How severe is CVE-2026-55557?

No CVSS score has been assigned to CVE-2026-55557 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-55557?

CVE-2026-55557 affects the following packages: browse-mcp (npm). Ecosystems affected: npm.

How do I fix CVE-2026-55557?

Update browse-mcp to 0.8.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55557 is resolved across your whole dependency graph.

How do I detect CVE-2026-55557 in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for browse-mcp, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-55557 if there is no patch (or I can't update yet)?

Resolve every user-supplied path to its canonical form and reject anything that escapes the intended directory, and run the component under an account that has no read or write access outside the directory it legitimately serves.

Is CVE-2026-55557 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-55557 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-55557?

CVE-2026-55557 has an EPSS (Exploit Prediction Scoring System) score of 0.2%, placing it in the 14th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-55557?

CVE-2026-55557 is classified as Path Traversal (CWE-22). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2026-55557 published, and has it been updated?

CVE-2026-55557 was published on August 25, 2026 and was last updated on September 11, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦 npm

Not in CISA KEV

CVE-2026-55557 — browse-mcp

Fix: That1Drifter/browse-mcp@5352a4a

CVE-2026-55557 is a Path Traversal vulnerability in browse-mcp. A fix is available for browse-mcp — see the affected versions and patch details below.

browse-mcp: Arbitrary file write via unconfined download and state paths

Also known asGHSA-m9mq-7m7q-xc6p

Published

Aug 25, 2026

Updated

Sep 11, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 8, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

A successful exploit gives an attacker total control of the affected component, not partial access.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-55557.

EPSS Exploitation Probability

via FIRST.org ↗

0.2%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs14th percentile — riskier than 14% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

0other npm packages depend on this — each one inherits the vulnerability until it's patched upstream

browse-mcpnpm

32downloads / week

Description

Impact

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.

Patches

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.

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.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`browse-mcp`	all versions	0.8.2`npm install browse-mcp@0.8.2`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for browse-mcp, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update browse-mcp to 0.8.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55557 is resolved across your whole dependency graph.
Workarounds
Resolve every user-supplied path to its canonical form and reject anything that escapes the intended directory, and run the component under an account that has no read or write access outside the directory it legitimately serves.

Frequently Asked Questions

### Impact `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

O3 Security · Impact-Aware SCA

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