Android SSL Re-Pinning

Two kinds of SSL Pinning implementations can be found in Android apps: the home-made and the proper one. The former is usually a single method, performing all the certificate checks (possibly using custom libraries), that returns a Boolean value. This means that this approach can be easily bypassed by identifying the interesting method and flipping the return value. The following example is a simplified version of a Frida JavaScript script:

After we identify the offending method (hint: logcat) we basically hijack it and let it always return true.

When SSL Pinning is instead performed according to the official Android documentation, well… things get tougher. There are many excellent solutions out there, being custom android images, underlying frameworks, socket.relaxsslcheck=yes , etc. Almost every attempt at bypassing SSL Pinning is based on manipulating the SSLContext. Can we manipulate the SSLContext with Frida? What we wanted was a generic/universal approach and we wanted to do it with a Frida JavaScript script.

The idea here is to do exactly what the official documentation suggests doing so we’ve ported the SSL Pinning Java code to Frida JavaScript.

How it works:

Load our rogue CAs cert from device Create our own KeyStore containing our trusted CAs Create a TrustManager that trusts the CAs in our KeyStore

When the application initializes its SSLContext we hijack the SSLContext.init() method and when it gets called, we swap the 2nd parameter, which is the application TrustManager, with our own TrustManager we previously prepared. (SSLContext.init(KeyManager, TrustManager, SecuRandom)).

This way we basically re-pinn the application to our own CA!

Example

$ adb push burpca-cert-der.crt /data/local/tmp/cert-der.crt

$ adb shell "/data/local/tmp/frida-server &"



$ frida -U -f it.app.mobile -l frida-android-repinning.js --no-pause

[…]

[USB::Samsung GT-31337::['it.app.mobile']]->

[.] 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!

[o] App invoked javax.net.ssl.SSLContext.init...

[+] SSLContext initialized with our custom TrustManager!

[o] App invoked javax.net.ssl.SSLContext.init...

[+] SSLContext initialized with our custom TrustManager!

[o] App invoked javax.net.ssl.SSLContext.init...

[+] SSLContext initialized with our custom TrustManager!

In this case the application invoked SSLContext.init four times which means it verified four different certs (two of which were used by 3rd party tracking libs).

Download here: frida-android-repinning_sa.js or from Frida Codeshare here https://codeshare.frida.re/@pcipolloni/universal-android-ssl-pinning-bypass-with-frida/

Frida & Android https://www.frida.re/docs/android/