The browser trust boundary is the line between what users think a browser feature or extension will do and what the installed software can actually control. In practice, a browser add-on may look like a simple helper, but if it has powerful permissions it can read pages, alter requests, inject content, or reroute traffic.
This matters because attackers often hide inside that boundary. A fake VPN or proxy extension can present a privacy brand while silently sending traffic through another proxy network, changing the path data takes outside the user’s direct control. Defenders watch for this by reviewing extension permissions, publisher identity, network behavior, and signs that a browser is acting as part of the network path. Treating extensions as privileged software, not harmless UI extras, helps reduce the risk.