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PYSEC-2026-3617

Vulnerability from pysec - Published: 2026-08-04 11:34 - Updated: 2026-08-04 13:36
VLAI
Details

Summary

The ALLOWED_SOURCES configuration is meant to restrict which hosts Thumbor's HTTP loader may fetch images from. Plain-string entries in that list (the overwhelming majority of real-world and documented configurations) are passed directly to re.match() without escaping. Because . is a regex wildcard, every dot in a domain name becomes a bypass vector: s.glbimg.com silently matches sXglbimgYcom, sAglbimg.com, and any other hostname that differs only at a dot position. This undermines the primary SSRF defence that ALLOWED_SOURCES is intended to provide.

Affected component

thumbor/loaders/http_loader.py — validate()

Proof of concept

import re
from thumbor.config import Config
from thumbor.context import Context
from thumbor.loaders import http_loader as loader

config = Config()
config.ALLOWED_SOURCES = ["s.glbimg.com"]   # typical user config
ctx = Context(None, config, None)

# These should be blocked — both return True due to the unescaped dot
print(loader.validate(ctx, "http://sXglbimgYcom/secret.jpg"))  # True ← bypass
print(loader.validate(ctx, "http://sAglbimg.com/secret.jpg"))  # True ← bypass

# Legitimate origin — correctly allowed
print(loader.validate(ctx, "http://s.glbimg.com/logo.jpg"))    # True ← correct

Root cause

thumbor/loaders/http_loader.py (before fix):

for pattern in context.config.ALLOWED_SOURCES:
    if isinstance(pattern, Pattern):
        match = url
    else:
        pattern = f"^{pattern}$"   # <-- dots not escaped, act as regex wildcard
        match = res.hostname

    if re.match(pattern, match):
        return True

Impact

An attacker who can influence the image source URL passed to Thumbor can fetch images from arbitrary hosts, bypassing the ALLOWED_SOURCES allowlist.

Preconditions:

  • ALLOWED_SOURCES contains at least one plain-string entry (the common case; all official documentation examples use plain strings).
  • The attacker can supply or influence the image URL — true whenever ALLOW_UNSAFE_URL = True (the default), or when the application forwards user input to a signed URL endpoint.

Fix

Apply re.escape() to plain-string patterns before compiling them, so every character is matched literally:

else:
    pattern = f"^{re.escape(pattern)}$"   # dots and other metacharacters are now literal
    match = res.hostname

This is a one-call addition with no breaking change for correctly written configurations. Users who need real regular-expression behaviour should supply a compiled pattern (re.compile(r"s\.glbimg\.com")), which is already handled by the existing isinstance(pattern, Pattern) branch and is unaffected by this change.

The ALLOWED_SOURCES docstring in config.py was also updated to document the two-mode behaviour explicitly.

Severity
8.2 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:L
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-2026-53500",
    "GHSA-6x26-6r6f-m537"
  ],
  "details": "## Summary\n\nThe `ALLOWED_SOURCES` configuration is meant to restrict which hosts Thumbor\u0027s HTTP loader may fetch images from. Plain-string entries in that list (the overwhelming majority of real-world and documented configurations) are passed directly to `re.match()` without escaping. Because `.` is a regex wildcard, every dot in a domain name becomes a bypass vector: `s.glbimg.com` silently matches `sXglbimgYcom`, `sAglbimg.com`, and any other hostname that differs only at a dot position. This undermines the primary SSRF defence that `ALLOWED_SOURCES` is intended to provide.\n\n## Affected component\n\n`thumbor/loaders/http_loader.py` \u2014 `validate()`\n\n## Proof of concept\n\n```python\nimport re\nfrom thumbor.config import Config\nfrom thumbor.context import Context\nfrom thumbor.loaders import http_loader as loader\n\nconfig = Config()\nconfig.ALLOWED_SOURCES = [\"s.glbimg.com\"]   # typical user config\nctx = Context(None, config, None)\n\n# These should be blocked \u2014 both return True due to the unescaped dot\nprint(loader.validate(ctx, \"http://sXglbimgYcom/secret.jpg\"))  # True \u2190 bypass\nprint(loader.validate(ctx, \"http://sAglbimg.com/secret.jpg\"))  # True \u2190 bypass\n\n# Legitimate origin \u2014 correctly allowed\nprint(loader.validate(ctx, \"http://s.glbimg.com/logo.jpg\"))    # True \u2190 correct\n```\n\n## Root cause\n\n`thumbor/loaders/http_loader.py` (before fix):\n\n```python\nfor pattern in context.config.ALLOWED_SOURCES:\n    if isinstance(pattern, Pattern):\n        match = url\n    else:\n        pattern = f\"^{pattern}$\"   # \u003c-- dots not escaped, act as regex wildcard\n        match = res.hostname\n\n    if re.match(pattern, match):\n        return True\n```\n\n## Impact\n\nAn attacker who can influence the image source URL passed to Thumbor can fetch images from arbitrary hosts, bypassing the `ALLOWED_SOURCES` allowlist.\n\n**Preconditions:**\n\n- `ALLOWED_SOURCES` contains at least one plain-string entry (the common case; all official documentation examples use plain strings).\n- The attacker can supply or influence the image URL \u2014 true whenever  `ALLOW_UNSAFE_URL = True` (the default), or when the application forwards user input to a signed URL endpoint.\n\n## Fix\n\nApply `re.escape()` to plain-string patterns before compiling them, so every\ncharacter is matched literally:\n\n```python\nelse:\n    pattern = f\"^{re.escape(pattern)}$\"   # dots and other metacharacters are now literal\n    match = res.hostname\n```\n\nThis is a one-call addition with no breaking change for correctly written configurations. Users who need real regular-expression behaviour should supply a compiled pattern (`re.compile(r\"s\\.glbimg\\.com\")`), which is already handled by the existing `isinstance(pattern, Pattern)` branch and is unaffected by this change.\n\nThe `ALLOWED_SOURCES` docstring in `config.py` was also updated to document the two-mode behaviour explicitly.",
  "id": "PYSEC-2026-3617",
  "modified": "2026-08-04T13:36:37.987077Z",
  "published": "2026-08-04T11:34:46.220271Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/thumbor/thumbor/security/advisories/GHSA-6x26-6r6f-m537"
    },
    {
      "type": "WEB",
      "url": "https://github.com/thumbor/thumbor/commit/68876715350c6c8f49c324e5515e64908830aed7"
    },
    {
      "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-6x26-6r6f-m537"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-53500"
    }
  ],
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Thumbor treats ALLOWED_SOURCES string patterns as unescaped regex, allowing hostname bypass via wildcard dot"
}



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