Fast Browser crosses the boundary of an authenticated Google Chrome profile. The extension can expose pages, cookies' effects, account data, form contents, and actions available to the signed-in user through the local MCP runtime. Treat access to Fast Browser as access to that browser profile.

browser_run_code_unsafe intentionally runs arbitrary Playwright-side JavaScript. Arbitrary page-derived or instruction-derived code passed to that tool is remote-code-execution-equivalent in the MCP process: it can use the process's permissions and interact with authenticated pages. A website, page content, downloaded text, or model instruction is not a trusted code source. Review code before execution and never turn untrusted page text directly into executable JavaScript.

Use a dedicated Chrome profile where possible. Keep the default safe profile, manual connection, explicit tool approvals, and session recording disabled unless the task requires more access. Select only the host you use. Grant the macOS account, Chrome profile, MCP process, and visited sites the least privilege practical for the task.

Automatic pairing stores the reconnect token in macOS Keychain as the dev.mattstack.fast-browser service and chrome-extension account. The secure Keychain prompt owns token input; Fast Browser does not accept a token flag or print the value. When automatic connection is active, the CLI reads the item and scopes the token to the launched MCP child process. Anyone who can control that process or the signed-in macOS account may be able to use the paired browser access.

Never disclose a reconnect token in logs, shell history, screenshots, macros, session records, support requests, or vulnerability reports. Re-pair after any suspected exposure. Ordinary uninstall and data purge currently retain the Keychain item; remove it in Keychain Access when it should no longer exist.

Recorded sessions may retain page content, URLs, user inputs, screenshots, and authenticated workflow details. Macros are executable JavaScript and can encode private selectors, identifiers, values, and business logic. Store both as sensitive local data, review macros before reuse, keep retention short, and purge data when it is no longer required.

This repository's available reporting mechanism is GitHub Issues: https://github.com/m4ttstack/fast-browser/issues/new

Open a minimal issue describing the affected Fast Browser version, impact, and safe reproduction outline. Do not include tokens, credentials, private page content, session recordings, or working exploit details in a public issue. If sensitive coordination is needed, ask the maintainer in that issue to establish a private channel; this repository does not publish a separate security contact in the candidate.