31 Mar 2022 | Reading time: ~5 min

Bypass certificate pinning with Frida and Xposed

Pinned - HackTheBox

#HackTheBox #Challenge #Android #reversing #bypass-certificate-pinning #frida #Xposed #TrustMeAlready #SSLUnpinning

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Table of contents

1. Introduction
   1. Improved skills
   2. Used tools
2. Video
3. Notes
   1. Universal Android SSL Pinning Bypass (from codeshare.frida.re)
   2. Hook application functions with Frida

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Introduction

Learn how to bypass a certificate pinning protection using Frida and Xposed in order to intercept application’s requests and obtain the secret flag.

Improved skills

• Disassemble APK
• Decompile .dex file
• Reverse Engineering Android applications
• Hook Java functions
• Bypass certificate pinning security implementations

Used tools

• APKTool
• bytecode-viewer
• Frida
• Xposed, TrustMeAlready, SSLUnpinning
• Burpsuite

Video

Notes

Universal Android SSL Pinning Bypass (from codeshare.frida.re)

https://codeshare.frida.re/@pcipolloni/universal-android-ssl-pinning-bypass-with-frida/

/* 
   Android SSL Re-pinning frida script v0.2 030417-pier 

   $ adb push burpca-cert-der.crt /data/local/tmp/cert-der.crt
   $ frida -U -f it.app.mobile -l frida-android-repinning.js --no-pause

   https://techblog.mediaservice.net/2017/07/universal-android-ssl-pinning-bypass-with-frida/
   
   UPDATE 20191605: Fixed undeclared var. Thanks to @oleavr and @ehsanpc9999 !
*/

setTimeout(function(){
    Java.perform(function (){
    	console.log("");
	    console.log("[.] Cert Pinning Bypass/Re-Pinning");

	    var CertificateFactory = Java.use("java.security.cert.CertificateFactory");
	    var FileInputStream = Java.use("java.io.FileInputStream");
	    var BufferedInputStream = Java.use("java.io.BufferedInputStream");
	    var X509Certificate = Java.use("java.security.cert.X509Certificate");
	    var KeyStore = Java.use("java.security.KeyStore");
	    var TrustManagerFactory = Java.use("javax.net.ssl.TrustManagerFactory");
	    var SSLContext = Java.use("javax.net.ssl.SSLContext");

	    // Load CAs from an InputStream
	    console.log("[+] Loading our CA...")
	    var cf = CertificateFactory.getInstance("X.509");
	    
	    try {
	    	var fileInputStream = FileInputStream.$new("/data/local/tmp/cert-der.crt");
	    }
	    catch(err) {
	    	console.log("[o] " + err);
	    }
	    
	    var bufferedInputStream = BufferedInputStream.$new(fileInputStream);
	  	var ca = cf.generateCertificate(bufferedInputStream);
	    bufferedInputStream.close();

		var certInfo = Java.cast(ca, X509Certificate);
	    console.log("[o] Our CA Info: " + certInfo.getSubjectDN());

	    // Create a KeyStore containing our trusted CAs
	    console.log("[+] Creating a KeyStore for our CA...");
	    var keyStoreType = KeyStore.getDefaultType();
	    var keyStore = KeyStore.getInstance(keyStoreType);
	    keyStore.load(null, null);
	    keyStore.setCertificateEntry("ca", ca);
	    
	    // Create a TrustManager that trusts the CAs in our KeyStore
	    console.log("[+] Creating a TrustManager that trusts the CA in our KeyStore...");
	    var tmfAlgorithm = TrustManagerFactory.getDefaultAlgorithm();
	    var tmf = TrustManagerFactory.getInstance(tmfAlgorithm);
	    tmf.init(keyStore);
	    console.log("[+] Our TrustManager is ready...");

	    console.log("[+] Hijacking SSLContext methods now...")
	    console.log("[-] Waiting for the app to invoke SSLContext.init()...")

	   	SSLContext.init.overload("[Ljavax.net.ssl.KeyManager;", "[Ljavax.net.ssl.TrustManager;", "java.security.SecureRandom").implementation = function(a,b,c) {
	   		console.log("[o] App invoked javax.net.ssl.SSLContext.init...");
	   		SSLContext.init.overload("[Ljavax.net.ssl.KeyManager;", "[Ljavax.net.ssl.TrustManager;", "java.security.SecureRandom").call(this, a, tmf.getTrustManagers(), c);
	   		console.log("[+] SSLContext initialized with our custom TrustManager!");
	   	}
    });
},0);

Hook application functions with Frida

# Already pushed and started frida server
$ adb push burpca-cert-der.crt /data/local/tmp/cert-der.crt

$ frida -U -f com.example.pinned -l unpin.js --no-pause
     ____
    / _  |   Frida 15.1.17 - A world-class dynamic instrumentation toolkit
   | (_| |
    > _  |   Commands:
   /_/ |_|       help      -> Displays the help system
   . . . .       object?   -> Display information about 'object'
   . . . .       exit/quit -> Exit
   . . . .
   . . . .   More info at https://frida.re/docs/home/
   . . . .
   . . . .   Connected to Google Nexus 5X (id=192.168.164.104:5555)
Spawned `com.example.pinned`. Resuming main thread!
[Google Nexus 5X::com.example.pinned ]->
[.] Cert Pinning Bypass/Re-Pinning
[+] Loading our CA...
[o] Our CA Info: CN=PortSwigger CA, OU=PortSwigger CA, O=PortSwigger, L=PortSwigger, ST=PortSwigger, C=PortSwigger
[+] Creating a KeyStore for our CA...
[+] Creating a TrustManager that trusts the CA in our KeyStore...
[+] Our TrustManager is ready...
[+] Hijacking SSLContext methods now...
[-] Waiting for the app to invoke SSLContext.init()...
[o] App invoked javax.net.ssl.SSLContext.init...
[+] SSLContext initialized with our custom TrustManager!

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