CVE-2026-55557: browse-mcp has an arbitrary file write via unconfined download and state paths

browser_download wrote a fetched file to join(save_dir, filename) with no validation of save_dir, and browser_save_state / browser_load_state honored an explicit path unchanged. The MCP caller controls these arguments (a malicious MCP client, or an autonomous agent steered by indirect prompt injection on a visited page), so an attacker could supply an arbitrary save_dir (or state path) together with a URL whose response body became the file contents, writing attacker-controlled bytes to any path the process can reach (for example ~/.bashrc, an autostart entry, or a cron file). That is an arbitrary file write that can lead to host code execution. The force_fetch fallback additionally used a raw fetch() that bypassed the BROWSE_MCP_ALLOWED_ORIGINS origin fence.

Estimated severity: CVSS 3.1 around 7.8 (High) for the local / agent-mediated case.