SSRF Vulnerability on assetlinks_check(act_name, well_knowns)

Summary

While examining the "App Link assetlinks.json file could not be found" vulnerability detected by MobSF, we, as the Trendyol Application Security team, noticed that a GET request was sent to the "/.well-known/assetlinks.json" endpoint for all hosts written with "android:host". In the AndroidManifest.xml file.

Since MobSF does not perform any input validation when extracting the hostnames in "android:host", requests can also be sent to local hostnames. This may cause SSRF vulnerability.

Details

Example <intent-filter structure in AndroidManifest.xml:

<intent-filter android:autoVerify="true">
<action android:name="android.intent.action.VIEW" />
<category android:name="android.intent.category.DEFAULT" />
<category android:name="android.intent.category.BROWSABLE" />
<data android:host="192.168.1.102/user/delete/1#" android:scheme="http" />
</intent-filter>

We defined it as android:host="192.168.1.102/user/delete/1#". Here, the "#" character at the end of the host prevents requests from being sent to the "/.well-known/assetlinks.json" endpoint and ensures that requests are sent to the endpoint before it.

PoC

https://drive.google.com/file/d/1nbKMd2sKosbJef5Mh4DxjcHcQ8Hw0BNR/view?usp=share_link

Impact

The attacker can cause the server to make a connection to internal-only services within the organization's infrastructure.