Racing for TLS Certificate Validation: A Hijacker's Guide to the Android TLS Galaxy

Besides developers' code, current Android apps usually integrate code from third-party libraries, all of which may include code for TLS validation. We analyze well-known improper TLS certificate validation issues in popular Android apps, and attribute the validation issues to the offending code/party in a fine-grained manner, unlike existing work labelling an entire app for validation failures. Surprisingly, we discovered a widely used practice of overriding the global default validation functions with improper validation logic, or simply performing no validation at all, affecting the entire app's TLS connections, which we call validation hijacking. We design and implement an automated dynamic analysis tool called Marvin to identify TLS validation failures, including validation hijacking, and the responsible parties behind such dangerous practice. We use Marvin to analyze 6315 apps from a Chinese app store and Google Play, and find many occurrences of insecure TLS certificate validation instances (55.7% of the Chinese apps and 4.6% of the Google Play apps). Validation hijacking happens in 34.3% of the insecure apps from the Chinese app store and 20.0% of insecure Google Play apps. A network attacker can exploit these insecure connections in various ways, e.g., to compromise PII, app login and SSO credentials, to launch phishing and other content modification attacks, including code injection. We found that most of these vulnerabilities are related to third-party libraries used by the apps, not the app code created by app developers. The technical root cause enabling validation hijacking appears to be the specific modifications made by Google in the OkHttp library integrated with the Android OS, which is used by many developers by default, without being aware of its potential dangers. Overall, our findings provide valuable insights into the responsible parties for TLS validation issues in Android, including the validation hijacking problem.