Deep Dive: How AmneziaVPN Fixed a tun0 Data Leak on Android

AmneziaVPN developers have released the second part of their report on fixing a critical vulnerability in their Android client involving traffic leaks via the tun0 interface. The issue allowed apps excluded from the VPN to bind sockets to the tunnel, potentially exposing the server address. The fix involved implementing UID-based packet filtering, requiring extensive work across the Go codebase, JNI bridge, and rigorous testing. The article details how the Linux kernel handles packet routing, the challenges of identifying socket owners, and the complexities of integrating these filters into the existing architecture. While the solution successfully blocked bypass attempts, the team continues to refine the implementation and invites the community to review their approach, aiming to help other VPN developers address similar vulnerabilities in their own clients.