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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

8.6 · CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-55557: 0.24% estimated exploitation activity in the next 30 days; percentile 14.96 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact browserdownload wrote a fetched file to join(savedir, filename) with no validation of savedir, and browsersavestate / browserloadstate 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 savedir (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 forcefetch fallback additionally used a raw fetch() that bypassed the BROWSEMCPALLOWEDORIGINS origin fence. Estimated severity: CVSS 3.1 around 7.8 (High) for the local / agent-mediated case. Patches Fixed in 0.8.2. savedir is confined under the download root (~/.browse-mcp/downloads) and the explicit state path under ~/.browse-mcp/state;…

Published by the source

25 Aug 2026, 16:28 UTC

Last modified by the source

25 Aug 2026, 16:28 UTC

Source last checked

26 Oct 2026, 13:01 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-m9mq-7m7q-xc6p

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# Patch Next advisory brief: CVE-2026-55557

Advisory: browse-mcp has an arbitrary file write via unconfined download and state paths
IDs: CVE-2026-55557, GHSA-m9mq-7m7q-xc6p
Record: https://patchnext.com/advisories/cve-2026-55557 (revision 1, source checked 26 Oct 2026, 13:01 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.6)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: browse-mcp 0.8.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm browse-mcp: affected <= 0.8.1; fixed in 0.8.2

## Suggested inspection (read-only)
- `npm ls browse-mcp --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on browse-mcp.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-m9mq-7m7q-xc6p (GitHub-reviewed advisory GHSA-m9mq-7m7q-xc6p lists affected ranges: npm browse-mcp >=0 <=0.8.1)

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