MASTG-TECH-0110: Intercepting Flutter HTTPS Traffic

Flutter is an open-source UI software development kit (SDK) created by Google. It is used for building natively compiled applications for mobile, web, and desktop from a single codebase. Flutter uses Dart, which is not proxy-aware and uses its own certificate store. The application doesn't use the proxy configuration of the system and sends the data directly to the server. Connections are verified against built-in certificates, so any certificates installed on the system are simply ignored. Due to this, it is not possible to intercept HTTPS requests as the certificate of the proxy will never be trusted.

In order to intercept Flutter HTTPS traffic, we need to deal with two problems:

Make sure the traffic is sent to the proxy.
Disable the TLS verification of any HTTPS connection.

There are generally two approaches to this: reFlutter and Frida for iOS.

reFlutter: This tool creates a modified version of the Flutter module which is then repackaged into the IPA. It configures the internal libraries to use a specified proxy and disable the TLS verification.
Frida: The disable-flutter-tls.js script can dynamically remove the TLS verification without the need for repackaging. As it doesn't modify the proxy configuration, additional steps are needed (e.g. VPN, DNS, iptables, WIFI hotspot).

Intercepting Traffic using reFlutter¶

Patch the app to enable traffic interception.

Run the command to patch the app and select the option Traffic monitoring and interception and then enter the IP of the machine on which the interception proxy is running.
```
$ reflutter demo.ipa

Choose an option:

    Traffic monitoring and interception
    Display absolute code offset for functions

[1/2]? 1

Example: (192.168.1.154) etc.
Please enter your BurpSuite IP: 192.168.29.216
```
This will create a release.RE.ipa file in the output folder.
Sign ( Repackaging and Re-Signing) the patched release.RE.ipa with the Apple certificates. This will create a signed ".ipa" file in the output folder.
Install the signed patched app on the mobile device.
Configure the interception proxy. For example, in Burp:
Under Proxy -> Proxy settings -> Add new Proxy setting.
Bind listening Port to 8083.
Select Bind to address to All interfaces.
Request Handling -> support for invisible proxying.
Open the app and start intercepting traffic.

Intercepting Traffic using WIFI Hotspot / openVPN with Frida¶

Configure using WIFI hotspot / openVPN method to redirect requests to Burp.
Install the iOS UnCrackable L1 on the mobile device.
Configure the interception proxy. For example, in Burp:
Under Proxy -> Proxy settings -> Add new Proxy setting.
Bind listening Port to 8080.
Select Bind to address to All interfaces.
Request Handling -> support for invisible proxying.

Run the disable-flutter-tls-verification Frida script.

frida -U -f eu.nviso.flutterPinning -l disable-flutter-tls.js

Start intercepting traffic.