could not be done by JEB <4.29:

//
// dex code:
//

package a.b;

class X {
  ...
  native String decrypt(char[] array, int key1, int key2);
  ...
  void f() {
    return decrypt(new char[]{'K', 'F', 'C'}, 4, 3);
  }
  ...
}

//
// native code:
//

// pseudo-code for method `dec` mapping to `a.b.X.decrypt`
jstring dec(JNIEnv* env, jobject this, jcharArray array, int a, int b) {
  int len = (*env)->GetArrayLength(env, array);
  uint16_t out[len];
  for(int i = 0; i < len; i++) {
    out[i] = array[i] - (a - b);
  }
  return (*env)->NewString(env, out, len);
}

JEB used to decompile X.f() to:

void f() {
  return decrypt(new char[]{'K', 'F', 'C'}, 4, 3);
}

JEB 4.29, if the native emulator is enabled, is able to return a simpler version:

void f() {
  return "JEB";
}

Preparation

Currently, the native emulator is disabled by default. In order to let dexdec use it, edit your dexdec-emu.cfg file (located in your coreplugins/ folder, or in the GUI, Android menu, handler Emulator Settings…):

  • Mandatory: set enable_native_code_emulator to true
  • Recommended: increase the values of emu_max_duration and emu_max_itercount (the reason being the the analysis of native images by the native code plugins can be quite time-consuming).

You will also need a JEB Pro license to use this feature.

Output

As usual, the auto-decryption of an item will also emit an event, which can be collected programmatically, and visible in the Decompiler’s “Events” fragment in the GUI.

Items whose address is formatted as @LIB:<lib.so>@NativeAddress are decrypted native items that were found in the SO image at some point.

Decrypted strings collected by the decompiler

Similarly, decrypted items found in decompiled code are rendered using a purple’ish pink (by default) in the GUI.

If native code was involved in the decryption, the on-hover pop-up will let you know:

Decryption of that string required emulation of native code

API

The native emulator(s) managed by a dexdec‘s IDState can be customized with the following newly-added methods and types:

  • enableNativeCodeEmulator / isNativeCodeEmulatorEnabled : enable or disable the native emulator (the master setting is pulled from your config file, dexdec-emu.cfg)
  • registerNativeEmulatorHooks / unregisterNativeEmulatorHooks : hooks into the evaluation (emulation) of the native code – refer to the appropriate hooks interfaces. The hooks receives a reference to the controlling EEmulator.
  • unregisterNativeEmulatorHooks / ununregisterNativeEmulatorHooks : hooks into the memory accesses of the emulator’s state – refer to the appropriate hooks interfaces. The hooks receives a reference to the target EState object.

Conclusion

Interfacing both emulators offers many possibilities to improve the reverse-engineering experience of complex binaries and applications.

There is more that can be done, which will be discussed further blog posts:

  • Retrieval of statically registered natives (through JNIEnv’s RegisterNatives) as opposed to native routines automatically resolved using the JNI naming conventions.
  • Automatic unpacking of native code.
  • Use of the native emulator in custom scripts and plugins.

Note that this feature is currently limited to JEB Pro.

The JNI native code emulator will work with x86, x64, and arm64 code (we may add support for arm in the near future). Needless to say, it is still in experimental mode! Therefore, you may encounter strange results or problems while analyzing code making use of it. Please send us error reports to support@pnfsoftware.com.

Until next time, and once again, thank you to our amazing users for their continued support and kind words 🙂 — Nicolas.

Reversing an Android app Protector, Part 3 – Code Virtualization

In this series: Part 1, Part 2, Part 3

The third part of this series is about bytecode virtualization. The analyses that follow were done statically.

Bytecode virtualization is the most interesting and technically challenging feature of this protector.

TL;DR:
– JEB Pro can un-virtualize protected methods.
– A Global Analysis (Android menu) will point you to p-code VM routines.
– Make sure to disable Parse Exceptions when decompiling such methods.
– For even clearer results, rename opaque predicates of the method to guard0/guard1 (refer part 1 of this blog for details)

What Is Code Virtualization

Relatively novel, code virtualization is possibly one of the most effective protection technique there is 1. With it come relatively heavy disadvantages, such as hampered speed of execution 2 and the difficulty to troubleshoot production code. The advantages are heightened reverse-engineering hurdles over other more traditional software protection techniques.

Virtualization in the context of code protection means:

  • Generating a virtual machine M1
  • Translating an original code object C0 meant to be executed on a machine M0 3, into a semantically-equivalent code object C1, to be run on M1.

While the general features of M1 are likely to be fixed (e.g., all generations of M1 are stack machines with such and such characteristics), the Instruction Set Architecture (ISA) of M1 may not necessarily be. For example, opcodes, microcodes and their implementation may vary from generation to generation. As for C1, the characteristics of a generation are only constrained by the capabilities of the converter. Needless to say, standard obfuscation techniques can be applied on C1. The virtualization process can possibly be recursive (C1 could be a VM implementing the specifications of a machine M2, executing a code object C2, emulating the original behavior of C0, etc.).

All in all, in practice, this makes M1 and C1 unique and hard to reverse-engineer.

Before and after virtualization of a code object C0 into C1

Example of a Protected Method

Note: all identifier names had been obfuscated. They were renamed for clarity and understanding.

Below, the class VClass was found to be “virtualized”. A virtualized class means that all non-constructor (all but <init>(*)V and <clinit>()V) methods were virtualized.

Interestingly, the constructors were not virtualized

The method d(byte[])byte[] is virtualized:

  • It was converted into an interpreter loop over two large switch constructs that branch on pseudo-code entries stored in the local array pcode.
  • A PCodeVM class was added. It is a modified stack-based virtual machine (more below) that performs basic load/store operations, custom loads/stores, as well as some arithmetic, binary and logical operations.
Virtualized method. Note the pcode array. The opcode handlers are located in two switches. This picture shows the second switch, used to handle specific operations and API calls.

A snippet of the p-code VM class. Full code here, also contains the virtualized class.

The generic interpreter is called via vm.exec(opcode). Execution falls back to a second switch entry, in the virtualized method, if the operation was not handled.

Please refer to the gist linked above for a full list of “generic” VM operations. Three examples, including one showing that the operations are not as generic as the term implies:

(specific to this VM) opcode 6, used to peek the most recently pushed object
(specific to this VM) opcode 8, a push-int operation
(specific to this VM) opcode 23 is relatively specialized, it implements an add-xor stack operation (pop, pop, push). It is quite interesting to see that the protection system does not simply generate one-to-one, dalvik-to-VM opcodes. Instead, the target routine is thoroughly analyzed, most likely lifted, high-level (compounded) arithmetic operations isolated, and pseudo-generic (in PCodeVM) or specialized (in the virtualized method) opcodes generated.

As said, negative opcodes represent custom operations specific to a virtualized method, including control flow changes. An example:

opcode -25: a if(a >=b) goto LABEL operation (first, call into opcode 55 to do a GE operation on the top two integers; then, use the result to do conditional branching)

Characteristics of the P-code VM

From the analysis of that code as well as virtualized methods found in other binaries, the characteristics of the p-code VM generated by the app protector can be inferred:

  • The VM is a hybrid stack machine that uses 5 parallel stacks of the same height, stored in arrays of:
    • java.lang.Object (accommodating all objects, including arrays)
    • int (accommodating all small integers, including boolean and char)
    • long
    • float
    • double
  • For each one of the 5 stack types above, the VM uses two additional registers for storing and loading
  • Two stack pointers are used: one indicates the stack TOP, the other one seems to be used more liberally, and is akin to a peek register
  • The stack has a reserved area to store the virtualized method parameters (including this if the method is non-static)
  • The ISA encoding is trivial: each instruction is exactly one-word long, it is the opcode of the p-code instruction to be executed. There is no concept of register, index, or immediate value embedded into the instruction, as most stack machine ISA’s have.
  • Because the ISA is so simple, the implementation of the semantics of an instruction falls almost entirely on the p-code handler. For this reason, they were grouped into two categories:
    • Semi-generic VM operations (load/store, arithmetic, binary, tests) are handled by the VM class and have a positive id. (A VM object is used by every virtualized method in a virtualized class.)
    • Operations specific to a given virtualized method (e.g., method invocations) use negative ids and are handled within the virtualized method itself.

P-code obfuscation: junk insertion, spaghetti code

While the PCodeVM opcodes are all “useful”, many specific opcodes of a virtualized method (negative ids) achieve nothing but the execution of code semantically equivalent to NOP or GOTO.

opcodes -2, -1: essentially branching instructions. A substantial amount of those can be found, including some branching to blocks with no other input but that source (i.e., an unnecessary GOTO – =spaghetti code -, or a NOP operation if the next block is the follow.)

Rebuilding Virtualized Methods

Below, we explain the process used to rebuild a virtualized method. The CFG’s presented are IR-CFG’s (Intermediate Representations) used by the dexdec 4 pipeline. Note that unlike gendec‘s IR 5, dexdec‘s IR is not exposed publicly, but its textual representation is mostly self-explanatory.

Overall, a virtualized routine, once processed by dexdec like any other routine, looks like the following: A loop over p-code entries (stored in x8 below), processed by a() at 0xE first, or by the large routine switch.

Virtualized method, optimized, virtualized

The routine a() is PCodeVM.exec(), and its optimized IR boils down to a large single switch. 6

PCodeVM.exec()

The unvirtualizer needs to identify key items in order to get started, such as the p-code entries, identifiers used as indices into the p-code array, etc. Once they have been gathered, concolic execution of the virtualized routine becomes possible, and allows rebuilding a raw version of the original execution flow. Multiple caveats need to be taken care of, such as p-code inlining, branching, or flow termination. In its current state, the unvirtualizer disregards exceptional control flow.

Below, a raw version of the unflattened CFG. Note that all operations are stack-based; the code itself has not been modified at this point, it still consists of VM stack-based operations.

Virtualized method after unflattening, raw

dexdec’s standard IR optimization passes (dead-code removal, constant and variable propagation, folding, arithmetic simplification, flow simplifications, etc.) clean up the code substantially:

Virtualized method after unflattening and IR optimizations (opt1)

At this stage, all operations are stack-based. The high-level code generated from the above would be quite unwieldy and difficult to analyze, although substantially better than the original double-switch.

The next stage is to analyze stack-based operations to recover stack slots uses and convert them back to identifiers (which can be viewed as virtual registers; essentially, we realize the conversion of stack-based operations into register-based ones). Stack analysis can be done in a variety of ways, for example, using fixed-point analysis. Again, several caveats apply, and the need to properly identify stacks as well as their indices is crucial for this operations.

Virtualized method after unflattening, IR optimizations, VM stack analysis (opt2)

After another round of optimizations:

Virtualized method after unflattening, IR optimizations, VM stack analysis, IR optimizations (opt2_1)

Once the stack analysis is complete, we can replace stack slot accesses by identifier accesses.

Virtualized method after unflattening, IR optimizations, VM stack analysis, IR optimizations, virtual registers insertion (opt3)

After a round of optimizations:

Virtualized method after unflattening, IR optimizations, VM stack analysis, IR optimizations, virtual registers insertion, IR optimizations (opt3)

At this point, the “original” CFG is essentially reconstructed, and other advanced deobfuscation passes (e.g., emulated-based deobfuscators) can be applied.

The high-level code generation yields a clean, unvirtualized routine:

High-level code, unvirtualized, unmarked

After reversing, it appears to be a modified RC4 algorithm. Note the +3/+4 added to the key.

High-level code, unvirtualized, marked

Detecting Virtualized Methods

All versions of JEB detect virtualized methods and classes: run Global Analysis (GUI menu: Android) on your APK/DEX and look for those special events:

dexdec event:
“Found Virtualized routine handler (P-Code VM)”

JEB Pro version 3.22 7 ships with the unvirtualizer module.

Tips:

  • Make sure to enable the Obfuscators, and enable Unvirtualization (enabled by default in the options).
  • The try-blocks analysis must be disabled for the class to unvirtualize. (Use MOD1+TAB to redecompile, untick “Parse Exception Blocks”).
  • After a first decompilation pass, it may be easier to identify guard0/guard1, rename, and recompile, else OP obfuscation will remain and make the code unnecessarily difficult to read. (Refer to part 1 of this series to learn about what renaming those fields to those special names means and does when a protected app is detected.)

Conclusion

We hope you enjoyed this third installment on code (un)virtualization.

There may be a fourth and final chapter to this series on native code protection. Until next time!

  1. On a personal note, my first foray into VM-based protection dates back to 2009 with the analysis of Trojan.Clampi, a Windows malware protected with VMProtect
  2. Although one could argue that with current hardware (fast x64/ARM64 processors) and software (JIT’er and AOT compilers), that drawback may not be as relevant as it used to be.
  3. Machine here may be understood as physical machine or virtual machine
  4. dexdec is JEB’s dex decompiler engine
  5. gendec is JEB’s generic decompilation pipeline
  6. Note the similarities with CFG flattened by chenxification and similar techniques. One key difference here is that the next block may be determined using the p-code array, instead of a key variable, updated after each operation. I.e., is the FSM – controlling what the next state (= the next basic block) is – embedded in the flattened code itself, or implemented as a p-code array.
  7. JEB Android and JEB demo builds do not ship the unvirtualizer module. I initially wrote this module as a proof-of-concept not intended for release, but eventually decided to offer it to our professional users who have legitimate (non malicious) use cases, e.g. code audits and black-box assessments.

Reversing an Android app Protector, Part 2 – Assets and Code Encryption

In this series: Part 1, Part 2, Part 3

The second part of this series focuses on encryption:

  • Asset encryption
  • Class encryption
  • Full application encryption

Those analyses were done statically using JEB 3.21.

Asset Encryption

Assets can be encrypted, while combining other techniques, such as class encryption (seen in several high-profile apps), and bytecode obfuscation (control-flow obfuscation, string encryption, reflected API access). With most bytecode obfuscation being automatically cleaned up, Assets are being accessed in the following way:

Purple and cyan tokens represent auto-decrypted code. The assets decryptor method was renamed to ‘dec’, it provides a FilterInputStream that transparently decrypts contents.
The DecryptorFilterStream (renamed) factory method

The DecryptorFilterStream object implements a variant of TEA (Tiny Encryption Algorithm), known for its simplicity of implementation and great performance 1.

Note the convoluted generation of Q_w, instead of hard-coding the immediate 0x9E37. Incidentally, a variant of that constant is also used by RC5 and RC6.
read() decrypts and buffers 64 bits of data at a time. The decryption loop consists of a variable number of rounds, between 5 and 16. Note that Q_w is used as a multiplier instead of an offset, as TEA/XTEA normally does.

It seems reasonable to assume that the encryption and decryption algorithms may not always be the same as this one. This app protector making extensive use of polymorphism throughout its protection layers, it could be the case that during the protection phase, the encryption primitive is either user-selected or selected semi-randomly.

JEB can automatically emulate throughout this code and extract assets, and in fact, this is how encrypted classes, described in the next section, were extracted for analysis. However, this functionality is not present in current JEB Release builds. Since the vast majority of uses are legitimate, we thought that shipping one-click auto-decryptors for data and code at this time was unnecessary, and would jeopardize the app security of several high-profile vendors.

Class Encryption

Class encryption, as seen in multiple recent apps as well, works as follows:

  • The class to be protected, CP, is encrypted, compressed, and stored in a file within the app folder. (The filename is random and seems to be terminated by a dot, although that could easily change.) Once decrypted, the file is a JAR containing a DEX holding CP and related classes.
  • CP is managed by a custom ClassLoader, CL.
  • CL is also encrypted, compressed, and stored in a file within the app folder. Once decrypted, the file is a JAR containing a DEX holding the custom class loader CL.
  • Within the application, code using CP (that is, any client that loads CP, invokes CP methods, or accesses CP fields) is replaced by code using CM, a class manager responsible for extracting CP and CL, and loading CL. CM offers bridge methods to the clients of CP, in order to achieve the original functionality.

The following diagram summarizes this mechanism:

Class encryption mechanism

Since protected applications use the extensive RASP (Runtime Application Self-Protection) facility to validate the environment they’re running on, the dynamic retrieval of CL and CP may prove difficult. In this analysis, it was retrieved statically by JEB.

Below, some client code using CM to create an encrypted-class object CP and execute a method on it. Everything is done via reflection. Items were renamed for enhanced clarity.

Encrypted class loading and virtual method invocation

CM is a heavy class, highly obfuscated. The first step in understanding it is to:

With auto-decryption and auto-unreflection enabled, the result is quite readable. A few snippets follow:

Decrypted files are deleted after loading. On older devices, loading is done with DexFile; on newer devices, it is done using InMemoryDexClassLoader.
In this case, the first encrypted JAR file (holding CL) is stored as “/e.”.
In this case, the second encrypted JAR file (holding CP and related) is stored as “/f.”.
The application held two additional couples, (“/a.”, “/b.”) and (“/c.”, “/d.”)

Once retrieved, those additional files can easily be “added” to the current DEX unit with IDexUnit.addDex() of your JEB project. Switch to the Terminal fragment, activate the Python interpreter (use py), and issue a command like:

Using Jython’s to add code to an existing DEX unit
The bnz class (CL) is a ClassLoader for the protected class (CP).

The protected class CP and other related classes, stored in “/f.” contained… anti-tampering verification code, which is part of the RASP facility! In other instances that were looked at, the protected classes contained: encrypted assets manager, custom code, API key maps, more RASP code, etc.

Full Application Encryption

“Full” encryption is taking class encryption to the extreme by encrypting almost all classes of an application. A custom Application object is generated, which simply overloads attachBaseContext(). On execution, the encrypted class manager will be called to decrypt and load the “original” application (all other protections still apply).

Custom application object used to provide full program encryption.

Note that activities can be encrypted as well. In the above case, the main activity is part of the encrypted jar.

Conclusion

That’s it for part 2. We focused on the encryption features. Both offer relatively limited protection for reverse-engineers willing to go the extra mile to retrieve original assets and bytecodes.

In Part 3, we will present what I think is the most interesting feature of this protector, code virtualization.

Until next time!

  1. The TEA encryption family is used by many win32 packers

Reversing an Android app Protector, Part 1 – Code Obfuscation & RASP

In this series: Part 1, Part 2, Part 3

What started as a ProGuard + basic string encryption + code reflection tool evolved into a multi-platform, complex solution including: control-flow obfuscation, complex and varied data and resources encryption, bytecode encryption, virtual environment and rooted system detection, application signature and certificate pinning enforcement, native code protection, as well as bytecode virtualization 1, and more.

This article presents the obfuscation techniques used by this app protector, as well as facility made available at runtime to protected programs 2. The analysis that follows was done statically, with JEB 3.20.

Identification

Identifying apps protected by this protector is relatively easy. It seems the default bytecode obfuscation settings place most classes in the o package, and some will be renamed to invalid names on a Windows system, such as con or aux. Closer inspection of the code will reveal stronger hints than obfuscated names: decryption stubs, specific encrypted data, the presence of some so library files, are all tell tale signs, as shown below.

Running a Global Analysis

Let’s run a Global Analysis (menu Android, Global analysis…) with standard settings on the file and see what gets auto-decrypted and auto-unreflected:

Results subset of Global Analysis (redacted areas are meant to keep the analyzed program anonymous; it is a clean app, whose business logic is irrelevant to the analysis of the app protector)

Lots of strings were decrypted, many of them specific to the app’s business logic itself, others related to RASP – that is, library code embedded within the APK, responsible for performing app signature verification for instance. That gives us valuable pointers into where we should be looking at if we’d like to focus on the protection code specifically.

Deobfuscating Code

The first section of this blog focuses on bytecode obfuscation and how JEB deals with it. It is mostly automated, but a final step requires manual assistance to achieve the best results.

Most obfuscated routines exhibit the following characteristics:

  • Dynamically generated strings via the use of per-class decryption routines
  • Most calls to external routines are done via reflection
  • Flow obfuscation via the use of a couple of opaque integer fields – let’s call them OPI0, OPI1. They are class fields generally initialized to 0 and 1.
  • Arithmetic operation obfuscation
  • Garbage code insertion
  • Unusual protected block structure, leading to fragmented try-blocks, unavoidable to produce semantically accurate raw code

As an example, the following class is used to perform app certificate validation in order, for instance, to prevent resigned apps from functioning. A few items were renamed for clarity; decompilation is done with disabled Deobfuscators (MOD1+TAB, untick “Enable deobfuscators”):

Take #1 (snippet) – The protected class is decompiled without deobfuscation in order to show semi-raw output (a few optimizers doing all sort of code cleanup are not categorized as deobfuscators internally, and will perform even if Deobfuscation is disabled). Note that a few items were also renamed for clarity.

In practice, such code is quite hard to comprehend on complex methods. With obfuscators enabled (the default setting), most of the above will be cleared.

See the re-decompilation of the same class, below.

  • strings are decrypted…
  • …enabling unreflection
  • most obfuscation is removed…
  • except for some control flow obfuscation that remains because JEB was unable to process OPI0/OPI1 directly (below,
Take #2 (full routine) – obfuscators enabled (default). The red blocks highlight use of opaque variables used to obfuscate control flow.

Let’s give a hint to JEB as to what OPI0/OPI1 are.

  • When analyzing protected apps, you can rename OPI0 and OPI1 to guard0 and guard1, respectively, to allow JEB go aggressively clean the code
  • Redecompile the class after renaming the fields
Take #3 (full routine) – with explicit guard0/guard1

That final output is clean and readable.

Other obfuscation techniques not exposed in this short routine above are arithmetic obfuscation and other operation complexification techniques. JEB will seamlessly deal with many of them. Example:

is optimized to

To summarize bytecode obfuscation:

  • decryption and unreflection is done automatically 3
  • garbage clean-up, code clean-up is also generic and done automatically
  • control flow deobfuscation needs a bit of guidance to operate (guard0/guard1 renaming)

Runtime Verification

RASP library routines are used at the developers’ discretion. They consist of a set of classes that the application code can call at any time, to perform tasks such as:

  • App signing verification
  • Debuggability/debugger detection
  • Emulator detection
  • Root detection
  • Instrumentation toolkits detection
  • Certificate pinning
  • Manifest check
  • Permission checks

The client decides when and where to use them as well as what action should be taken on the results. The code itself is protected, that goes without saying.

App Signing Verification

  • Certificate verification uses the PackageManager to retrieve app’s signatures: PackageManager.getPackageInfo(packageName, GET_SIGNATURES).signatures
  • The signatures are hashed and compared to caller-provided values in an IntBuffer or LongBuffer.

Debug Detection

Debuggability check

The following checks must pass:

  • assert that Context.ctx.getApplicationInfo().flags & ApplicationInfo.FLAG_DEBUGGABLE is false
  • check the ro.debuggable property, in two ways to ensure consistency
    • using android.os.SystemProperties.get() (private API)
    • using the getprop‘s binary
  • verify that no hooking framework is detected (see specific section below)

Debugging session check

The following checks must pass:

  • assert that android.os.Debug.isDebuggerConnected() is false
  • verify no tracer process: tracerpid entry in /proc/<pid>/status must be <= 0
  • verify that no hooking framework is detected (see specific section below)

Debug key signing

  • enumerate the app’s signatures via PackageInfo.signatures
  • use getSubjectX500Principal() to verify that no certificate has a subject distinguished name (DN) equals to "CN=Android Debug,O=Android,C=US", which is the standard DN for debug certificates generated by the SDK tools

Emulator Detection

Emulator detection is done by checking any of the below.

1) All properties defined in system/build.prop are retrieved, hashed, and matched against a small set of hard-coded hashes:

86701cb958c69d64cd59322dfebacede -> property ???
19385aafbb452f39b5079513f668bbeb -> property ???
24ad686ec83d904347c5a916acbe1779 -> property ???
b8c8255febc6c46a3e43b369225ded3e -> property ???
d76386ddf2c96a9a92fc4bc8f829173c -> property ???
15fed45d5ca405da4e6aa9805daf2fbf -> property ??? (unused)

Unfortunately, we were not able to reverse those hashes back to known property strings – however, it was tried only on AOSP emulator images. If anybody wants to help and run the below on other build.prop files, feel free to let us know what property strings those hashes match to. Here is the hash verification source, to be run be on build.prop files.

2) The following file is readable:

/sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq

3) Verify if any of those qemu, genymotion and bluestacks emulator files exist and are readable:

/dev/qemu_pipe
/dev/socket/baseband_genyd
/dev/socket/genyd
/dev/socket/qemud
/sys/qemu_trace
/system/lib/libc_malloc_debug_qemu.so
/dev/bst_gps
/dev/bst_time
/dev/socket/bstfolderd
/system/lib/libbstfolder_jni.so

4) Check for the presence of wired network interfaces: (via NetworkInterface.getNetworkInterfaces)

eth0
eth1

5) If the app has the permission READ_PHONE_STATE, telephony information is verified, an emulator is detected if any of the below matches (standard emulator image settings):

- "getLine1Number": "15555215554", "15555215556", "15555215558", "15555215560", "15555215562", "15555215564", "15555215566", "15555215568", "15555215570", "15555215572", "15555215574", "15555215576", "15555215578", "15555215580", "15555215582", "15555215584"
- "getNetworkOperatorName": "android"
- "getSimSerialNumber": "89014103211118510720"
- "getSubscriberId": "310260000000000"
- "getDeviceId": "000000000000000", "e21833235b6eef10", "012345678912345"

6) /proc checks:

/proc/ioports: entry "0ff :" (unknown port, likely used by some emulators)
/proc/self/maps: entry "gralloc.goldfish.so" (GF: older emulator kernel name)

7) Property checks (done in multiple ways with a consistency checks, as explained earlier), failed if any entry is found and start with one of the provided values:

- "ro.product.manufacturer": "Genymotion", "unknown", "chromium"
- "ro.product.device": "vbox86p", "generic", "generic_x86", "generic_x86_64"
- "ro.product.model": "sdk", "emulator", "App Runtime for Chrome", "Android SDK built for x86", "Android SDK built for x86_64"
- "ro.hardware": "goldfish", "vbox86", "ranchu"
- "ro.product.brand": "generic", "chromium"
- "ro.kernel.qemu": "1"
- "ro.secure": "0"
- "ro.build.product": "sdk", "vbox86p", "full_x86", "generic_x86", "generic_x86_64"
- "ro.build.fingerprint": "generic/sdk/generic", "generic_x86/sdk_x86/generic_x86", "generic/google_sdk/generic", "generic/vbox86p/vbox86p", "google/sdk_gphone_x86/generic_x86"
- "ro.bootloader": "unknown"
- "ro.bootimage.build.fingerprint": "Android-x86"
- "ro.build.display.id": "test-"
- "init.svc.qemu-props" (any value)
- "qemu.hw.mainkeys" (any value)
- "qemu.sf.fake_camera" (any value)
- "qemu.sf.lcd_density" (any value)
- "ro.kernel.android.qemud" (any value)

Hooking Systems Detection

The term covers a wide range of techniques designed to intercept regular control flow in order to examine and/or modify execution.

1) Xposed instrumentation framework detection, by attempting to load any of the classes:

de.robv.android.xposed.XposedBridge
de.robv.android.xposed.XC_MethodHook

Class loading is done in different ways in an attempt to circumvent hooking itself, using Class.forName with a variety of class loaders, custom class loaders and ClassLoader.getLoadedClass, as well as lower-level private methods, such as Class.classForName.

2) Cydia Substrate instrumentation framework detection.

3) ADBI (Android Dynamic Binary Instrumentation) detection

4) Stack frame verification: an exception is generated in order to retrieve a stack frame. The callers are hashed and compared to an expected hard-coded value.

5) Native code checks. This will be detailed in another blog, if time allows.

Root Detection

While root detection overlaps with most of the above, it is still another layer of security a determined attacker would have to jump over (or walk around) in order to get protected apps to run on unusual systems. Checks are plenty, and as is the case for all the code described here, heavily obfuscated. If you are analyzing such files, keeping the Deobfuscators enabled and providing guard0/guard1 hints is key to a smooth analysis.

Static initializer of the principal root detection class. Most artifacts indicative of a rooted device are searched for by hash.

Build.prop checks. As was described in emulator detection.

su execution. Attempt to execute su, and verify whether su -c id == root

su presence. su is looked up in the following locations:

/data/local/
/data/local/bin/
/data/local/xbin/
/sbin/
/system/bin/
/system/bin/.ext/
/system/bin/failsafe/
/system/sd/xbin/
/system/usr/we-need-root/
/system/xbin/

Magisk detection through mount. Check whether mount can be executed and contains databases/su.db (indicative of Magisk) or whether /proc/mounts contains references to databases/su.db.

Read-only system partitions. Check if any system partition is mounted as read-write (when it should be read-only). The result of mount is examined for any of the following entries marked rw:

/system
/system/bin
/system/sbin
/system/xbin
/vendor/bin
/sbin
/etc

Verify installed apps in the hope of finding one whose package name hashes to the hard-coded value:

0x9E6AE9309DBE9ECFL

Unfortunately, that value was not reversed, let us know if you find which package name generates this hash – see the algorithm below:

    public static long hashstring(String str) {
        long h = 0L;
        for(int i = 0; i < str.length(); i++) {
            int c = str.charAt(i);
            h = h << 5 ^ (0xFFFFFFFFF8000000L & h) >> 27 ^ ((long)c);
        }
        return h;
    }

NOTE: App enumeration is performed in two ways to maximize chances of evading partial hooks.

  • Straightforward: PackageManager.getInstalledApplications
  • More convoluted: iterate over all known MAIN intents: PackageManager.queryIntentActivities(new Intent("android.intent.action.MAIN")), derive the package name from the intent via ResolveInfo.activityInfo.packageName

SElinux verification. If the file /sys/fs/selinux/policy cannot be read, the check immediately passes. If it is readable, the policy is examined and hints indicative of a rooted device are looked for by hash comparison:

472001035L
-601740789L

The hashing algorithm is extremely simple, see below. For each byte of the file, the crc is updated and compared to hard-coded values.

long h = 0L;
//for each byte:
    h = (h << 5 ^ ((long)(((char)b)))) & 0x3FFFFFFFL;
    // check h against known list

Running processes checks. All running processes and their command-lines are enumerated and hashed, and specific values are indirectly looked up by comparing against hard-coded lists.

APK Check

This verifier parses compressed entries in the APK (zip) file and compares them against well-known, hard-coded CRC values.

Manifest Check

Consistency checks on the application Manifest consists of enumerating the entries using two different ways and comparing results. Discrepancies are reported.

  • Open the archive’s MANIFEST.MF file via Context.getAssets(), parse manually
  • Use JarFile(Context.getPackageCodePath()).getManifest().getEntries()

Discrepancies in the Manifest could indicate system hooks attempting to conceal files added to the application.

Permissions Check

This routine checks for permission discrepancies between what’s declared by the app and what the system grant the app.

  • Set A: App permission gathering: all permissions requested and defined by the app, as well as all permissions offered by the system, plus the INTERACT_ACROSS_USERS and INTERACT_ACROSS_USERS_FULL permissions,
  • Set B: Retrieve all permissions that exist on the system
  • Define set C = B – A
  • For every permission in C, use checkCallingOrSelfPermission (API 22-) or checkSelfPermission (API 23+) to verify that the permission is not granted.

Permission discrepancies could be used to find out system hooks or unorthodox execution environments.

Note the “X & -(A+1) | ~X & A” checks. Several opaque arithmetic/binary expressions attempt to complicate the control flow. Here, that expression is never equals to v2, and therefore, the if-check will always fail. JEB 3.20 does not clean all those artifacts.

Miscellaneous

Other runtime components include library code to perform SSL certificate pinning, as well as obfuscated wrappers around web view clients. None of those are of particular interest.

Wrapper for android.webkit.WebViewClient. Make sure to enable deobfuscators and provide guardX hints. When this is done, most methods will be crystal clear. In fact, the majority of them are simple forwarders.

Conclusion

That’s it for the obfuscation and runtime protection facility. Key take-away to analyze such protected code:

  • Keep the obfuscators enabled
  • Locate the opaque integers, rename them to guard0/guard1 to give JEB a hint on where control flow deobfuscation should be performed, and redecompile the class

The second part in the series presents bytecode encryption and assets encryption.

  1. VM in VM, repeat ad nauseam – something not new to code protection systems, it’s existed on x86 for more than a decade, but new on Android, and other players in this field, commercial and otherwise, seem to be implementing similar solutions.
  2. So-called “RASP”, a relatively new acronym for Runtime Application Self-Protection
  3. Decryption and unreflection are generic processes of dexdec (the DEX Decompiler plugin); there is nothing specific to this protector here. The vast majority or encrypted data, regardless of the protection system in place, will be decrypted.

JEB Android Updates – Generic String Decryption, Lambda Recovery, Unreflecting Code, and More

Updated on March 11.

A note about 2020 Q1 updates (versions 3.10 to 3.16) regarding the DEX/Dalvik decompiler modules:

  • Generic String Decryption
  • Lambda Recovery
  • Unreflecting Code
  • Decompiling Java Bytecode
  • Auto-Rename All

Generic String Decryption

JEB ships with a generic deobfuscator that can perform on-the-fly string decryption and other complex optimizations. Although this optimizer performs safe (i.e., guaranteed) optimizations in most cases, it is unsafe in the general case case and therefore, may be disabled in the options. Refer to the Engines options .parsers.dcmp_dex.EnableDeobfuscators and .parsers.dcmp_dex.EmulationSupport.

Many code protectors offer options to replace immediate string constants by method invocations that perform on-the-fly decryption.

A variety of techniques exist, ranging from simple one-off trivial decryptor methods, to complex schemes involving object(s) creation, complicated decryptors injected in third-party packages, non-trivial logic, junk code meant to slow down analyzers, use of opaque predicates, etc. They are implemented in an infinite number of ways. JEB’s generic deobfuscator can perform quick, safe emulation of the intermediate representation to provide a replacement. It may sometimes fail or bail out due to several reasons, such as performance or pitfalls like anti-emulation and anti-sandboxing techniques.

Example 1

The string decryptor is a static method reading encrypted string data in a class byte array. Also note that code reflection is used.

The above code (blue box) ends up being deobfuscated to:

Example 2:

The decryption methods were injected into library packages, e.g. Gson’s

The above code is deobfuscated to:

With the generic string decryptor optimizer ON

Below, a decryptor that had been injected into the com.google.gson.Gson() class:

The concat(String, int) method is not part of standard Gson, of course. It was injected by the protector and is used by (some) code to perform on-the-fly string decryption.

Example 3:

One last example, which was involuntarily – yet, quite timely! – provided by a user:

Static fields initialized with decrypted strings.
After decryption.

Decrypting all strings: The decryptor kicks in when decompiling methods only. At the moment, if a string happens to be successfully decrypted, the optimizer does not attempt to recover all similarly encrypted strings in the code, although it is most certainly an addition that will make it in a future software update.

Rendering: You may quickly identify decrypted strings in the client as they are rendered using a special color associated with the itemId STRING_GENERATED, by default rendered in a flashy pink color in light and dark themes. Hovering over such items will bring up a pop-up with additional origin information, like the underlying code that would have generated that string:

Auto-decrypted strings in pink; overlays with source/origin information.

API:
– From a DEX perspective: Generated strings are artificial. Therefore, IDexString.isArtificial() would return true.
– From a Java/AST perspective: IJavaConstant objects that embed origin information do so using the “origin” tag. Use IJavaConstant.getTags().get("origin") to retrieve it.

Lambda Recovery

JEB attempts to perform Java 8 style lambda recovery and reconstruction.

Desugared Lambdas

Recovery and reconstruction does not rely on any type of metadata 1, such as special prefixes -$$Lambda$ for classes and methods implementing desugared lambdas in dex 37-.

You may therefore see constructs like this:

This DEX file contains desugared, non-obfuscated lambdas.
This DEX file contained desugared, obfuscated lambdas

Options: Lambda reconstruction can be disabled in the options (Edit, Options, Engines, …). Lambda rendering can also be disabled in the options, as well as on-demand by right-clicking a decompiled view, Rendering Options….

Lambdas options

API Note: In the above cases, the underlying Java AST may be a IJavaNew or IJavaStaticField node. This is not the case for real (not desugared) lambdas, which map to an IJavaCall node – see below.

Real Lambdas

Lambda reconstruction also takes place when the code has not been desugared (which is rare!), i.e. code relying on dex38’s invoke-custom and invoke-polymorphic.

This DEX file contains real lambdas implemented via invoke-custom

API Note: Such lambdas map to an IJavaCall node for which isCustomCall() will return true.

Unreflecting Code

Many code protectors make heavy use of reflection – combined with string encryption, as we’ll see below – to obfuscate code. In practice, reflection is limited to method invocation (static and virtual), static and non-static field setting and getting, and new instance creation. A few examples:

v = Class.forName("java.lang.Integer").getMethod("valueOf",
        String.class).invoke(null, str);
// instead of 
v = Integer.valueOf(str);
Class.forName("SomeClassName").getField("b").setInt(x, 4);
// instead of 
x.b = 4;
Class.forName("java.lang.String").getConstructor(byte[].class)
        .newInstance(val);
// instead of
new String(arg6);

Such code is generally protected by a catch-all handler that forwards the cause of any exception raised by a reflection issue:

try {
    // ...
}
catch(Throwable e) {
    throw e.getCause();
}

By default, JEB will attempt to unreflect code. This deobfuscator is potentially unsafe and may be disabled in the options. Note that you always have the ability to choose, for a particular decompilation, whether some options should be temporarily enabled or disabled, by pressing CTRL+TAB (or COMMAND+TAB on macOS) to decompile (same as menu Action, Decompile with options…).

Unsafe deobfuscators can be globally disabled.

So, in a nutshell, code normally decompiled to:

Reflection, not cleaned (malware was obfuscated)

will be decompiled to:

With reflection cleaned

Technical Note: This optimizer works on the Intermediate Representation manipulated by the decompiler, not to be confused with the AST rendered as its output. (The AST cleaner that was described in an older post is more limited than this IR optimizer.)

Last-step failures: Successfully unreflecting code eventually depends on being able to find the intended target method or field matching the provided description (method parameter types or field type). Failure to do so will generate a log like "A candidate field/method/constructor for unreflection was not found".

Decompiling Java Bytecode

JEB supports JLS bytecode decompilation for *.class files and jar-like archives (jar, war, ear, etc.). The Java bytecode is converted to Dalvik using Android’s dx by default. Users may choose to use d8 (not recommended for now) instead by selecting so in the Options.

The resulting DEX file(s) are processed as usual.

You may use this to decompile Android Library files (*.aar files) in JEB.

Examining the android-arch-core-runtime library

Auto-Rename All

JEB 3.13 introduced a new generic action, Auto-Rename All. Its implementation is at the discretion of code plugins. The DEX plugin implements it, therefore users may execute Action, Auto-Rename All… at any time (generally after processing an obfuscated file) in order to rename code items such as field, method, or class names, to something more easily processable for our -limited- human brains.

Look at this horrendous obfuscation scheme below. It’s using right-to-left unicode characters to seriously mess up rendering:

Obfuscated name using RTL Arabic characters

Let’s run Action, Auto-Rename All… on this file:

Auto-Renaming capabilities are provided (optionally) by plugins.
After auto-renaming code items in the above file. Not clearer in terms of meaning, but at least, it’s something we can start working on.

As usual, feel free to join us on Slack, message us on Twitter, or email us privately at support@pnfsoftware.com.

Until next time!

  1. Relying on metadata leads to false negatives in the best case – e.g., when the code has been minified by something like ProGuard; it leads to false positives in the worst case – e.g. forged metadata to incite the decompiler to generate inaccurate or wrong code.

Android metaresources.arsc

One of our users recently reported an Android resources.arsc file seemingly unprocessed by JEB. Upon closer inspection, it turned out this file was not a regular binary resources file, but instead, a compressed resources container serving as a generator for localized resources.arsc. Older versions of Google Play (eg, com.android.vending 11.6.18) and other official Google applications have been using this type of file, which is stored as a raw asset and sometimes named metaresources.arsc.

I decided to have a quick look. However, for better or worse, what was planned as a superficial exploration turned into a deep-dive into the rabbit-hole that was the “meta-arsc” parsing code.

Those files, as said above, are used to generate localized (non-English) resources.arsc files. That means that the client application can generate lightweight resources files on the fly. And presumably, APKs as well. Since this mechanism seems to be primarily used by the Play Store app, a reasonable use case could be Dynamic Delivery.

  • Full support was added into JEB (3.5-Beta)
  • A brief description of the file format can be found below
  • The fully annotated JDB2 is here (as well as the source apk) if you’d like to write your own implementation of a parser and localized arsc generator. The parser and generator have been thoroughly deobfuscated and commented out where need be. Package: com.google.d.a.a.a.a. Client code: FilteredResourceHelper.
    Drop both files (jdb2, apk) in a folder and open the JDB2 file in JEB

What does it look like when metaresources.arsc is processed in JEB?

JEB arsc_meta plugin, here seen processing a metaresources.arsc file. All localized resources.arsc files are generated and attached as children of the original meta file.
A french localized resources generated from a metaresources container. JEB processes those files as regular, stand-alone arsc files, and provides textual output similar to the one generated by the aapt2-dump tool from the Android SDK.

Binary Format

Disclaimer: Specification is a work-in-progress. Refer to the JDB2 annotation and code to fill in the gaps.

metaresources.arsc=
BE_UINT32                     cnt           count of languages
BE_UINT16[cnt]                langs         2-char language codes
MetaEntry[cnt]                metaentries   meta entries matching the language codes
CompressedResourceTableChunk  restab        a compressed resource table (code: 0x1002)
EOF                           -             not necessarily the EOF, but all metares
files examined contained a single resource chunk, which is a compressed resource tab MetaEntry= BE_UINT32 magic the value 'META' BE_UINT32 entrysize complete entry size (including the above
magic) in bytes BE_UINT16 lang language code VAR_INT32 cnt1 . VAR_INT32[cnt1] offsets1 a custom serialization of java.util.BitSet
(refer to JDB2 for details) holding
positions for strings and string styles
stored in string pool chunks VAR_INT32 cnt2 . VAR_INT32[cnt2] offsets2 offsets to Table Package chunk entries
(types, typespecs) => Compressed entries: - 5 types exist, basically non-XML chunk types - Their type code is the same as arsc's with the 0x1000 bit set - List of chunks: StringPool= refer to JDB2, class CompressedStringPoolChunk ResourceTable= refer to JDB2, class CompressedResourceTableChunk Package= refer to JDB2, class CompressedPackageChunk Type= refer to JDB2, class CompressedTypeChunk TypeSpec= refer to JDB2, class CompressedTypeSpecChunk

New version of Androsig

This post is a follow-up on a previous article: we have updated the Androsig plugin and the pre-generated set of library signatures.

Reminder: Androsig is a JEB plugin used to sign and match library code for Android applications.

The purpose of the plugin is to help deobfuscate lightly-obfuscated applications that perform name mangling and hierarchy flattening (such as Proguard and other common Java and Dalvik protectors). Using our collection of signatures for common libraries, library code can be recognized; methods and classes can be renamed; package hierarchies can be rebuilt

Examples

Below, an example of what that looks like on a test app:

Matched libraries on a sample app bundling the Android Support package

Another example: running Androsig on a large app (Vidmate 4.0809), see the reconstructed glide/… sub-packages below:

Matched libraries on a PlayStore app

Installation

1) Download the latest version of the compiled binary plugin and drop it into the JEB coreplugins/ folder. If you are running JEB 3.4+, the plugin should come bundled with your .

Link: JebAndroidSigPlugin-1.1.x.jar

This single JAR offers two plugin entry-points, as can be seen in the picture below:

2) Then download and extract the latest signatures package to your [JEB]/coreplugins/android_sigs/ folder.

Link: androsig_1.1_db_20190515.zip

The user interface was unchanged so you can refer to previous article for matching, generating, results and parameters.

Debugging Android apps on Android Pie and above

Update (March 2020): It looks like the problem highlighted below regarding the impossibility to read locals that do not have associated DebugLocalInfo on Android P and Q was fixed in Android R (verified with the Developer Preview 1 released on Feb 19). The patch solving this issue is likely this one.

Lower-level components of the Dalvik debugging stack, namely JDWP, JVM TI, and JVM DI implementations, were upgraded in Android Pie. It is something we indirectly noticed after installing P.beta-1 in the Spring of 2018. For lack of time, and because our recommendation is to debug apps (non-debuggable and debuggable alike) using API levels 21 (Lollipop) to 27 (Oreo)1, reversers could easily avoid road blocks which manifested in JEB as the following:

  • An empty local variable panel (with the exception of this for non-static methods)
  • Type 35 JDWP errors reported in the console, indicating that an invalid slot was being accessed

Since JEB 3.2 is out, I decided to revisit that error before jumping into anything else2

A type 35 error in this context means an invalid local slot is being accessed. In the example shown above, it would mean accessing a slot outside of [0, 10] (per the .registers directive) since the method declares a frame of 11 registers.

The second type of noticeable errors (not visible in the screenshot) were mix-ups between variable indices. Normally, and up to the JDWP implementation used in Android Pie, indices used to access slots were Java-style parameter indices (represented in Dalvik as pX), instead of Dalvik-style indices (vX). Converting from one to the other is trivial assuming the method staticity and prototype is known. It is a matter of generating pX so that they end up at the bottom of the frame. In the case above:

v0    p2
v1    p3
v2    p4
..
v9
v10   p0
v11   p1

When issuing a JDWP request (16,1) to read frame slots, we would normally use pX indices. It is no longer the case with Android P and Q: vX indices are to be used.

Open up JEB, start debugging any APK, switch to the Terminal view, and type ‘info’. In the context of JDWP, this JEB Debugger command issues Info and Sizes requests. Notice the differences:

=> On Android Oreo (API 27):

VM> info
Debuggee is running on ?
VM information: JDWP:"Android Runtime 2.1.0" v1.6 (VM:Dalvik v1.6.0)
VM identifier sizes: f=8,m=8,o=8,rt=8,fr=8

=> On Android Pie (API 28) and Android Q:

VM> info
Debuggee is running on ?
VM information: JDWP:"Java Debug Wire Protocol (Reference Implementation) version 1.8
JVM Debug Interface version 1.2
JVM version 0 (Dalvik, )" v1.8 (VM:Dalvik v0)
VM identifier sizes: f=4,m=4,o=8,rt=8,fr=8

Notice the reported version 1.2 for JVM DI, previously unspecified, and reported version 1.8 for JDWP, likely the cause of the breakage. Also note ID encoding size updates. JDWP had been reported a 1.6 version number, as well as field and method IDs encoded on 8 bytes, for as long as I can remember.

The vX/pX index issue was easily solved. It took a little while to crack the second issue. A superficial browsing of AOSP did not show anything fruitful, but after digging around, it seemed clear that this updated implementation of JDWP used CodeItem variables’ debug information to determine which variables are worth checking, and using what type.

In JEB, right-click and select Rendering Options, tick Show Debug Directives to display variable definition and re-definition information. In the example above, the APK holds information stating that v0 is being using as a boolean starting at address 2, and v1 a String starting at address 4. Android P+’s JDWP implementation does use this information to validate local variables accesses.

See below: at address 2, v0 has been declared and is rendered. v1 has not been declared yet, the debugger cannot read it (we’ll get error 35).

Single-step: at address 4, v1 is declared. Although it is uninitialized, the debugger can successfully read the var:

So – Up until P, this metadata information, when present (almost all Release-type builds of legitimate and malware files alike discard it), had been considered indicative. Now, the debugger takes it literally. There are multiple candidate reasons as of why, but an obvious one is Safety. JDWP has been known to have the potential to crash the VM when receiving reading requests for frame variables using a bad type. E.g., requesting to read an integer-holding slot as a reference would most likely crash the target VM. Using type information providing in metadata, a debugger server can now ensure that a debugger requesting to read a slot as type T is indeed a valid request – assuming the metadata is legitimate, and since the primary use case is to debug applications inside IDE, which hold source information used to generate valid debug metadata, the assumption is fair.

Validating access to local vars has the interesting side-effect to act as an anti-debugging feature. While debugging the app remains possible, not being able to easily read some locals (parameters pX are always readable though), can be quite an annoyance.

In the future, how could we work around the JDWP limitation? Well, aside from the obvious cop out “use Oreo or below”, an idea would be to extend JEB’s –makeapkdebug option (that generates a debuggable version of a non-debuggable APK) to insert DEX metadata information specifying that all variables of a frame are used and of a given type. That may not work depending on the type of validation performed by the DEX verifier, but it’s something worth exploring. Maybe more simply, an alternative could be a custom AOSP build that disabled that feature.3 Or better yet, finding if a system property exists to disable/enable that JDWP functionality.

A final note: debugging non-debuggable APKs on Android Pie or above also proved more difficult, if not practically impossible, than on Oreo and below. Assuming your phone is rooted, here’s a solution (found when browsing around AOSP commits). On a rooted phone:

> adb root
> adb shell setprop dalvik.vm.dex2oat-flags --debuggable
> adb shell stop
> adb shell start

Until next time – Thanks!

  1. Still a valid recommendation.
  2. Native decompiler upgrades are around the corner!
  3. If a charitable soul could point us to where in AOSP that would be, we’d greatly appreciate it? 🙂