{"id":"CVE-2026-55557","aliases":["GHSA-m9mq-7m7q-xc6p"],"url":"https://o3.security/vulnerability/CVE-2026-55557","summary":"browse-mcp: Arbitrary file write via unconfined download and state paths","details":"### Impact\n`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.\n\nEstimated severity: CVSS 3.1 around 7.8 (High) for the local / agent-mediated case.\n\n### Patches\nFixed 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.\n\n### Workarounds\nRestrict 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.\n\n- Reported privately by novice-22.","published":"2026-08-25T16:28:07.368Z","modified":"2026-09-11T03:30:56.008040008Z","cvss":null,"epss":{"score":0.00189,"percentile":0.08509,"asOf":"2026-08-27"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"browse-mcp","fixedVersion":"0.8.2"}],"fix":{"url":"https://github.com/That1Drifter/browse-mcp/commit/5352a4a56f626254b445bfa07e4bb48c5aad15c1","label":"That1Drifter/browse-mcp@5352a4a"},"references":[{"type":"WEB","url":"https://github.com/That1Drifter/browse-mcp/blob/v0.8.2/CHANGELOG.md#082---2026-06-13"},{"type":"WEB","url":"https://github.com/That1Drifter/browse-mcp/releases/tag/v0.8.2"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55557.json"},{"type":"ADVISORY","url":"https://github.com/That1Drifter/browse-mcp/security/advisories/GHSA-m9mq-7m7q-xc6p"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-55557"},{"type":"FIX","url":"https://github.com/That1Drifter/browse-mcp/commit/5352a4a56f626254b445bfa07e4bb48c5aad15c1"},{"type":"FIX","url":"https://github.com/That1Drifter/browse-mcp/pull/58"},{"type":"PACKAGE","url":"https://github.com/That1Drifter/browse-mcp"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-09-11T03:30:56.008040008Z"}}