GHSA-hq37-853p-g5cf
Vulnerability from github
Published
2021-01-06 16:57
Modified
2024-09-13 17:42
Severity ?
7.5 (High) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
7.7 (High) - CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:P
Summary
Regular Expression Denial of Service in CairoSVG
Details

Doyensec Vulnerability Advisory

  • Regular Expression Denial of Service (REDoS) in cairosvg
  • Affected Product: CairoSVG v2.0.0+
  • Vendor: https://github.com/Kozea
  • Severity: Medium
  • Vulnerability Class: Denial of Service
  • Author(s): Ben Caller (Doyensec)

Summary

When processing SVG files, the python package CairoSVG uses two regular expressions which are vulnerable to Regular Expression Denial of Service (REDoS). If an attacker provides a malicious SVG, it can make cairosvg get stuck processing the file for a very long time.

Technical description

The vulnerable regular expressions are

https://github.com/Kozea/CairoSVG/blob/9c4a982b9a021280ad90e89707eacc1d114e4ac4/cairosvg/colors.py#L190-L191

The section between 'rgb(' and the final ')' contains multiple overlapping groups.

Since all three infinitely repeating groups accept spaces, a long string of spaces causes catastrophic backtracking when it is not followed by a closing parenthesis.

The complexity is cubic, so doubling the length of the malicious string of spaces makes processing take 8 times as long.

Reproduction steps

Create a malicious SVG of the form:

<svg width="1" height="1"><rect fill="rgb(                     ;"/></svg>

with the following code:

'<svg width="1" height="1"><rect fill="rgb(' + (' ' * 3456) + ';"/></svg>'

Note that there is no closing parenthesis before the semi-colon.

Run cairosvg e.g.:

cairosvg cairo-redos.svg -o x.png

and notice that it hangs at 100% CPU. Increasing the number of spaces increases the processing time with cubic complexity.

Remediation

Fix the regexes to avoid overlapping parts. Perhaps remove the [ \n\r\t]* groups from the regex, and use .strip() on the returned capture group.

Disclosure timeline

  • 2020-12-30: Vulnerability disclosed via email to CourtBouillon
Show details on source website

To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "CairoSVG"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.5.1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2021-21236"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-400"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2021-01-06T16:57:38Z",
    "nvd_published_at": "2021-01-06T17:15:00Z",
    "severity": "HIGH"
  },
  "details": "# Doyensec Vulnerability Advisory \n\n* Regular Expression Denial of Service (REDoS) in cairosvg\n* Affected Product: CairoSVG v2.0.0+\n* Vendor: https://github.com/Kozea\n* Severity: Medium\n* Vulnerability Class: Denial of Service\n* Author(s): Ben Caller ([Doyensec](https://doyensec.com))\n\n## Summary\n\nWhen processing SVG files, the python package CairoSVG uses two regular expressions which are vulnerable to Regular Expression Denial of Service (REDoS).\nIf an attacker provides a malicious SVG, it can make cairosvg get stuck processing the file for a very long time.\n\n## Technical description\n\nThe vulnerable regular expressions are\n\nhttps://github.com/Kozea/CairoSVG/blob/9c4a982b9a021280ad90e89707eacc1d114e4ac4/cairosvg/colors.py#L190-L191\n\nThe section between \u0027rgb(\u0027 and the final \u0027)\u0027 contains multiple overlapping groups.\n\nSince all three infinitely repeating groups accept spaces, a long string of spaces causes catastrophic backtracking when it is not followed by a closing parenthesis.\n\nThe complexity is cubic, so doubling the length of the malicious string of spaces makes processing take 8 times as long.\n\n## Reproduction steps\n\nCreate a malicious SVG of the form:\n\n    \u003csvg width=\"1\" height=\"1\"\u003e\u003crect fill=\"rgb(                     ;\"/\u003e\u003c/svg\u003e\n\nwith the following code:\n\n    \u0027\u003csvg width=\"1\" height=\"1\"\u003e\u003crect fill=\"rgb(\u0027 + (\u0027 \u0027 * 3456) + \u0027;\"/\u003e\u003c/svg\u003e\u0027\n\nNote that there is no closing parenthesis before the semi-colon.\n\nRun cairosvg e.g.:\n\n    cairosvg cairo-redos.svg -o x.png\n\nand notice that it hangs at 100% CPU. Increasing the number of spaces increases the processing time with cubic complexity.\n\n## Remediation\n\nFix the regexes to avoid overlapping parts. Perhaps remove the [ \\n\\r\\t]* groups from the regex, and use .strip() on the returned capture group.\n\n## Disclosure timeline\n\n- 2020-12-30: Vulnerability disclosed via email to CourtBouillon",
  "id": "GHSA-hq37-853p-g5cf",
  "modified": "2024-09-13T17:42:15Z",
  "published": "2021-01-06T16:57:50Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/Kozea/CairoSVG/security/advisories/GHSA-hq37-853p-g5cf"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-21236"
    },
    {
      "type": "WEB",
      "url": "https://github.com/Kozea/CairoSVG/commit/cfc9175e590531d90384aa88845052de53d94bf3"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/Kozea/CairoSVG"
    },
    {
      "type": "WEB",
      "url": "https://github.com/Kozea/CairoSVG/releases/tag/2.5.1"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/advisory-database/tree/main/vulns/cairosvg/PYSEC-2021-5.yaml"
    },
    {
      "type": "WEB",
      "url": "https://pypi.org/project/CairoSVG"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    },
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:P",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Regular Expression Denial of Service in CairoSVG"
}


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