CVE-2026-55557

browse-mcp has an arbitrary file write via unconfined download and state paths

Workarounds

Restrict the exposed tools with BROWSE_MCP_TOOLS to a set that excludes browser_download, browser_save_state, and browser_load_state (for example the hardened recipe in SECURITY.md). Note that the allowlist hides the tools from an agent but does not stop a malicious MCP client from calling them by name, so upgrading is the real fix.

• Reported privately by novice-22.

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.

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

Kodem intelligence

Already deployed Kodem?

Fixed in 0.8.2. save_dir is confined under the download root (~/.browse-mcp/downloads) and the explicit state path under ~/.browse-mcp/state; absolute paths and .. escapes are rejected, and download filenames are reduced to a bare basename. force_fetch now also honors the origin fence. Data roots remain relocatable via BROWSE_MCP_HOME. Upgrade to browse-mcp 0.8.2.

1. What is CVE-2026-55557? CVE-2026-55557 is a high-severity path traversal vulnerability in browse-mcp (npm), affecting versions <= 0.8.1. It is fixed in 0.8.2. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. Which versions of browse-mcp are affected by CVE-2026-55557? browse-mcp (npm) versions <= 0.8.1 is affected.
3. Is there a fix for CVE-2026-55557? Yes. CVE-2026-55557 is fixed in 0.8.2. Upgrade to this version or later.
4. Is CVE-2026-55557 exploitable, and should I be worried? Whether CVE-2026-55557 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-55557 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-55557? Upgrade browse-mcp to 0.8.2 or later.