What Is Frida?
Frida is a powerful dynamic instrumentation toolkit used for:
- reverse engineering
- mobile application analysis
- binary manipulation
- runtime function hooking
- debugging protected applications
Frida allows you to inject JavaScript into running processes on Windows, Linux, macOS, Android, and iOS.
In CTFs—especially Reverse, Pwn, and Mobile (Android/iOS) categories—Frida is one of the most commonly used tools for bypassing checks and extracting hidden values.
Why Frida Is Useful in CTF
Frida can:
- hook functions
- override return values
- bypass anti-debugging
- dump encryption keys
- modify program logic at runtime
- trace functions to understand behavior
- brute-force obfuscated logic dynamically
This makes it ideal for Android apps with obfuscated Java or native code, and for binaries where static analysis alone is not enough.
Basic Commands You Use in CTF
1. Check connected devices (Android)
frida-ls-devices
2. Attach to a running process
frida -U -n com.example.app
3. Run Frida script
frida -U -f com.ctf.challenge -l script.js --no-pause
-U: USB device-f: spawn target-l: load script
Common Frida Scripts in CTF
1. Hooking Java Methods (Android)
Hook a Java class method:
Java.perform(function() {
var Cls = Java.use("com.example.SecretClass");
Cls.getFlag.implementation = function() {
console.log("getFlag called!");
return "FLAG{override}";
};
});
Used for:
- Bypassing license checks
- Returning flags directly
- Overriding authentication routines
2. Printing Parameters and Return Values
Java.perform(function() {
var Cls = Java.use("com.example.Login");
Cls.checkPassword.implementation = function(pwd) {
console.log("Password checked:", pwd);
var result = this.checkPassword(pwd);
console.log("Result:", result);
return result;
};
});
Used for:
- Finding expected passwords
- Revealing hidden values
- Tracking logic flow
3. Hooking Native (C/C++) Functions
For NDK-based apps or native challenges:
Interceptor.attach(Module.findExportByName(null, "strcmp"), {
onEnter(args) {
console.log("strcmp called:", args[0].readUtf8String(), args[1].readUtf8String());
}
});
Or hook functions in a specific library:
Interceptor.attach(Module.findExportByName("libnative.so", "check_flag"), {
onLeave(retval) {
console.log("Return value:", retval.toInt32());
}
});
4. Bypassing Root/Jailbreak/Debug Checks
Many APKs contain checks like _isDeviceRooted() or isDebuggerConnected().
Override them:
Java.perform(function() {
var RootCheck = Java.use("com.security.CheckRoot");
RootCheck.isRooted.implementation = function() {
return false;
};
});
5. Dumping Dynamic Values or Keys
Extract AES keys, API tokens, or hidden strings:
Java.perform(function() {
var Crypto = Java.use("javax.crypto.spec.SecretKeySpec");
Crypto.$init.implementation = function(buf, algo) {
console.log("Key:", buf.toString());
return this.$init(buf, algo);
};
});
How Frida Is Used in CTF Challenges
1. Bypass license / password checks
Hook the function checking input → return true.
2. Extract the correct flag from obfuscated logic
Obfuscated APKs hide the flag in Java or native code.
Frida reveals:
- decryption keys
- final plaintext
- the actual return value of
getFlag() - hidden strings in memory
3. Analyze native libraries (.so files)
Sometimes binaries have ARM/NDK code such as:
if (strcmp(input, "supersecret") == 0)
Frida hooks strcmp() and prints expected values.
4. Bypass anti-debugging
Typical anti-debug code:
if (Debug.isDebuggerConnected()) { exit() }
Frida bypasses it using override technique.
5. Altering runtime behavior
Modify program logic to skip:
- checksum validation
- signature verification
- encryption rounds
- anti-reverse tampering protection
6. Dynamic tracing
Frida helps map unknown code behavior:
- log function calls
- print caller addresses
- inspect stack traces
Useful for fast understanding of complex binaries.
Common Challenge Patterns
| Pattern | Description | How Frida Helps |
|---|---|---|
| Obfuscated APK | Hard to statically analyze | Hook functions & print values |
| Hardcoded but encrypted flag | Hidden in runtime | Dump decryption routines |
| Anti-debugging | Prevents analysis | Override checks |
| Native library puzzles | ARM/NDK code | Hook native functions |
| Password-check logic | input vs expected | Reveal expected input |
| Multi-step verification | Hashing then comparing | Dump intermediate values |
| Custom ciphers | Complicated logic | Instrument decrypt function |
| XOR / custom ops | Performed at runtime | Log arguments & outputs |
Recommended Workflow for CTF Using Frida
- Install Frida & tools
pip install frida-tools
- Analyze the app statically
Look for classes, strings, and native functions. - Attach with Frida
Use-ffor spawning the process. - Identify target methods
Check Java or native functions to hook. - Hook & log values
Create simple scripts to dump internal operations. - Override logic when needed
Force checks to always return true. - Extract flag
Often found in a return value or decrypted buffer.
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