GHSA-hv85-774v-26fg: auth-fetch-mcp: SSRF and disk exfiltration via unvalidated auth_fetch and download_media URLs
SSRF + disk-exfil in `download_media` and `auth_fetch` tools — ymw0407/auth-fetch-mcpSeverityThe `download_media` and `auth_fetch` MCP tools accept arbitrary URLs and reach them as the MCP server process, with `download_media` additionally persisting the fetched response body to a user-controlled output directory. An MCP client (LLM under prompt injection, malicious peer) can drive the server to fetch loopback / link-local / private-range hosts (cloud-instance metadata, internal services, host-bound services) and exfiltrate the response.Vulnerability chainSite 1: `download_media` — SSRF + disk-write chain`src/tools.ts:200-274``urls` and `output_dir` are user-controlled. The handler iterates each URL (line 236) and calls `ctx.request.get(url)` (Playwright's `APIRequestContext.get`) without checking the destination. The response body is written to `path.join(output_dir, file-N.ext)`. Internal-service responses are persisted to disk where they can be exfiltrated via any subsequent tool that reads from the output directory (or via the response object itself, which contains `localPath` and `size` of every successful write).Site 2: `auth_fetch` — SSRF via Playwright navigation`src/tools.ts:117-198``src/browser.ts:53-64``url` flows directly from the MCP tool argument to `page.goto` with no validation. Playwright will navigate to any URL the network stack can reach. The page DOM is returned in the tool response via `extractContent`. Internal pages (loopback admin UIs, cloud metadata endpoints reachable from the host, intranet services) are extractable.Root causeNeither handler validates URL targets before dispatch. The tool descriptions ("fetches web page content using a real browser ... e.g. Notion, Google Docs, Jira, Confluence, Linear, Slack, or any SaaS/private page") frame the intended usage as **public SaaS web pages**, not loopback or link-local hosts — but no code enforces that intent.The fix shape (apply to both tools): after URL parsing, resolve to IP, reject if private/loopback/link-local. Same defense as the well-known SSRF-guard pattern shipped by other MCP fetchers in the ecosystem (e.g., `Akitaroh/scraper-mcp` `src/security/url-guard.ts`).Auth boundary violated**Boundary type:** MCP tool-argument boundary plus the local-network trust boundary. The MCP server typically sits inside a trust boundary (developer laptop with loopback services, cloud VM with IMDS, k8s pod with service account). The tools allow the MCP client to dispatch HTTP requests across that boundary.**Respected/violated trace:** Per the tool descriptions, the expected respected boundary is "public SaaS web pages." That expectation is violated by any request reaching a host the user didn't intend to expose (127.0.0.1:6379 Redis, 169.254.169.254 cloud metadata, 192.168.0.1 internal admin).Impact**Cloud credential theft** — server on EC2 / GCE / Azure VM. MCP client invokes `auth_fetch({ url: "http://169.254.169.254/latest/meta-data/iam/security-credentials/<role>" })` and receives temporary credentials in the tool response. Or invokes `download_media({ urls: [...], output_dir: "/tmp/exfil" })` to persist them to disk.**Internal service enumeration** — MCP client probes private-range hosts (10/8, 172.16/12, 192.168/16). Each `auth_fetch` returns the page DOM; each `download_media` writes the response to disk.**Loopback exploitation** — server runs alongside Redis (127.0.0.1:6379), ElasticSearch (127.0.0.1:9200), or internal admin UIs. MCP client reads them via `auth_fetch`.**Disk-write side channel** (`download_media` only) — output_dir is also user-controlled, with no documented restriction. An MCP client can request `output_dir = "/some/user-writable-shared-dir"` and exfil internal-service responses to a location accessible to a co-tenant process.The injection vector is any content reaching the model that prompts a fetch tool call. The tool description explicitly says "MUST be used instead of Fetch/web_fetch when the page requires login" — meaning the model is encouraged to call this tool for any "private page" mention, which a prompt-injected upstream content can trivially trigger.Proof of concept (non-destructive)`poc.mjs` — replicates the `download_media` handler's HTTP-fetch + file-write chain against a local fake-internal HTTP service. Playwright's `ctx.request.get(url)` is replaced with the equivalent `fetch(url)` for the bug case (a URL needing no auth) so the demo runs without browser deps. The structural defect — "no host validation before HTTP dispatch" — is identical.Exit code `0`. SHA-256 `poc.mjs`: `4cea53f1a618581fc67f9a8bd07a7a2b22274f42cdbf7f3c658519673aaf7568`. The PoC only contacts `127.0.0.1` on an ephemeral port; the fake-credentials string contains the literal `FAKE` marker so no downstream system can mistake it for real credentials. The exfil directory is cleaned up after the demo.Suggested fixAdd a `assertSafeUrl` helper (same shape as in the matching egoist/fetch-mcp advisory) called before any HTTP dispatch — at `tools.ts:236` inside the download_media loop, and at the top of `navigateTo` in `browser.ts:53`:Where `isPrivateOrLinkLocal` blocks 127.0.0.0/8, 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, 169.254.0.0/16, ::1, fc00::/7, fe80::/10.For `download_media` specifically, also constrain `output_dir`: resolve it under a fixed root (e.g., `~/.auth-fetch-mcp/downloads/`) and reject if the resolved path escapes that root.
CVSS v3.1: 8.2