GHSA-chwh-f6gm-r836: Gotenberg: Server-Side Request Forgery via Chromium URL Endpoint with Redirect-Based Deny-List Bypass
A review of 4 published Gotenberg security advisories exposed an SSRF issue. GHSA-pjrr-jgp4-v2fm covers SSRF via the `downloadFrom` endpoint. GHSA-pcrp-7g9h-7qhp covers SSRF via the `webhook` endpoint. Neither advisory addresses SSRF through the primary Chromium URL-to-PDF conversion endpoint (`/forms/chromium/convert/url`), which has no default deny-list for HTTP/HTTPS targets. The redirect-based deny-list bypass described here also applies to `downloadFrom` and `webhook` but is a separate finding from the initial request validation those advisories cover.SummaryGotenberg's Chromium URL-to-PDF endpoint (`/forms/chromium/convert/url`) has no default protection against HTTP/HTTPS-based SSRF. The default deny-list regex only blocks `file://` URIs. An unauthenticated attacker can point Chromium at any internal IP — including loopback, RFC 1918 ranges, and cloud metadata endpoints — and receive the response rendered as a PDF.Additionally, even when operators configure a custom deny-list, the protection is bypassed via HTTP redirects. Gotenberg's Chromium instance follows `302` redirects from an attacker-controlled external URL to internal targets without re-validating the redirect destination against the deny-list.What makes this particularly notable is that Gotenberg's secondary features — `downloadFrom` and `webhook` — ship with default deny-lists that explicitly block RFC 1918 and link-local addresses. The primary feature, the one that literally takes a URL and fetches it server-side, does not.Details**Finding 1: Zero default SSRF protection on Chromium URL endpoint**The Chromium URL endpoint is the core feature of Gotenberg. It accepts a URL, tells headless Chromium to fetch it, and returns the rendered page as a PDF. The default deny-list is configured in `pkg/modules/chromium/chromium.go` and the value shipped in Docker is:This regex only blocks `file://` URIs outside of `/tmp/`. HTTP and HTTPS requests to any host — including `127.0.0.1`, `10.x.x.x`, `192.168.x.x`, and `169.254.169.254` — are not filtered at all.Meanwhile, the `downloadFrom` and `webhook` endpoints use deny-lists that explicitly block loopback, RFC 1918, and cloud metadata IPs. The developer clearly understood the SSRF risk but the protection was not applied to the main Chromium conversion endpoint.**Finding 2: Redirect-based SSRF bypass on all endpoints**Both `downloadFrom` and `webhook` use Go's default `http.Client{}` with no `CheckRedirect` function. Go follows up to 10 redirects automatically. The deny-list is a pre-flight check on the initial URL only. Once the request is in flight, redirects are followed transparently and the application never re-validates the destination.The Chromium browser similarly follows redirects without restriction. Even if an operator configures a custom deny-list on the Chromium URL endpoint, an attacker hosts a redirect server that passes initial validation and then redirects Chromium to an internal target.PoCTested on Docker using `gotenberg/gotenberg:8` (v8.30.1) on `localhost:3000`. No authentication is required on any endpoint.**Environment:****1. Control — external URL works as expected:****2. Control — `file://` protocol is correctly blocked by default deny-list:****3. SSRF to localhost — NOT blocked:**Chromium fetched its own `/health` endpoint and rendered the response as a PDF. The request succeeded because the default deny-list does not cover HTTP to loopback.**4. Cloud metadata IP — NOT blocked:**The request timed out because there is no metadata service running in the Docker test environment. The critical observation is that Gotenberg did not block or reject the request. In a cloud deployment (AWS, GCP, Azure), this would return IAM credentials rendered as a PDF.**5. Redirect-based bypass — Chromium follows 302 to internal target:**Redirect server on the host (port 9999):Chromium followed the 302 redirect from `http://172.17.0.1:9999/` (external, passes any deny-list) to `http://127.0.0.1:3000/health` (internal). The internal response was rendered as a PDF and returned to the caller. No validation occurred on the redirect destination.The Chromium endpoint accepted all HTTP/HTTPS URLs including loopback and cloud metadata addresses. Only `file://` URIs were blocked by the default deny-list. The redirect from an external server to `127.0.0.1` was also followed without any check on the redirect target.ImpactAny user who can reach the Gotenberg API — which requires no authentication by default — can make the server fetch arbitrary internal resources and receive the rendered content as a PDF. Gotenberg is typically deployed as a backend service in infrastructure that has broad internal network access.Practical attack scenarios:**Cloud credential theft**: Request `http://169.254.169.254/latest/meta-data/iam/security-credentials/` to exfiltrate AWS IAM role credentials. The same applies to GCP and Azure metadata endpoints.**Internal service access**: Reach any HTTP service on the internal network that the Gotenberg container can route to — admin panels, databases with HTTP interfaces, monitoring dashboards.**Internal port scanning**: Use response timing and content differences to map internal infrastructure.**Deny-list bypass via redirect**: Even deployments that have configured custom deny-lists for the initial URL are vulnerable. An attacker hosts a redirect server at `https://attacker.com/r` that responds with `302 → http://169.254.169.254/latest/meta-data/`. The deny-list validates the initial URL, Chromium follows the redirect, and the cloud metadata is returned as a PDF.The redirect bypass also affects the `downloadFrom` and `webhook` endpoints, which use Go's `http.Client{}` with no `CheckRedirect` function. Their RFC 1918 deny-lists are rendered ineffective by a single redirect hop.
CVSS v3.1: 8.6