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ullvad VPN has discovered that Android leaks traffic every time the device connects to a WiFi network, even if the “Block connections without VPN,” or “Always-on VPN,” features is enabled.
The data being leaked outside VPN tunnels includes source IP addresses, DNS lookups, HTTPS traffic, and likely also NTP traffic.
This behavior is built into the Android operating system and is a design choice. However, Android users likely didn’t know this until now due to the inaccurate description of the “VPN Lockdown” features in Android’s documentation.
Mullvad discovered the issue during a security audit that hasn’t been published yet, issuing a warning yesterday to raise awareness on the matter and apply additional pressure on Google.
VPNs on Android
VPNs (virtual private networks) are protected network connections that encrypt internet traffic over public networks. When connected to a VPN, all your Internet connections will use the IP address of your VPN service rather than your public IP address.
This allows users to bypass censorship and throttling, and maintain privacy and anonymity while browsing the web, as the remote hosts will never see your actual IP address.
Android offers a setting under “Network & Internet” to block network connections unless you’re using a VPN. This feature is designed to prevent accidental leaks of the user’s actual IP address if the VPN connection is interrupted or drops suddenly.
Unfortunately, this feature is undercut by the need to accommodate special cases like identifying captive portals (like hotel WiFi) that must be checked before the user can log in or when using split-tunnel features.
This is why Android is configured to leak some data upon connecting to a new WiFi network, regardless of whether you enabled the “Block connections without VPN” setting.
Mullvad reported the issue to Google, requesting the addition of an option to disable connectivity checks.