Android Anti-Reversing Defenses
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In the context of anti-reversing, the goal of root detection is to make running the app on a rooted device a bit more difficult, which in turn blocks some of the tools and techniques reverse engineers like to use. Like most other defenses, root detection is not very effective by itself, but implementing multiple root checks that are scattered throughout the app can improve the effectiveness of the overall anti-tampering scheme.
For Android, we define "root detection" a bit more broadly, including custom ROMs detection, i.e., determining whether the device is a stock Android build or a custom build.
In the following section, we list some common root detection methods you'll encounter. You'll find some of these methods implemented in the that accompany the OWASP Mobile Testing Guide.
Root detection can also be implemented through libraries such as .
SafetyNet is an Android API that provides a set of services and creates profiles of devices according to software and hardware information. This profile is then compared to a list of accepted device models that have passed Android compatibility testing. Google using the feature as "an additional in-depth defense signal as part of an anti-abuse system".
How exactly SafetyNet works is not well documented and may change at any time. When you call this API, SafetyNet downloads a binary package containing the device validation code provided from Google, and the code is then dynamically executed via reflection. An [analysis by John Kozyrakis]( "SafetyNet: Google's tamper detection for Android") showed that SafetyNet also attempts to detect whether the device is rooted, but exactly how that's determined is unclear.
To use the API, an app may call the SafetyNetApi.attest method (which returns a JWS message with the Attestation Result) and then check the following fields:
ctsProfileMatch: If 'true', the device profile matches one of Google's listed devices.
basicIntegrity: If 'true', the device running the app likely hasn't been tampered with.
nonces: To match the response to its request.
timestampMs: To check how much time has passed since you made the request and you got the response. A delayed response may suggest suspicious activity.
apkPackageName, apkCertificateDigestSha256, apkDigestSha256: Provide information about the APK, which is used to verify the identity of the calling app. These parameters are absent if the API cannot reliably determine the APK information.
The following is a sample attestation result:
ctsProfileMatch Vs basicIntegrity
The SafetyNet Attestation API initially provided a single value called basicIntegrity to help developers determine the integrity of a device. As the API evolved, Google introduced a new, stricter check whose results appear in a value called ctsProfileMatch, which allows developers to more finely evaluate the devices on which their app is running.
In broad terms, basicIntegrity gives you a signal about the general integrity of the device and its API. Many Rooted devices fail basicIntegrity, as do emulators, virtual devices, and devices with signs of tampering, such as API hooks.
On the other hand, ctsProfileMatch gives you a much stricter signal about the compatibility of the device. Only unmodified devices that have been certified by Google can pass ctsProfileMatch. Devices that will fail ctsProfileMatch include the following:
Devices that fail basicIntegrity
Devices with an unlocked bootloader
Devices with a custom system image (custom ROM)
Devices for which the manufacturer didn't apply for, or pass, Google certification
Devices with a system image built directly from the Android Open Source Program source files
Devices with a system image distributed as part of a beta or developer preview program (including the Android Beta Program)
Recommendations when using SafetyNetApi.attest
Create a large (16 bytes or longer) random number on your server using a cryptographically-secure random function so that a malicious user can not reuse a successful attestation result in place of an unsuccessful result
Trust APK information (apkPackageName, apkCertificateDigestSha256 and apkDigestSha256) only if the value of ctsProfileMatch is true.
The entire JWS response should be sent to your server, using a secure connection, for verification. It isn't recommended to perform the verification directly in the app because, in that case, there is no guarantee that the verification logic itself hasn't been modified.
The SafetyNet Attestation API gives you a snapshot of the state of a device at the moment when the attestation request was made. A successful attestation doesn't necessarily mean that the device would have passed attestation in the past, or that it will in the future. It's recommended to plan a strategy to use the least amount of attestations required to satisfy the use case.
Programmatic Detection
File existence checks
Perhaps the most widely used method of programmatic detection is checking for files typically found on rooted devices, such as package files of common rooting apps and their associated files and directories, including the following:
Detection code also often looks for binaries that are usually installed once a device has been rooted. These searches include checking for busybox and attempting to open the su binary at different locations:
Checking whether su is on the PATH also works:
Executing su and other commands
Another way of determining whether su exists is attempting to execute it through the Runtime.getRuntime.exec method. An IOException will be thrown if su is not on the PATH. The same method can be used to check for other programs often found on rooted devices, such as busybox and the symbolic links that typically point to it.
Checking running processes
Checking installed app packages
You can use the Android package manager to obtain a list of installed packages. The following package names belong to popular rooting tools:
Checking for writable partitions and system directories
Unusual permissions on system directories may indicate a customized or rooted device. Although the system and data directories are normally mounted read-only, you'll sometimes find them mounted read-write when the device is rooted. Look for these filesystems mounted with the "rw" flag or try to create a file in the data directories.
Checking for custom Android builds
Bypassing Root Detection
To bypass these checks, you can use several techniques, most of which were introduced in the "Reverse Engineering and Tampering" chapter:
Renaming binaries. For example, in some cases simply renaming the su binary is enough to defeat root detection (try not to break your environment though!).
Unmounting /proc to prevent reading of process lists. Sometimes, the unavailability of /proc is enough to bypass such checks.
Using Frida or Xposed to hook APIs on the Java and native layers. This hides files and processes, hides the contents of files, and returns all kinds of bogus values that the app requests.
Hooking low-level APIs by using kernel modules.
Patching the app to remove the checks.
Check for root detection mechanisms, including the following criteria:
Multiple detection methods are scattered throughout the app (as opposed to putting everything into a single method).
The root detection mechanisms operate on multiple API layers (Java APIs, native library functions, assembler/system calls).
The mechanisms are somehow original (they're not copied and pasted from StackOverflow or other sources).
Develop bypass methods for the root detection mechanisms and answer the following questions:
Can the mechanisms be easily bypassed with standard tools, such as RootCloak?
Is static/dynamic analysis necessary to handle the root detection?
Do you need to write custom code?
How long did successfully bypassing the mechanisms take?
What is your assessment of the difficulty of bypassing the mechanisms?
If root detection is missing or too easily bypassed, make suggestions in line with the effectiveness criteria listed above. These suggestions may include more detection mechanisms and better integration of existing mechanisms with other defenses.
Debugging is a highly effective way to analyze runtime app behavior. It allows the reverse engineer to step through the code, stop app execution at arbitrary points, inspect the state of variables, read and modify memory, and a lot more.
Anti-debugging features can be preventive or reactive. As the name implies, preventive anti-debugging prevents the debugger from attaching in the first place; reactive anti-debugging involves detecting debuggers and reacting to them in some way (e.g., terminating the app or triggering hidden behavior). The "more-is-better" rule applies: to maximize effectiveness, defenders combine multiple methods of prevention and detection that operate on different API layers and are well distributed throughout the app.
As mentioned in the "Reverse Engineering and Tampering" chapter, we have to deal with two debugging protocols on Android: we can debug on the Java level with JDWP or on the native layer via a ptrace-based debugger. A good anti-debugging scheme should defend against both types of debugging.
In the chapter "Reverse Engineering and Tampering", we talked about JDWP, the protocol used for communication between the debugger and the Java Virtual Machine. We showed that it is easy to enable debugging for any app by patching its manifest file, and changing the ro.debuggable system property which enables debugging for all apps. Let's look at a few things developers do to detect and disable JDWP debuggers.
Checking the Debuggable Flag in ApplicationInfo
We have already encountered the android:debuggable attribute. This flag in the Android Manifest determines whether the JDWP thread is started for the app. Its value can be determined programmatically, via the app's ApplicationInfo object. If the flag is set, the manifest has been tampered with and allows debugging.
isDebuggerConnected
While this might be pretty obvious to circumvent for a reverse engineer, you can use isDebuggerConnected from the android.os.Debug class to determine whether a debugger is connected.
The same API can be called via native code by accessing the DvmGlobals global structure.
Timer Checks
Messing with JDWP-Related Data Structures
In Dalvik, the global virtual machine state is accessible via the DvmGlobals structure. The global variable gDvm holds a pointer to this structure. DvmGlobals contains various variables and pointers that are important for JDWP debugging and can be tampered with.
One way to overwrite the method pointers is to overwrite the address of the function jdwpAdbState::ProcessIncoming with the address of JdwpAdbState::Shutdown. This will cause the debugger to disconnect immediately.
Checking TracerPid
Remember that this only applies to native code. If you're debugging a Java/Kotlin-only app the value of the "TracerPid" field should be 0.
You can see how the status file of com.example.hellojni (PID=11657) contains a TracerPID of 11839, which we can identify as the lldb-server process.
Using Fork and ptrace
You can prevent debugging of a process by forking a child process and attaching it to the parent as a debugger via code similar to the following simple example code:
With the child attached, further attempts to attach to the parent will fail. We can verify this by compiling the code into a JNI function and packing it into an app we run on the device.
Attempting to attach to the parent process with gdbserver fails with an error:
You can easily bypass this failure, however, by killing the child and "freeing" the parent from being traced. You'll therefore usually find more elaborate schemes, involving multiple processes and threads as well as some form of monitoring to impede tampering. Common methods include
forking multiple processes that trace one another,
keeping track of running processes to make sure the children stay alive,
monitoring values in the /proc filesystem, such as TracerPID in /proc/pid/status.
Let's look at a simple improvement for the method above. After the initial fork, we launch in the parent an extra thread that continually monitors the child's status. Depending on whether the app has been built in debug or release mode (which is indicated by the android:debuggable flag in the manifest), the child process should do one of the following things:
In release mode: The call to ptrace fails and the child crashes immediately with a segmentation fault (exit code 11).
In debug mode: The call to ptrace works and the child should run indefinitely. Consequently, a call to waitpid(child_pid) should never return. If it does, something is fishy and we would kill the whole process group.
The following is the complete code for implementing this improvement with a JNI function:
Again, we pack this into an Android app to see if it works. Just as before, two processes show up when we run the app's debug build.
However, if we terminate the child process at this point, the parent exits as well:
To bypass this, we must modify the app's behavior slightly (the easiest ways to do so are patching the call to _exit with NOPs and hooking the function _exit in libc.so). At this point, we have entered the proverbial "arms race": implementing more intricate forms of this defense as well as bypassing it are always possible.
There's no generic way to bypass anti-debugging: the best method depends on the particular mechanism(s) used to prevent or detect debugging and the other defenses in the overall protection scheme. For example, if there are no integrity checks or you've already deactivated them, patching the app might be the easiest method. In other cases, a hooking framework or kernel modules might be preferable. The following methods describe different approaches to bypass debugger detection:
Patching the anti-debugging functionality: Disable the unwanted behavior by simply overwriting it with NOP instructions. Note that more complex patches may be required if the anti-debugging mechanism is well designed.
Using Frida or Xposed to hook APIs on the Java and native layers: manipulate the return values of functions such as isDebuggable and isDebuggerConnected to hide the debugger.
Changing the environment: Android is an open environment. If nothing else works, you can modify the operating system to subvert the assumptions the developers made when designing the anti-debugging tricks.
Bypassing Example: UnCrackable App for Android Level 2
When dealing with obfuscated apps, you'll often find that developers purposely "hide away" data and functionality in native libraries. You'll find an example of this in level 2 of the "UnCrackable App for Android".
At first glance, the code looks like the prior challenge. A class called CodeCheck is responsible for verifying the code entered by the user. The actual check appears to occur in the bar method, which is declared as a native method.
Check for anti-debugging mechanisms, including the following criteria:
Attaching jdb and ptrace-based debuggers fails or causes the app to terminate or malfunction.
Multiple detection methods are scattered throughout the app's source code (as opposed to their all being in a single method or function).
The anti-debugging defenses operate on multiple API layers (Java, native library functions, assembler/system calls).
The mechanisms are somehow original (as opposed to being copied and pasted from StackOverflow or other sources).
Work on bypassing the anti-debugging defenses and answer the following questions:
Can the mechanisms be bypassed trivially (e.g., by hooking a single API function)?
How difficult is identifying the anti-debugging code via static and dynamic analysis?
Did you need to write custom code to disable the defenses? How much time did you need?
What is your subjective assessment of the difficulty of bypassing the mechanisms?
If anti-debugging mechanisms are missing or too easily bypassed, make suggestions in line with the effectiveness criteria above. These suggestions may include adding more detection mechanisms and better integration of existing mechanisms with other defenses.
There are two topics related to file integrity:
The file storage integrity checks: The integrity of files that the application stores on the SD card or public storage and the integrity of key-value pairs that are stored in SharedPreferences should be protected.
Sample Implementation - Application Source Code
Integrity checks often calculate a checksum or hash over selected files. Commonly protected files include
AndroidManifest.xml,
class files *.dex,
native libraries (*.so).
Sample Implementation - Storage
When providing integrity on the storage itself, you can either create an HMAC over a given key-value pair (as for the Android SharedPreferences) or create an HMAC over a complete file that's provided by the file system.
Complete the following procedure when generating an HMAC with BouncyCastle:
Make sure BouncyCastle or SpongyCastle is registered as a security provider.
Initialize the HMAC with a key (which can be stored in a keystore).
Get the byte array of the content that needs an HMAC.
Call doFinal on the HMAC with the bytecode.
Append the HMAC to the bytearray obtained in step 3.
Store the result of step 5.
Complete the following procedure when verifying the HMAC with BouncyCastle:
Make sure that BouncyCastle or SpongyCastle is registered as a security provider.
Extract the message and the HMAC-bytes as separate arrays.
Repeat steps 1-4 of the procedure for generating an HMAC.
Compare the extracted HMAC-bytes to the result of step 3.
The following is a convenient HMAC implementation without AndroidKeyStore:
Another way to provide integrity is to sign the byte array you obtained and add the signature to the original byte array.
Bypassing File Integrity Checks
Bypassing the application-source integrity checks
Patch the anti-debugging functionality. Disable the unwanted behavior by simply overwriting the associated bytecode or native code with NOP instructions.
Use Frida or Xposed to hook file system APIs on the Java and native layers. Return a handle to the original file instead of the modified file.
Use the kernel module to intercept file-related system calls. When the process attempts to open the modified file, return a file descriptor for the unmodified version of the file.
Bypassing the storage integrity checks
Retrieve the data from the device, as described in the "Testing Device Binding" section.
Alter the retrieved data and then put it back into storage.
For application-source integrity checks
Run the app in an unmodified state and make sure that everything works. Apply simple patches to classes.dex and any .so libraries in the app package. Re-package and re-sign the app as described in the "Basic Security Testing" chapter, then run the app. The app should detect the modification and respond in some way. At the very least, the app should alert the user and/or terminate. Work on bypassing the defenses and answer the following questions:
Can the mechanisms be bypassed trivially (e.g., by hooking a single API function)?
How difficult is identifying the anti-debugging code via static and dynamic analysis?
Did you need to write custom code to disable the defenses? How much time did you need?
What is your assessment of the difficulty of bypassing the mechanisms?
For storage integrity checks
An approach similar to that for application-source integrity checks applies. Answer the following questions:
Can the mechanisms be bypassed trivially (e.g., by changing the contents of a file or a key-value)?
How difficult is getting the HMAC key or the asymmetric private key?
Did you need to write custom code to disable the defenses? How much time did you need?
What is your assessment of the difficulty of bypassing the mechanisms?
The presence of tools, frameworks and apps commonly used by reverse engineers may indicate an attempt to reverse engineer the app. Some of these tools can only run on a rooted device, while others force the app into debugging mode or depend on starting a background service on the mobile phone. Therefore, there are different ways that an app may implement to detect a reverse engineering attack and react to it, e.g. by terminating itself.
You can detect popular reverse engineering tools that have been installed in an unmodified form by looking for associated application packages, files, processes, or other tool-specific modifications and artifacts. In the following examples, we'll discuss different ways to detect the Frida instrumentation framework, which is used extensively in this guide. Other tools, such as Substrate and Xposed, can be detected similarly. Note that DBI/injection/hooking tools can often be detected implicitly, through runtime integrity checks, which are discussed below.
For instance, in its default configuration on a rooted device, Frida runs on the device as frida-server. When you explicitly attach to a target app (e.g. via frida-trace or the Frida REPL), Frida injects a frida-agent into the memory of the app. Therefore, you may expect to find it there after attaching to the app (and not before). If you check /proc/<pid>/maps you'll find the frida-agent as frida-agent-64.so:
Looking at these two traces that Frida lefts behind, you might already imagine that detecting those would be a trivial task. And actually, so trivial will be bypassing that detection. But things can get much more complicated. The following table shortly presents a set of some typical Frida detection methods and a short discussion on their effectiveness.
Method
Description
Discussion
Checking the App Signature
This is unfortunately too trivial to bypass, e.g. by patching the APK or performing system call hooking.
Check The Environment For Related Artifacts
Since Android 7.0 (API level 24), inspecting the running services/processes won't show you daemons like the frida-server as it is not being started by the app itself. Even if it would be possible, bypassing this would be as easy just renaming the corresponding Frida artifact (frida-server/frida-gadget/frida-agent).
Checking For Open TCP Ports
The frida-server process binds to TCP port 27042 by default. Check whether this port is open is another method of detecting the daemon.
This method detects frida-server in its default mode, but the listening port can be changed via a command line argument, so bypassing this is a little too trivial.
Checking For Ports Responding To D-Bus Auth
frida-server uses the D-Bus protocol to communicate, so you can expect it to respond to D-Bus AUTH. Send a D-Bus AUTH message to every open port and check for an answer, hoping that frida-server will reveal itself.
This is a fairly robust method of detecting frida-server, but Frida offers alternative modes of operation that don't require frida-server.
Scanning Process Memory for Known Artifacts
Scan the memory for artifacts found in Frida's libraries, e.g. the string "LIBFRIDA" present in all versions of frida-gadget and frida-agent. For example, use Runtime.getRuntime().exec and iterate through the memory mappings listed in /proc/self/maps or /proc/<pid>/maps (depending on the Android version) searching for the string.
It is important to note that these controls are only increasing the complexity of the reverse engineering process. If used, the best approach is to combine the controls cleverly instead of using them individually. However, none of them can assure a 100% effectiveness, as the reverse engineer will always have full access to the device and will therefore always win! You also have to consider that integrating some of the controls into your app might increase the complexity of your app and even have an impact on its performance.
Launch the app with various reverse engineering tools and frameworks installed in your test device. Include at least the following: Frida, Xposed, Substrate for Android, Drozer, RootCloak, Android SSL Trust Killer.
The app should respond in some way to the presence of those tools. For example by:
Alerting the user and asking for accepting liability.
Preventing execution by gracefully terminating.
Securely wiping any sensitive data stored on the device.
Reporting to a backend server, e.g, for fraud detection.
Next, work on bypassing the detection of the reverse engineering tools and answer the following questions:
Can the mechanisms be bypassed trivially (e.g., by hooking a single API function)?
How difficult is identifying the anti reverse engineering code via static and dynamic analysis?
Did you need to write custom code to disable the defenses? How much time did you need?
What is your assessment of the difficulty of bypassing the mechanisms?
The following steps should guide you when bypassing detection of reverse engineering tools:
Patch the anti reverse engineering functionality. Disable the unwanted behavior by simply overwriting the associated bytecode or native code with NOP instructions.
Use Frida or Xposed to hook file system APIs on the Java and native layers. Return a handle to the original file, not the modified file.
Use a kernel module to intercept file-related system calls. When the process attempts to open the modified file, return a file descriptor for the unmodified version of the file.
In the context of anti-reversing, the goal of emulator detection is to increase the difficulty of running the app on an emulated device, which impedes some tools and techniques reverse engineers like to use. This increased difficulty forces the reverse engineer to defeat the emulator checks or utilize the physical device, thereby barring the access required for large-scale device analysis.
There are several indicators that the device in question is being emulated. Although all these API calls can be hooked, these indicators provide a modest first line of defense.
The first set of indicators are in the file build.prop.
You can edit the file build.prop on a rooted Android device or modify it while compiling AOSP from source. Both techniques will allow you to bypass the static string checks above.
The next set of static indicators utilize the Telephony manager. All Android emulators have fixed values that this API can query.
Keep in mind that a hooking framework, such as Xposed or Frida, can hook this API to provide false data.
Patch the emulator detection functionality. Disable the unwanted behavior by simply overwriting the associated bytecode or native code with NOP instructions.
Use Frida or Xposed APIs to hook file system APIs on the Java and native layers. Return innocent-looking values (preferably taken from a real device) instead of the telltale emulator values. For example, you can override the TelephonyManager.getDeviceID method to return an IMEI value.
Install and run the app in the emulator. The app should detect that it is being executed in an emulator and terminate or refuse to execute the functionality that's meant to be protected.
Work on bypassing the defenses and answer the following questions:
How difficult is identifying the emulator detection code via static and dynamic analysis?
Can the detection mechanisms be bypassed trivially (e.g., by hooking a single API function)?
Did you need to write custom code to disable the anti-emulation feature(s)? How much time did you need?
What is your assessment of the difficulty of bypassing the mechanisms?
Controls in this category verify the integrity of the app's memory space to defend the app against memory patches applied during runtime. Such patches include unwanted changes to binary code, bytecode, function pointer tables, and important data structures, as well as rogue code loaded into process memory. Integrity can be verified by:
comparing the contents of memory or a checksum over the contents to good values,
searching memory for the signatures of unwanted modifications.
There's some overlap with the category "detecting reverse engineering tools and frameworks", and, in fact, we demonstrated the signature-based approach in that chapter when we showed how to search process memory for Frida-related strings. Below are a few more examples of various kinds of integrity monitoring.
Runtime Integrity Check Examples
Detecting tampering with the Java Runtime**
Detecting Native Hooks
By using ELF binaries, native function hooks can be installed by overwriting function pointers in memory (e.g., Global Offset Table or PLT hooking) or patching parts of the function code itself (inline hooking). Checking the integrity of the respective memory regions is one way to detect this kind of hook.
The Global Offset Table (GOT) is used to resolve library functions. During runtime, the dynamic linker patches this table with the absolute addresses of global symbols. GOT hooks overwrite the stored function addresses and redirect legitimate function calls to adversary-controlled code. This type of hook can be detected by enumerating the process memory map and verifying that each GOT entry points to a legitimately loaded library.
In contrast to GNU ld, which resolves symbol addresses only after they are needed for the first time (lazy binding), the Android linker resolves all external functions and writes the respective GOT entries immediately after a library is loaded (immediate binding). You can therefore expect all GOT entries to point to valid memory locations in the code sections of their respective libraries during runtime. GOT hook detection methods usually walk the GOT and verify this.
Inline hooks work by overwriting a few instructions at the beginning or end of the function code. During runtime, this so-called trampoline redirects execution to the injected code. You can detect inline hooks by inspecting the prologues and epilogues of library functions for suspect instructions, such as far jumps to locations outside the library.
Make sure that all file-based detection of reverse engineering tools is disabled. Then, inject code by using Xposed, Frida, and Substrate, and attempt to install native hooks and Java method hooks. The app should detect the "hostile" code in its memory and respond accordingly.
Work on bypassing the checks with the following techniques:
Patch the integrity checks. Disable the unwanted behavior by overwriting the respective bytecode or native code with NOP instructions.
Use Frida or Xposed to hook the APIs used for detection and return fake values.
Obfuscation is the process of transforming code and data to make it more difficult to comprehend. It is an integral part of every software protection scheme. What's important to understand is that obfuscation isn't something that can be simply turned on or off. Programs can be made incomprehensible, in whole or in part, in many ways and to different degrees.
In the test case "Make Sure That Free Security Features Are Activated (MSTG-CODE-9)" in chapter "Code Quality and Build Settings of Android Apps", we describe a few basic obfuscation techniques that are commonly used on Android with R8 and Pro-Guard.
Attempt to decompile the bytecode, disassemble any included library files, and perform static analysis. At the very least, the app's core functionality (i.e., the functionality meant to be obfuscated) shouldn't be easily discerned. Verify that
meaningful identifiers, such as class names, method names, and variable names, have been discarded,
string resources and strings in binaries are encrypted,
code and data related to the protected functionality is encrypted, packed, or otherwise concealed.
For a more detailed assessment, you need a detailed understanding of the relevant threats and the obfuscation methods used.
The goal of device binding is to impede an attacker who tries to both copy an app and its state from device A to device B and continue executing the app on device B. After device A has been determined trustworthy, it may have more privileges than device B. These differential privileges should not change when an app is copied from device A to device B.
Before we describe the usable identifiers, let's quickly discuss how they can be used for binding. There are three methods that allow device binding:
Augmenting the credentials used for authentication with device identifiers. This make sense if the application needs to re-authenticate itself and/or the user frequently.
Encrypting the data stored in the device with the key material which is strongly bound to the device can strengthen the device binding. The Android Keystore offers non-exportable private keys which we can use for this. When a malicious actor would extract such data from a device, it wouldn't be possible to decrypt the data, as the key is not accessible. Implementing this, takes the following steps:
Generate the key pair in the Android Keystore using KeyGenParameterSpec API.
Generating a secret key for AES-GCM:
Encrypt the authentication data and other sensitive data stored by the application using a secret key through AES-GCM cipher and use device specific parameters such as Instance ID, etc. as associated data:
Encrypt the secret key using the public key stored in Android Keystore and store the encrypted secret key in the private storage of the application.
Whenever authentication data such as access tokens or other sensitive data is required, decrypt the secret key using private key stored in Android Keystore and then use the decrypted secret key to decrypt the ciphertext.
Use token-based device authentication (Instance ID) to make sure that the same instance of the app is used.
use the Advertising ID (AdvertisingIdClient.Info) when it comes to advertising -so that the user has the option to decline.
use the Instance ID (FirebaseInstanceId) for device identification.
use the SSAID only for fraud detection and for sharing state between apps signed by the same developer.
Note that the Instance ID and the Advertising ID are not stable across device upgrades and device-resets. However, the Instance ID will at least allow to identify the current software installation on a device.
There are a few key terms you can look for when the source code is available:
Unique identifiers that will no longer work:
Build.SERIAL without Build.getSerial
htc.camera.sensor.front_SN for HTC devices
persist.service.bdroid.bdadd
Settings.Secure.bluetooth_address or WifiInfo.getMacAddress from WifiManager, unless the system permission LOCAL_MAC_ADDRESS is enabled in the manifest.
ANDROID_ID used only as an identifier. This will influence the binding quality over time for older devices.
The absence of Instance ID, Build.SERIAL, and the IMEI.
The creation of private keys in the AndroidKeyStore using the KeyPairGeneratorSpec or KeyGenParameterSpec APIs.
To be sure that the identifiers can be used, check AndroidManifest.xml for usage of the IMEI and Build.Serial. The file should contain the permission <uses-permission android:name="android.permission.READ_PHONE_STATE" />.
Apps for Android 8.0 (API level 26) will get the result "UNKNOWN" when they request
Build.Serial.
There are several ways to test the application binding:
Dynamic Analysis with an Emulator
Run the application on an emulator.
Make sure you can raise the trust in the application instance (e.g., authenticate in the app).
Retrieve the data from the emulator according to the following steps:
SSH into your simulator via an ADB shell.
Execute run-as <your app-id>. Your app-id is the package described in the AndroidManifest.xml.
chmod 777 the contents of cache and shared-preferences.
Exit the current user from the the app-id.
Copy the contents of /data/data/<your appid>/cache and shared-preferences to the SD card.
Use ADB or the DDMS to pull the contents.
Install the application on another emulator.
In the application's data folder, overwrite the data from step 3.
Copy the data from step 3 to the second emulator's SD card.
SSH into your simulator via an ADB shell.
Execute run-as <your app-id>. Your app-id is the package described in AndroidManifest.xml.
chmod 777 the folder's cache and shared-preferences.
Copy the older contents of the SD card to /data/data/<your appid>/cache and shared-preferences.
Can you continue in an authenticated state? If so, binding may not be working properly.
Google Instance ID
Configure your Instance ID for the given application in your Google Developer Console. This includes managing the PROJECT_ID.
Setup Google Play services. In the file build.gradle, add
Get an Instance ID.
Generate a token.
Make sure that you can handle callbacks from Instance ID, in case of invalid device information, security issues, etc. This requires extending Instance IDListenerService and handling the callbacks there:
Register the service in your Android manifest:
When you submit the Instance ID (iid) and the tokens to your server, you can use that server with the Instance ID Cloud Service to validate the tokens and the iid. When the iid or token seems invalid, you can trigger a safeguard procedure (e.g., informing the server of possible copying or security issues or removing the data from the app and asking for a re-registration).
IMEI & Serial
Google recommends not using these identifiers unless the application is at a high risk.
For Android devices before Android 8.0 (API level 26), you can request the serial as follows:
For devices running Android version O and later, you can request the device's serial as follows:
Set the permission in your Android manifest:
Get the serial:
Retrieve the IMEI:
Set the required permission in your Android manifest:
Get the IMEI:
SSAID
Google recommends not using these identifiers unless the application is at a high risk. You can retrieve the SSAID as follows:
There are a few key terms you can look for when the source code is available:
Unique identifiers that will no longer work:
Build.SERIAL without Build.getSerial
htc.camera.sensor.front_SN for HTC devices
persist.service.bdroid.bdadd
Settings.Secure.bluetooth_address or WifiInfo.getMacAddress from WifiManager, unless the system permission LOCAL_MAC_ADDRESS is enabled in the manifest.
Usage of ANDROID_ID as an identifier only. Over time, this will influence the binding quality on older devices.
The absence of Instance ID, Build.SERIAL, and the IMEI.
To make sure that the identifiers can be used, check AndroidManifest.xml for usage of the IMEI and Build.Serial. The manifest should contain the permission <uses-permission android:name="android.permission.READ_PHONE_STATE" />.
There are a few ways to test device binding dynamically:
Using an Emulator
See section "Dynamic Analysis with an Emulator" above.
Using two different rooted devices
Run the application on your rooted device.
Make sure you can raise the trust (e.g., authenticate in the app) in the application instance.
Retrieve the data from the first rooted device.
Install the application on the second rooted device.
In the application's data folder, overwrite the data from step 3.
Can you continue in an authenticated state? If so, binding may not be working properly.
MSTG-RESILIENCE-1: "The app detects, and responds to, the presence of a rooted or jailbroken device either by alerting the user or terminating the app."
MSTG-RESILIENCE-2: "The app prevents debugging and/or detects, and responds to, a debugger being attached. All available debugging protocols must be covered."
MSTG-RESILIENCE-3: "The app detects, and responds to, tampering with executable files and critical data within its own sandbox."
MSTG-RESILIENCE-4: "The app detects, and responds to, the presence of widely used reverse engineering tools and frameworks on the device."
MSTG-RESILIENCE-5: "The app detects, and responds to, being run in an emulator."
MSTG-RESILIENCE-6: "The app detects, and responds to, tampering the code and data in its own memory space."
MSTG-RESILIENCE-9: "Obfuscation is applied to programmatic defenses, which in turn impede de-obfuscation via dynamic analysis."
MSTG-RESILIENCE-10: "The app implements a 'device binding' functionality using a device fingerprint derived from multiple properties unique to the device."
The verify method only validates that the JWS message was signed by SafetyNet. It doesn't verify that the payload of the verdict matches your expectations. As useful as this service may seem, it is designed for test purposes only, and it has very strict usage quotas of 10,000 requests per day, per project which will not be increased upon request. Hence, you should refer and implement the digital signature verification logic on your server in a way that it doesn't depend on Google's servers.
To prevent inadvertently reaching your SafetyNetApi.attest quota and getting attestation errors, you should build a system that monitors your usage of the API and warns you well before you reach your quota so you can get it increased. You should also be prepared to handle attestation failures because of an exceeded quota and avoid blocking all your users in this situation. If you are close to reaching your quota, or expect a short-term spike that may lead you to exceed your quota, you can submit this to request short or long-term increases to the quota for your API key. This process, as well as the additional quota, is free of charge.
Follow this to ensure that you've completed each of the steps needed to integrate the SafetyNetApi.attest API into the app.
File checks can be easily implemented in both Java and native code. The following JNI example (adapted from ) uses the stat system call to retrieve information about a file and returns "1" if the file exists.
Supersu-by far the most popular rooting tool-runs an authentication daemon named daemonsu, so the presence of this process is another sign of a rooted device. Running processes can be enumerated with the ActivityManager.getRunningAppProcesses and manager.getRunningServices APIs, the ps command, and browsing through the /proc directory. The following is an example implemented in :
Checking for signs of test builds and custom ROMs is also helpful. One way to do this is to check the BUILD tag for test-keys, which normally . :
Missing Google Over-The-Air (OTA) certificates is another sign of a custom ROM: on stock Android builds, .
Run execution traces with jdb, , strace, and/or kernel modules to find out what the app is doing. You'll usually see all kinds of suspect interactions with the operating system, such as opening su for reading and obtaining a list of processes. These interactions are surefire signs of root detection. Identify and deactivate the root detection mechanisms, one at a time. If you're performing a black box resilience assessment, disabling the root detection mechanisms is your first step.
Debug.threadCpuTimeNanos indicates the amount of time that the current thread has been executing code. Because debugging slows down process execution, .
For example, :
You can disable debugging by using similar techniques in ART even though the gDvm variable is not available. The ART runtime exports some of the vtables of JDWP-related classes as global symbols (in C++, vtables are tables that hold pointers to class methods). This includes the vtables of the classes JdwpSocketState and JdwpAdbState, which handle JDWP connections via network sockets and ADB, respectively. You can manipulate the behavior of the debugging runtime (archived).
On Linux, the is used to observe and control the execution of a process (the tracee) and to examine and change that process' memory and registers. ptrace is the primary way to implement system call tracing and breakpoint debugging in native code. Most JDWP anti-debugging tricks (which may be safe for timer-based checks) won't catch classical debuggers based on ptrace and therefore, many Android anti-debugging tricks include ptrace, often exploiting the fact that only one debugger at a time can attach to a process.
When you debug an app and set a breakpoint on native code, Android Studio will copy the needed files to the target device and start the lldb-server which will use ptrace to attach to the process. From this moment on, if you inspect the of the debugged process (/proc/<pid>/status or /proc/self/status), you will see that the "TracerPid" field has a value different from 0, which is a sign of debugging.
This technique is usually applied within the JNI native libraries in C, as shown in implementation of the IsDebuggerAttached method. However, if you prefer to include this check as part of your Java/Kotlin code you can refer to this Java implementation of the hasTracerPid method from .
When trying to implement such a method yourself, you can manually check the value of TracerPid with ADB. The following listing uses Google's NDK sample app to perform the check after attaching Android Studio's debugger:
Please see in GitHub.
Code integrity checks: In the "" chapter, we discussed Android's APK code signature check. We also saw that determined reverse engineers can easily bypass this check by re-packaging and re-signing an app. To make this bypassing process more involved, a protection scheme can be augmented with CRC checks on the app bytecode, native libraries, and important data files. These checks can be implemented on both the Java and the native layer. The idea is to have additional controls in place so that the app only runs correctly in its unmodified state, even if the code signature is valid.
The following calculates a CRC over classes.dex and compares it to the expected value.
When using an HMAC, you can .
When generating the HMAC based on the , then it is best to only do this for Android 6.0 (API level 23) and higher.
Refer to the "" chapter for examples of patching, code injection, and kernel modules.
The other method (which also works for non-rooted devices) consists of embedding a into the APK and forcing the app to load it as one of its native libraries. If you inspect the app memory maps after starting the app (no need to attach explicitly to it) you'll find the embedded frida-gadget as libfrida-gadget.so.
Some of the following detection methods are presented in the article (archived). Please refer to it for more details and for example code snippets.
In order to embed the frida-gadget within the APK, it would need to be repackaged and resigned. You could check the signature of the APK when the app is starting (e.g. since API level 28) and compare it to the one you pinned in your APK.
Artifacts can be package files, binaries, libraries, processes, and temporary files. For Frida, this could be the frida-server running in the target (rooted) system (the daemon responsible for exposing Frida over TCP). Inspect the running services () and processes (ps) searching for one whose name is "frida-server". You could also walk through the list of loaded libraries and check for suspicious ones (e.g. those including "frida" in their names).
This method is a bit more effective, and it is difficult to bypass with Frida only, especially if some obfuscation has been added and if multiple artifacts are being scanned. However, the chosen artifacts might be patched in the Frida binaries. Find the source code on .
Please remember that this table is far from exhaustive. We could start talking about (used by frida-server for external communication), detecting (indirect jump vectors inserted at the prologue of functions), which would help detecting Substrate or Frida's Interceptor but, for example, won't be effective against Frida's Stalker; and many other, more or less, effective detection methods. Each of them will depend on whether you're using a rooted device, the specific version of the rooting method and/or the version of the tool itself. At the end, this is part of the cat and mouse game of protecting data being processed on an untrusted environment (an app running in the user device).
Refer to the "" chapter for examples of patching, code injection, and kernel modules.
Refer to the "" chapter for examples of patching, code injection, and kernel modules.
This detection code is from the .
Refer to the "" chapter for examples of patching, code injection, and kernel modules.
In the past, Android developers often relied on the Settings.Secure.ANDROID_ID (SSAID) and MAC addresses. This . As the MAC address is now often randomized when not connected to an access point and the SSAID is no longer a device bound ID. Instead, it became a value bound to the user, the device and the app signing key of the application which requests the SSAID. In addition, there are new in Google's SDK documentation. Basically, Google recommends to:
uses tokens to authenticate the running application instance. The moment the application is reset, uninstalled, etc., the Instance ID is reset, meaning that you'll have a new "instance" of the app. Go through the following steps for Instance ID:
Please note that .
Request the permission at runtime from the user: See for more details.
If you're using Android version Android 6 (API level 23) or later, request the permission at runtime from the user: See for more details.
The behavior of the SSAID and MAC addresses have . In addition, there are for identifiers in Google's SDK documentation. Because of this new behavior, we recommend that developers not rely on the SSAID alone. The identifier has become less stable. For example, the SSAID may change after a factory reset or when the app is reinstalled after the upgrade to Android 8.0 (API level 26). There are devices that have the same ANDROID_ID and/or have an ANDROID_ID that can be overridden. Therefore it is better to encrypt the ANDROID_ID with a randomly generated key from the AndroidKeyStore using AES_GCM encryption. The encrypted ANDROID_ID should then be stored in the SharedPreferences (privately). The moment the app-signature changes, the application can check for a delta and register the new ANDROID_ID. The moment this changes without a new application signing key, it should indicate that something else is wrong.
Developer Guideline -
SafetyNet Attestation Checklist -
Do's & Don'ts of SafetyNet Attestation -
SafetyNet Verification Samples -
SafetyNet Attestation API - Quota Request -