Gotenberg versions 8.29.1 up to but not including 8.31.0 contain an unauthenticated server-side request forgery (SSRF) vulnerability via an unfiltered webhook URL. An attacker with network access can cause Gotenberg to make arbitrary HTTP POST requests to internal or external destinations by supplying a crafted URL in the Gotenberg-Webhook-Url header. This blind SSRF allows internal network probing and forced POST requests to internal services, including cloud metadata endpoints, without exposing response bodies. The root cause is an insecure default in URL filtering that allows all URLs when no allowlist or denylist is configured. A fix is available in version 8.31.0.