{"id":"GHSA-m9mq-7m7q-xc6p","summary":"browse-mcp has an 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.","aliases":["CVE-2026-55557"],"modified":"2026-08-26T00:05:58.828785060Z","published":"2026-08-25T16:28:32Z","database_specific":{"cwe_ids":["CWE-22"],"severity":"HIGH","github_reviewed":true,"github_reviewed_at":"2026-08-25T16:28:32Z","nvd_published_at":null},"references":[{"type":"WEB","url":"https://github.com/That1Drifter/browse-mcp/security/advisories/GHSA-m9mq-7m7q-xc6p"},{"type":"WEB","url":"https://github.com/That1Drifter/browse-mcp/pull/58"},{"type":"WEB","url":"https://github.com/That1Drifter/browse-mcp/commit/5352a4a56f626254b445bfa07e4bb48c5aad15c1"},{"type":"PACKAGE","url":"https://github.com/That1Drifter/browse-mcp"},{"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"}],"affected":[{"package":{"name":"browse-mcp","ecosystem":"npm","purl":"pkg:npm/browse-mcp"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"0.8.2"}]}],"database_specific":{"last_known_affected_version_range":"\u003c= 0.8.1","source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/08/GHSA-m9mq-7m7q-xc6p/GHSA-m9mq-7m7q-xc6p.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"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"}]}