Android Leaks Traffic with Always-On VPN

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Understanding the Issue

Connectivity Checks

Android devices perform connectivity checks when connecting to new WiFi networks. These checks ensure the network can support internet access by sending data outside the secure VPN tunnel. This process, while necessary for functionality, poses a privacy risk.

Design Choice vs. User Expectations

Google designed Android to leak certain types of traffic, arguing that preventing these leaks would confuse users. However, security experts and users expect comprehensive protection from their VPNs, leading to skepticism about Google's explanation.

Implications of the Traffic Leak

Potential Threats

Leaked data can reveal sensitive information, such as the location of the WiFi access point. This information, combined with other data points, can de-anonymize users, posing significant risks for those relying on VPNs for privacy.

Exploitation Risks

Network administrators and sophisticated threat actors can exploit leaked traffic data. For example, attackers controlling connectivity check servers can observe and analyze traffic, potentially revealing user identities despite VPN usage.

Workarounds and Mitigations

Disable Connectivity Checks

One effective way to prevent traffic leakage is by disabling connectivity checks. Users can achieve this using the adb shell command to set captive_portal_mode to 0, reducing data leakage outside the VPN tunnel.

Custom ROMs and Specialized Operating Systems

Custom ROMs like GrapheneOS offer additional privacy and security features. GrapheneOS, for instance, provides the option to disable connectivity checks by default, minimizing data leakage risks.

Current State and Future Developments

Mullvad VPN has reported the issue to Google, suggesting updates to documentation and the ability to disable connectivity checks by default. Google's response has been that this behavior is intended, but this stance has faced criticism from security experts.

Recommendations for Users

1. Disable Connectivity Checks: Use the adb shell command to set captive_portal_mode to 0.
2. Use Custom ROMs: Consider custom ROMs like GrapheneOS for enhanced privacy and security.
3. Stay Updated: Keep VPN apps and operating systems updated with the latest security patches.
4. Be Aware: Understand VPN limitations and take necessary precautions to minimize risks.

By staying informed and taking proactive steps, users can better protect their privacy and security on Android devices despite inherent design choices leading to traffic leaks.