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00102d7 Jul 13, 2018
@b-mueller @commjoen @sushi2k @Sjord @juan-dambra @romualdszkudlarek @leonjza @bolot
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Android Anti-Reversing Defenses

Testing Root Detection

Overview

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.

Common Root Detection Methods

In the following section, we list some common root detection methods you'll encounter. You'll find some of these methods implemented in the crackme examples that accompany the OWASP Mobile Testing Guide.

Root detection can also be implemented through libraries such as RootBeer.

SafetyNet

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 whitelisted device models that have passed Android compatibility testing. Google recommends 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 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.

The following is a sample attestation result:

{
  "nonce": "R2Rra24fVm5xa2Mg",
  "timestampMs": 9860437986543,
  "apkPackageName": "com.package.name.of.requesting.app",
  "apkCertificateDigestSha256": ["base64 encoded, SHA-256 hash of the
                                  certificate used to sign requesting app"],
  "apkDigestSha256": "base64 encoded, SHA-256 hash of the app's APK",
  "ctsProfileMatch": true,
  "basicIntegrity": true,
}
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:

/system/app/Superuser.apk
/system/etc/init.d/99SuperSUDaemon
/dev/com.koushikdutta.superuser.daemon/
/system/xbin/daemonsu

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:

/system/xbin/busybox

/sbin/su
/system/bin/su
/system/xbin/su
/data/local/su
/data/local/xbin/su

Checking whether su is on the PATH also works:

    public static boolean checkRoot(){
        for(String pathDir : System.getenv("PATH").split(":")){
            if(new File(pathDir, "su").exists()) {
                return true;
            }
        }
        return false;
    }

File checks can be easily implemented in both Java and native code. The following JNI example (adapted from rootinspector) uses the stat system call to retrieve information about a file and returns "1" if the file exists.

jboolean Java_com_example_statfile(JNIEnv * env, jobject this, jstring filepath) {
  jboolean fileExists = 0;
  jboolean isCopy;
  const char * path = (*env)->GetStringUTFChars(env, filepath, &isCopy);
  struct stat fileattrib;
  if (stat(path, &fileattrib) < 0) {
    __android_log_print(ANDROID_LOG_DEBUG, DEBUG_TAG, "NATIVE: stat error: [%s]", strerror(errno));
  } else
  {
    __android_log_print(ANDROID_LOG_DEBUG, DEBUG_TAG, "NATIVE: stat success, access perms: [%d]", fileattrib.st_mode);
    return 1;
  }

  return 0;
}

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

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 rootinspector:

    public boolean checkRunningProcesses() {

      boolean returnValue = false;

      // Get currently running application processes
      List<RunningServiceInfo> list = manager.getRunningServices(300);

      if(list != null){
        String tempName;
        for(int i=0;i<list.size();++i){
          tempName = list.get(i).process;

          if(tempName.contains("supersu") || tempName.contains("superuser")){
            returnValue = true;
          }
        }
      }
      return returnValue;
    }

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:

com.thirdparty.superuser
eu.chainfire.supersu
com.noshufou.android.su
com.koushikdutta.superuser
com.zachspong.temprootremovejb
com.ramdroid.appquarantine

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

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 indicate a custom Android image. Check the BUILD tag as follows:

private boolean isTestKeyBuild()
{
String str = Build.TAGS;
if ((str != null) && (str.contains("test-keys")));
for (int i = 1; ; i = 0)
  return i;
}

Missing Google Over-The-Air (OTA) certificates is another sign of a custom ROM: on stock Android builds, OTA updates Google's public certificates.

Bypassing Root Detection

Run execution traces with JDB, DDMS, 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.

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.

Effectiveness Assessment

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.

Testing Anti-Debugging

Overview

Debugging is a highly effective way to analyze run-time 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.

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.

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 distributed throughout the app.

Anti-JDWP-Debugging Examples

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 app 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.

    public static boolean isDebuggable(Context context){

        return ((context.getApplicationContext().getApplicationInfo().flags & ApplicationInfo.FLAG_DEBUGGABLE) != 0);

    }
isDebuggerConnected

The Android Debug system class offers a static method to determine whether a debugger is connected. The method returns a boolean value.

    public static boolean detectDebugger() {
        return Debug.isDebuggerConnected();
    }

The same API can be called via native code by accessing the DvmGlobals global structure.

JNIEXPORT jboolean JNICALL Java_com_test_debugging_DebuggerConnectedJNI(JNIenv * env, jobject obj) {
    if (gDvm.debuggerConnected || gDvm.debuggerActive)
        return JNI_TRUE;
    return JNI_FALSE;
}
Timer Checks

Debug.threadCpuTimeNanos indicates the amount of time that the current thread has been executing code. Because debugging slows down process execution, you can use the difference in execution time to guess whether a debugger is attached.

static boolean detect_threadCpuTimeNanos(){
  long start = Debug.threadCpuTimeNanos();

  for(int i=0; i<1000000; ++i)
    continue;

  long stop = Debug.threadCpuTimeNanos();

  if(stop - start < 10000000) {
    return false;
  }
  else {
    return true;
  }
}
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.

struct DvmGlobals {
    /*
     * Some options that could be worth tampering with :)
     */

    bool        jdwpAllowed;        // debugging allowed for this process?
    bool        jdwpConfigured;     // has debugging info been provided?
    JdwpTransportType jdwpTransport;
    bool        jdwpServer;
    char*       jdwpHost;
    int         jdwpPort;
    bool        jdwpSuspend;

    Thread*     threadList;

    bool        nativeDebuggerActive;
    bool        debuggerConnected;      /* debugger or DDMS is connected */
    bool        debuggerActive;         /* debugger is making requests */
    JdwpState*  jdwpState;

};

For example, setting the gDvm.methDalvikDdmcServer_dispatch function pointer to NULL crashes the JDWP thread:

JNIEXPORT jboolean JNICALL Java_poc_c_crashOnInit ( JNIEnv* env , jobject ) {
  gDvm.methDalvikDdmcServer_dispatch = NULL;
}

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 by overwriting the method pointers in the associated vtables.

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.

#include <jni.h>
#include <string>
#include <android/log.h>
#include <dlfcn.h>
#include <sys/mman.h>
#include <jdwp/jdwp.h>

#define log(FMT, ...) __android_log_print(ANDROID_LOG_VERBOSE, "JDWPFun", FMT, ##__VA_ARGS__)

// Vtable structure. Just to make messing around with it more intuitive

struct VT_JdwpAdbState {
    unsigned long x;
    unsigned long y;
    void * JdwpSocketState_destructor;
    void * _JdwpSocketState_destructor;
    void * Accept;
    void * showmanyc;
    void * ShutDown;
    void * ProcessIncoming;
};

extern "C"

JNIEXPORT void JNICALL Java_sg_vantagepoint_jdwptest_MainActivity_JDWPfun(
        JNIEnv *env,
        jobject /* this */) {

    void* lib = dlopen("libart.so", RTLD_NOW);

    if (lib == NULL) {
        log("Error loading libart.so");
        dlerror();
    }else{

        struct VT_JdwpAdbState *vtable = ( struct VT_JdwpAdbState *)dlsym(lib, "_ZTVN3art4JDWP12JdwpAdbStateE");

        if (vtable == 0) {
            log("Couldn't resolve symbol '_ZTVN3art4JDWP12JdwpAdbStateE'.\n");
        }else {

            log("Vtable for JdwpAdbState at: %08x\n", vtable);

            // Let the fun begin!

            unsigned long pagesize = sysconf(_SC_PAGE_SIZE);
            unsigned long page = (unsigned long)vtable & ~(pagesize-1);

            mprotect((void *)page, pagesize, PROT_READ | PROT_WRITE);

            vtable->ProcessIncoming = vtable->ShutDown;

            // Reset permissions & flush cache

            mprotect((void *)page, pagesize, PROT_READ);

        }
    }
}
Anti-Native-Debugging Examples

Most Anti-JDWP tricks (which may be safe for timer-based checks) won't catch classical, ptrace-based debuggers, so other defenses are necessary. Many "traditional" Linux anti-debugging tricks are used in this situation.

Checking TracerPid

When the ptrace system call is used to attach to a process, the "TracerPid" field in the status file of the debugged process shows the PID of the attaching process. The default value of "TracerPid" is 0 (no process attached). Consequently, finding anything other than 0 in that field is a sign of debugging or other ptrace shenanigans.

The following implementation is from Tim Strazzere's Anti-Emulator project:

    public static boolean hasTracerPid() throws IOException {
        BufferedReader reader = null;
        try {
            reader = new BufferedReader(new InputStreamReader(new FileInputStream("/proc/self/status")), 1000);
            String line;

            while ((line = reader.readLine()) != null) {
                if (line.length() > tracerpid.length()) {
                    if (line.substring(0, tracerpid.length()).equalsIgnoreCase(tracerpid)) {
                        if (Integer.decode(line.substring(tracerpid.length() + 1).trim()) > 0) {
                            return true;
                        }
                        break;
                    }
                }
            }

        } catch (Exception exception) {
            exception.printStackTrace();
        } finally {
            reader.close();
        }
        return false;
    }

Ptrace variations*

On Linux, the ptrace system call 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 breakpoint debugging and system call tracing. Many anti-debugging tricks include ptrace, often exploiting the fact that only one debugger at a time can attach to a process.

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:

void fork_and_attach()
{
  int pid = fork();

  if (pid == 0)
    {
      int ppid = getppid();

      if (ptrace(PTRACE_ATTACH, ppid, NULL, NULL) == 0)
        {
          waitpid(ppid, NULL, 0);

          /* Continue the parent process */
          ptrace(PTRACE_CONT, NULL, NULL);
        }
    }
}

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.

root@android:/ # ps | grep -i anti
u0_a151   18190 201   1535844 54908 ffffffff b6e0f124 S sg.vantagepoint.antidebug
u0_a151   18224 18190 1495180 35824 c019a3ac b6e0ee5c S sg.vantagepoint.antidebug

Attempting to attach to the parent process with gdbserver fails with an error:

root@android:/ # ./gdbserver --attach localhost:12345 18190
warning: process 18190 is already traced by process 18224
Cannot attach to lwp 18190: Operation not permitted (1)
Exiting

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:

#include <jni.h>
#include <unistd.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <pthread.h>

static int child_pid;

void *monitor_pid() {

    int status;

    waitpid(child_pid, &status, 0);

    /* Child status should never change. */

    _exit(0); // Commit seppuku

}

void anti_debug() {

    child_pid = fork();

    if (child_pid == 0)
    {
        int ppid = getppid();
        int status;

        if (ptrace(PTRACE_ATTACH, ppid, NULL, NULL) == 0)
        {
            waitpid(ppid, &status, 0);

            ptrace(PTRACE_CONT, ppid, NULL, NULL);

            while (waitpid(ppid, &status, 0)) {

                if (WIFSTOPPED(status)) {
                    ptrace(PTRACE_CONT, ppid, NULL, NULL);
                } else {
                    // Process has exited
                    _exit(0);
                }
            }
        }

    } else {
        pthread_t t;

        /* Start the monitoring thread */
        pthread_create(&t, NULL, monitor_pid, (void *)NULL);
    }
}

JNIEXPORT void JNICALL
Java_sg_vantagepoint_antidebug_MainActivity_antidebug(JNIEnv *env, jobject instance) {

    anti_debug();
}

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.

root@android:/ # ps | grep -I anti-debug
u0_a152   20267 201   1552508 56796 ffffffff b6e0f124 S sg.vantagepoint.anti-debug
u0_a152   20301 20267 1495192 33980 c019a3ac b6e0ee5c S sg.vantagepoint.anti-debug

However, if we terminate the child process at this point, the parent exits as well:

root@android:/ # kill -9 20301
130|root@hammerhead:/ # cd /data/local/tmp                                     
root@android:/ # ./gdbserver --attach localhost:12345 20267   
gdbserver: unable to open /proc file '/proc/20267/status'
Cannot attach to lwp 20267: No such file or directory (2)
Exiting

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.

Bypassing Debugger Detection

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.

package sg.vantagepoint.uncrackable2;

public class CodeCheck {
    public CodeCheck() {
        super();
    }

    public boolean a(String arg2) {
        return this.bar(arg2.getBytes());
    }

    private native boolean bar(byte[] arg1) {
    }
}

    static {
        System.loadLibrary("foo");
    }

Please see different proposed solutions for the Android Crackme Level 2 in Github.

Effectiveness Assessment

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.

Testing File Integrity Checks

Overview

There are two topics related to file integrity:

  1. Code integrity checks: In the "Tampering and Reverse Engineering" 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 byte-code, 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.
  2. 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).

The following sample implementation from the Android Cracking Blog calculates a CRC over classes.dex and compares it to the expected value.

private void crcTest() throws IOException {
 boolean modified = false;
 // required dex crc value stored as a text string.
 // it could be any invisible layout element
 long dexCrc = Long.parseLong(Main.MyContext.getString(R.string.dex_crc));

 ZipFile zf = new ZipFile(Main.MyContext.getPackageCodePath());
 ZipEntry ze = zf.getEntry("classes.dex");

 if ( ze.getCrc() != dexCrc ) {
  // dex has been modified
  modified = true;
 }
 else {
  // dex not tampered with
  modified = false;
 }
}
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.

When using an HMAC, you can use a bouncy castle implementation or the AndroidKeyStore to HMAC the given content.

Complete the following procedure when generating an HMAC with BouncyCastle:

  1. Make sure BouncyCastle or SpongyCastle is registered as a security provider.
  2. Initialize the HMAC with a key (which can be stored in a keystore).
  3. Get the byte array of the content that needs an HMAC.
  4. Call doFinal on the HMAC with the byte-code.
  5. Append the HMAC to the bytearray obtained in step 3.
  6. Store the result of step 5.

Complete the following procedure when verifying the HMAC with BouncyCastle:

  1. Make sure that BouncyCastle or SpongyCastle is registered as a security provider.
  2. Extract the message and the hmacbytes as separate arrays.
  3. Repeat steps 1-4 of the procedure for generating an HMAC.
  4. Compare the extracted hmacbytes to the result of step 3.

When generating the HMAC based on the Android Keystore, then it is best to only do this for Android 6 and higher.

The following is a convenient HMAC implementation without AndroidKeyStore:

public enum HMACWrapper {
    HMAC_512("HMac-SHA512"), //please note that this is the spec for the BC provider
    HMAC_256("HMac-SHA256");

    private final String algorithm;

    private HMACWrapper(final String algorithm) {
        this.algorithm = algorithm;
    }

    public Mac createHMAC(final SecretKey key) {
        try {
            Mac e = Mac.getInstance(this.algorithm, "BC");
            SecretKeySpec secret = new SecretKeySpec(key.getKey().getEncoded(), this.algorithm);
            e.init(secret);
            return e;
        } catch (NoSuchProviderException | InvalidKeyException | NoSuchAlgorithmException e) {
            //handle them
        }
    }

    public byte[] hmac(byte[] message, SecretKey key) {
        Mac mac = this.createHMAC(key);
        return mac.doFinal(message);
    }

    public boolean verify(byte[] messageWithHMAC, SecretKey key) {
        Mac mac = this.createHMAC(key);
        byte[] checksum = extractChecksum(messageWithHMAC, mac.getMacLength());
        byte[] message = extractMessage(messageWithHMAC, mac.getMacLength());
        byte[] calculatedChecksum = this.hmac(message, key);
        int diff = checksum.length ^ calculatedChecksum.length;

        for (int i = 0; i < checksum.length && i < calculatedChecksum.length; ++i) {
            diff |= checksum[i] ^ calculatedChecksum[i];
        }

        return diff == 0;
    }

    public byte[] extractMessage(byte[] messageWithHMAC) {
        Mac hmac = this.createHMAC(SecretKey.newKey());
        return extractMessage(messageWithHMAC, hmac.getMacLength());
    }

    private static byte[] extractMessage(byte[] body, int checksumLength) {
        if (body.length >= checksumLength) {
            byte[] message = new byte[body.length - checksumLength];
            System.arraycopy(body, 0, message, 0, message.length);
            return message;
        } else {
            return new byte[0];
        }
    }

    private static byte[] extractChecksum(byte[] body, int checksumLength) {
        if (body.length >= checksumLength) {
            byte[] checksum = new byte[checksumLength];
            System.arraycopy(body, body.length - checksumLength, checksum, 0, checksumLength);
            return checksum;
        } else {
            return new byte[0];
        }
    }

    static {
        Security.addProvider(new BouncyCastleProvider());
    }
}

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

  1. Patch the anti-debugging functionality. Disable the unwanted behavior by simply overwriting the associated byte-code or native code with NOP instructions.
  2. 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.
  3. 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.

Refer to the "Tampering and Reverse Engineering" section for examples of patching, code injection, and kernel modules.

Bypassing the storage integrity checks

  1. Retrieve the data from the device, as described in the section on device binding.
  2. Alter the retrieved data and then put it back into storage.

Effectiveness Assessment

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?

Testing The Detection of Reverse Engineering Tools

Overview

Reverse engineers use a lot of tools, frameworks, and apps, many of which you've encountered in this guide. Consequently, the presence of such tools on the device may indicate that the user is attempting to reverse engineer the app. Users increase their risk by installing such tools.

Detection Methods

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 demonstrate 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 run time integrity checks, which are discussed below.

Example: Ways to Detect Frida

An obvious way to detect Frida and similar frameworks is to check the environment for related artifacts, such as package files, binaries, libraries, processes, and temporary files. As an example, I'll hone in on frida-server, the daemon responsible for exposing Frida over TCP. You can use a Java method that iterates through the list of running processes to determine whether frida-server is running:

public boolean checkRunningProcesses() {

  boolean returnValue = false;

  // Get currently running application processes
  List<RunningServiceInfo> list = manager.getRunningServices(300);

  if(list != null){
    String tempName;
    for(int i=0;i<list.size();++i){
      tempName = list.get(i).process;

      if(tempName.contains("fridaserver")) {
        returnValue = true;
      }
    }
  }
  return returnValue;
}

This works if Frida is run in its default configuration. Perhaps it's also enough to stump some script kiddies during their first steps in reverse engineering. It can, however, be easily bypassed by renaming the frida-server binary, so we should find a better method.

frida-server binds to TCP port 27047 by default, so checking whether this port is open is another method of detecting the daemon. The following native code implements this method:

boolean is_frida_server_listening() {
    struct sockaddr_in sa;

    memset(&sa, 0, sizeof(sa));
    sa.sin_family = AF_INET;
    sa.sin_port = htons(27047);
    inet_aton("127.0.0.1", &(sa.sin_addr));

    int sock = socket(AF_INET , SOCK_STREAM , 0);

    if (connect(sock , (struct sockaddr*)&sa , sizeof sa) != -1) {
      /* Frida server detected. Do something… */
    }

}   

Again, this code 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. This method can be improved with an nmap -sV. frida-server uses the D-Bus protocol to communicate, so we send a D-Bus AUTH message to every open port and check for an answer, hoping that frida-server will reveal itself.

/*
 * Mini-portscan to detect frida-server on any local port.
 */

for(i = 0 ; i <= 65535 ; i++) {

    sock = socket(AF_INET , SOCK_STREAM , 0);
    sa.sin_port = htons(i);

    if (connect(sock , (struct sockaddr*)&sa , sizeof sa) != -1) {

        __android_log_print(ANDROID_LOG_VERBOSE, APPNAME,  "FRIDA DETECTION [1]: Open Port: %d", i);

        memset(res, 0 , 7);

        // send a D-Bus AUTH message. Expected answer is "REJECT"

        send(sock, "\x00", 1, NULL);
        send(sock, "AUTH\r\n", 6, NULL);

        usleep(100);

        if (ret = recv(sock, res, 6, MSG_DONTWAIT) != -1) {

            if (strcmp(res, "REJECT") == 0) {
               /* Frida server detected. Do something… */
            }
        }
    }
    close(sock);
}

We now have a fairly robust method of detecting frida-server, but there are still some glaring issues. Most importantly, Frida offers alternative modes of operation that don't require frida-server! How do we detect those?

The common theme for all Frida's modes is code injection, so we can expect to have Frida libraries mapped into memory whenever Frida is used. The straightforward way to detect these libraries is to walk through the list of loaded libraries and check for suspicious ones:

char line[512];
FILE* fp;

fp = fopen("/proc/self/maps", "r");

if (fp) {
    while (fgets(line, 512, fp)) {
        if (strstr(line, "frida")) {
            /* Evil library is loaded. Do something… */
        }
    }

    fclose(fp);

    } else {
       /* Error opening /proc/self/maps. If this happens, something is of. */
    }
}

This detects any libraries whose names include "frida." This check works, but there are some major issues:

  • Remember that relying on frida-server being referred to as "fridaserver" wasn't a good idea? The same applies here; with some small modifications, the Frida agent libraries could simply be renamed.
  • Detection depends on standard library calls such as fopen and strstr. Essentially, we're attempting to detect Frida by using functions that can be easily hooked with-you guessed it-Frida. Obviously, this isn't a very solid strategy.

The first issue can be addressed by implementing a classic-virus-scanner-like strategy: scanning memory for "gadgets" found in Frida's libraries. I chose the string "LIBFRIDA," which appears to be in all versions of frida-gadget and frida-agent. Using the following code, we iterate through the memory mappings listed in /proc/self/maps and search for the string in every executable section. Although I omitted the most boring functions for the sake of brevity, you can find them on GitHub.

static char keyword[] = "LIBFRIDA";
num_found = 0;

int scan_executable_segments(char * map) {
    char buf[512];
    unsigned long start, end;

    sscanf(map, "%lx-%lx %s", &start, &end, buf);

    if (buf[2] == 'x') {
        return (find_mem_string(start, end, (char*)keyword, 8) == 1);
    } else {
        return 0;
    }
}

void scan() {

    if ((fd = my_openat(AT_FDCWD, "/proc/self/maps", O_RDONLY, 0)) >= 0) {

    while ((read_one_line(fd, map, MAX_LINE)) > 0) {
        if (scan_executable_segments(map) == 1) {
            num_found++;
        }
    }

    if (num_found > 1) {

        /* Frida Detected */
    }

}

Note the use of my_openat, etc., instead of the normal libc library functions. These are custom implementations that do the same things as their Bionic libc counterparts: they set up the arguments for the respective system call and execute the swi instruction (see the following code). Using these functions eliminates the reliance on public APIs, thus making them less susceptible to the typical libc hooks. The complete implementation is in syscall.S. The following is an assembler implementation of my_openat.

#include "bionic_asm.h"

.text
    .globl my_openat
    .type my_openat,function
my_openat:
    .cfi_startproc
    mov ip, r7
    .cfi_register r7, ip
    ldr r7, =__NR_openat
    swi #0
    mov r7, ip
    .cfi_restore r7
    cmn r0, #(4095 + 1)
    bxls lr
    neg r0, r0
    b __set_errno_internal
    .cfi_endproc

    .size my_openat, .-my_openat;

This implementation is a bit more effective, and it is difficult to bypass with Frida only, especially if some obfuscation has been added. Even so, there are of course many ways to bypass this. Patching and system call hooking come to mind. Remember, the reverse engineer always wins!

Bypassing Detection of Reverse Engineering Tools
  1. Patch the anti-debugging functionality. Disable the unwanted behavior by simply overwriting the associated byte-code or native code with NOP instructions.
  2. 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.
  3. 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.

Refer to the "Tampering and Reverse Engineering" section for examples of patching, code injection, and kernel modules.

Effectiveness Assessment

Launch the app with various apps and frameworks installed. Include at least the following:

  • Substrate for Android
  • Xposed
  • Frida
  • Introspy-Android
  • Drozer
  • RootCloak
  • Android SSL Trust Killer

The app should respond in some way to the presence of each of those tools. At the very least, the app should alert the user and/or terminate the app. 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-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?

Testing Emulator Detection

Overview

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.

Emulator Detection Examples

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.

API Method          Value           Meaning
Build.ABI           armeabi         possibly emulator
BUILD.ABI2          unknown         possibly emulator
Build.BOARD         unknown         emulator
Build.Brand         generic         emulator
Build.DEVICE        generic         emulator
Build.FINGERPRINT   generic         emulator
Build.Hardware      goldfish        emulator
Build.Host          android-test    possibly emulator
Build.ID            FRF91           emulator
Build.MANUFACTURER  unknown         emulator
Build.MODEL         sdk             emulator
Build.PRODUCT       sdk             emulator
Build.RADIO         unknown         possibly emulator
Build.SERIAL        null            emulator
Build.TAGS          test-keys       emulator
Build.USER          android-build   emulator

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.

API                                                     Value                   Meaning
TelephonyManager.getDeviceId()                          0's                     emulator
TelephonyManager.getLine1 Number()                      155552155               emulator
TelephonyManager.getNetworkCountryIso()                 us                      possibly emulator
TelephonyManager.getNetworkType()                       3                       possibly emulator
TelephonyManager.getNetworkOperator().substring(0,3)    310                     possibly emulator
TelephonyManager.getNetworkOperator().substring(3)      260                     possibly emulator
TelephonyManager.getPhoneType()                         1                       possibly emulator
TelephonyManager.getSimCountryIso()                     us                      possibly emulator
TelephonyManager.getSimSerial Number()                  89014103211118510720    emulator
TelephonyManager.getSubscriberId()                      310260000000000         emulator
TelephonyManager.getVoiceMailNumber()                   15552175049             emulator

Keep in mind that a hooking framework, such as Xposed or Frida, can hook this API to provide false data.

Bypassing Emulator Detection

  1. Patch the emulator detection functionality. Disable the unwanted behavior by simply overwriting the associated byte-code or native code with NOP instructions.
  2. 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.

Refer to the "Tampering and Reverse Engineering" section for examples of patching, code injection, and kernel modules.

Effectiveness Assessment

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?

Testing Run Time Integrity Checks

Overview

Controls in this category verify the integrity of the app's memory space to defend the app against memory patches applied during run time. Such patches include unwanted changes to binary code, byte-code, function pointer tables, and important data structures, as well as rogue code loaded into process memory. Integrity can be verified by

  1. comparing the contents of memory or a checksum over the contents to good values,
  2. 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.

Run Time Integrity Check Examples

Detecting tampering with the Java Runtime

This detection code is from the dead && end blog.

try {
  throw new Exception();
}
catch(Exception e) {
  int zygoteInitCallCount = 0;
  for(StackTraceElement stackTraceElement : e.getStackTrace()) {
    if(stackTraceElement.getClassName().equals("com.android.internal.os.ZygoteInit")) {
      zygoteInitCallCount++;
      if(zygoteInitCallCount == 2) {
        Log.wtf("HookDetection", "Substrate is active on the device.");
      }
    }
    if(stackTraceElement.getClassName().equals("com.saurik.substrate.MS$2") &&
        stackTraceElement.getMethodName().equals("invoked")) {
      Log.wtf("HookDetection", "A method on the stack trace has been hooked using Substrate.");
    }
    if(stackTraceElement.getClassName().equals("de.robv.android.xposed.XposedBridge") &&
        stackTraceElement.getMethodName().equals("main")) {
      Log.wtf("HookDetection", "Xposed is active on the device.");
    }
    if(stackTraceElement.getClassName().equals("de.robv.android.xposed.XposedBridge") &&
        stackTraceElement.getMethodName().equals("handleHookedMethod")) {
      Log.wtf("HookDetection", "A method on the stack trace has been hooked using Xposed.");
    }

  }
}

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 run time, 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 run time. 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 run time, 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.

Bypass and Effectiveness Assessment

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:

  1. Patch the integrity checks. Disable the unwanted behavior by overwriting the respective byte-code or native code with NOP instructions.
  2. Use Frida or Xposed to hook the APIs used for detection and return fake values.

Refer to the "Tampering and Reverse Engineering" section for examples of patching, code injection, and kernel modules.

Testing Device Binding

Overview

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.
  • Obfuscating the data stored on the device by using device identifiers as keys for encryption methods. This can help with binding to a device when the app does a lot of offline work or when access to APIs depends on access-tokens stored by the application.
  • Use token-based device authentication (Instance ID) to make sure that the same instance of the app is used.

Static Analysis

In the past, Android developers often relied on the Settings.Secure.ANDROID_ID (SSAID) and MAC addresses. However, the behavior of the SSAID has changed since Android O, and the behavior of MAC addresses changed with the release of Android N. In addition, there are new recommendations for identifiers in Google's SDK documentation.

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, 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.
  TelephonyManager tm = (TelephonyManager) context.getSystemService(Context.TELEPHONY_SERVICE);
  String IMEI = tm.getDeviceId();

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 O will get the result "UNKNOWN" when they request Build.Serial.

Dynamic Analysis

There are several ways to test the application binding:

Dynamic Analysis with an Emulator
  1. Run the application on an emulator.
  2. Make sure you can raise the trust in the application instance (e.g., authenticate in the app).
  3. 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.
  4. Install the application on another emulator.
  5. 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.
  6. Can you continue in an authenticated state? If so, binding may not be working properly.
Google Instance ID

Google Instance ID 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:

  1. Configure your Instance ID for the given application in your Google Developer Console. This includes managing the PROJECT_ID.

  2. Setup Google Play services. In the file build.gradle, add

  apply plugin: 'com.android.application'
    ...

    dependencies {
        compile 'com.google.android.gms:play-services-gcm:10.2.4'
    }
  1. Get an Instance ID.
  String iid = Instance ID.getInstance(context).getId();
  //now submit this iid to your server.
  1. Generate a token.
String authorizedEntity = PROJECT_ID; // Project id from Google Developer Console
String scope = "GCM"; // e.g. communicating using GCM, but you can use any
                      // URL-safe characters up to a maximum of 1000, or
                      // you can also leave it blank.
String token = Instance ID.getInstance(context).getToken(authorizedEntity,scope);
//now submit this token to the server.
  1. 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:
public class MyInstance IDService extends Instance IDListenerService {
  public void onTokenRefresh() {
    refreshAllTokens();
  }

  private void refreshAllTokens() {
    // assuming you have defined TokenList as
    // some generalized store for your tokens for the different scopes.
    // Please note that for application validation having just one token with one scopes can be enough.
    ArrayList<TokenList> tokenList = TokensList.get();
    Instance ID iid = Instance ID.getInstance(this);
    for(tokenItem : tokenList) {
      tokenItem.token =
        iid.getToken(tokenItem.authorizedEntity,tokenItem.scope,tokenItem.options);
      // send this tokenItem.token to your server
    }
  }
};
  1. Register the service in your Android manifest:
<service android:name=".MyInstance IDService" android:exported="false">
  <intent-filter>
        <action android:name="com.google.android.gms.iid.Instance ID"/>
  </intent-filter>
</service>

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).

Please note that [Firebase also supports Instance ID](https://firebase.google.com/docs/reference/android/com/google/firebase/iid/FirebaseInstance ID "Firebase Instance ID documentation").

IMEI & Serial

Google recommends not using these identifiers unless the application is at a high risk.

For pre-Android O devices, you can request the serial as follows:

   String serial = android.os.Build.SERIAL;

For devices running Android version O and later, you can request the device's serial as follows:

  1. Set the permission in your Android manifest:
  <uses-permission android:name="android.permission.READ_PHONE_STATE"/>
  <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
  1. Request the permission at run time from the user: See https://developer.android.com/training/permissions/requesting.html for more details.
  2. Get the serial:
  String serial = android.os.Build.getSerial();

Retrieve the IMEI:

  1. Set the required permission in your Android manifest:
  <uses-permission android:name="android.permission.READ_PHONE_STATE"/>
  1. If you're using Android version M or later, request the permission at run time from the user: See https://developer.android.com/training/permissions/requesting.html for more details.

  2. Get the IMEI:

  TelephonyManager tm = (TelephonyManager) context.getSystemService(Context.TELEPHONY_SERVICE);
  String IMEI = tm.getDeviceId();
SSAID

Google recommends not using these identifiers unless the application is at a high risk. You can retrieve the SSAID as follows:

  String SSAID = Settings.Secure.ANDROID_ID;

The behavior of the SSAID has changed since Android O, and the behavior of MAC addresses changed with the release of Android N. In addition, there are new recommendations 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 O. There are devices that have the same ANDROID_ID and/or have an ANDROID_ID that can be overridden.

Effectiveness Assessment

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, 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.

  TelephonyManager tm = (TelephonyManager) context.getSystemService(Context.TELEPHONY_SERVICE);
  String IMEI = tm.getDeviceId();

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
  1. Run the application on your rooted device.
  2. Make sure you can raise the trust (e.g., authenticate in the app) in the application instance.
  3. Retrieve the data from the first rooted device.
  4. Install the application on the second rooted device.
  5. In the application's data folder, overwrite the data from step 3.
  6. Can you continue in an authenticated state? If so, binding may not be working properly.

Testing Obfuscation

Overview

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 this test case, we describe a few basic obfuscation techniques that are commonly used on Android.

Effectiveness Assessment

Attempt to decompile the byte-code, 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.

References

OWASP Mobile Top 10 2016

OWASP MASVS

  • V8.1: "The app detects, and responds to, the presence of a rooted or jailbroken device either by alerting the user or terminating the app."
  • V8.2: "The app prevents debugging and/or detects, and responds to, a debugger being attached. All available debugging protocols must be covered."
  • V8.3: "The app detects, and responds to, tampering with executable files and critical data within its own sandbox."
  • V8.4: "The app detects, and responds to, the presence of widely used reverse engineering tools and frameworks on the device."
  • V8.5: "The app detects, and responds to, being run in an emulator."
  • V8.6: "The app detects, and responds to, tampering the code and data in its own memory space."
  • V8.9: "All executable files and libraries belonging to the app are either encrypted on the file level and/or important code and data segments inside the executables are encrypted or packed. Trivial static analysis doesn't reveal important code or data."
  • V8.10: "Obfuscation is applied to programmatic defenses, which in turn impede de-obfuscation via dynamic analysis."
  • V8.10: "The app implements a 'device binding' functionality using a device fingerprint derived from multiple properties unique to the device."

Tools