GHSA-VV2X-VRPJ-QQPQ

Vulnerability from github – Published: 2021-02-02 17:58 – Updated: 2025-03-20 18:50
VLAI?
Summary
Cross-site scripting in Bleach
Details

Impact

A mutation XSS affects users calling bleach.clean with all of:

  • svg or math in the allowed tags
  • p or br in allowed tags
  • style, title, noscript, script, textarea, noframes, iframe, or xmp in allowed tags
  • the keyword argument strip_comments=False

Note: none of the above tags are in the default allowed tags and strip_comments defaults to True.

Patches

Users are encouraged to upgrade to bleach v3.3.0 or greater.

Note: bleach v3.3.0 introduces a breaking change to escape HTML comments by default.

Workarounds

  • modify bleach.clean calls to at least one of:
  • not allow the style, title, noscript, script, textarea, noframes, iframe, or xmp tag
  • not allow svg or math tags
  • not allow p or br tags

  • set strip_comments=True

  • A strong Content-Security-Policy without unsafe-inline and unsafe-eval script-srcs) will also help mitigate the risk.

References

  • https://bugzilla.mozilla.org/show_bug.cgi?id=1689399
  • https://advisory.checkmarx.net/advisory/CX-2021-4303
  • https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-23980
  • https://cure53.de/fp170.pdf

Credits

  • Reported by Yaniv Nizry from the CxSCA AppSec group at Checkmarx
  • Additional eject tags not mentioned in the original advisory and the CSP mitigation line being truncated in the revised advisory reported by Michał Bentkowski at Securitum

For more information

If you have any questions or comments about this advisory:

Severity ?
6.1 (Medium)
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
6.9 (Medium)
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N
Show details on source website
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{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "bleach"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "3.3.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2021-23980"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2021-02-02T15:54:20Z",
    "nvd_published_at": "2023-02-16T22:15:00Z",
    "severity": "MODERATE"
  },
  "details": "### Impact                                                                                                                                                                                                                                                    \n\nA [mutation XSS](https://cure53.de/fp170.pdf) affects users calling `bleach.clean` with all of:\n\n* `svg` or `math` in the allowed tags\n* `p` or `br` in allowed tags\n* `style`, `title`, `noscript`, `script`, `textarea`, `noframes`, `iframe`, or `xmp` in allowed tags\n* the keyword argument `strip_comments=False`\n\nNote: none of the above tags are in the default allowed tags and `strip_comments` defaults to `True`.\n\n### Patches\n\nUsers are encouraged to upgrade to bleach v3.3.0 or greater.\n\nNote: bleach v3.3.0 introduces a breaking change to escape HTML comments by default.\n\n### Workarounds\n\n* modify `bleach.clean` calls to at least one of:\n  * not allow the `style`, `title`, `noscript`, `script`, `textarea`, `noframes`, `iframe`, or `xmp` tag\n  * not allow `svg` or `math` tags\n  * not allow `p` or `br` tags\n  * set `strip_comments=True`\n\n* A strong [Content-Security-Policy](https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP) without `unsafe-inline` and `unsafe-eval` [`script-src`s](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src)) will also help mitigate the risk.\n\n### References\n\n* https://bugzilla.mozilla.org/show_bug.cgi?id=1689399\n* https://advisory.checkmarx.net/advisory/CX-2021-4303\n* https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-23980\n* https://cure53.de/fp170.pdf\n\n### Credits\n\n* Reported by [Yaniv Nizry](https://twitter.com/ynizry) from the CxSCA AppSec group at Checkmarx\n* Additional eject tags not mentioned in the original advisory and the CSP mitigation line being truncated in the revised advisory reported by [Micha\u0142 Bentkowski](https://twitter.com/SecurityMB) at Securitum\n\n### For more information\n\nIf you have any questions or comments about this advisory:\n\n* Open an issue at [https://github.com/mozilla/bleach/issues](https://github.com/mozilla/bleach/issues)\n* Email us at [security@mozilla.org](mailto:security@mozilla.org)",
  "id": "GHSA-vv2x-vrpj-qqpq",
  "modified": "2025-03-20T18:50:31Z",
  "published": "2021-02-02T17:58:40Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/mozilla/bleach/security/advisories/GHSA-vv2x-vrpj-qqpq"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-23980"
    },
    {
      "type": "WEB",
      "url": "https://github.com/mozilla/bleach/commit/79b7a3c5e56a09d1d323a5006afa59b56162eb13"
    },
    {
      "type": "WEB",
      "url": "https://advisory.checkmarx.net/advisory/CX-2021-4303"
    },
    {
      "type": "WEB",
      "url": "https://bugzilla.mozilla.org/show_bug.cgi?id=1689399"
    },
    {
      "type": "WEB",
      "url": "https://bugzilla.mozilla.org/show_bug.cgi?id=CVE-2021-23980"
    },
    {
      "type": "WEB",
      "url": "https://cure53.de/fp170.pdf"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/mozilla/bleach"
    },
    {
      "type": "WEB",
      "url": "https://github.com/mozilla/bleach/blob/79b7a3c5e56a09d1d323a5006afa59b56162eb13/CHANGES#L4"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/advisory-database/tree/main/vulns/bleach/PYSEC-2021-865.yaml"
    },
    {
      "type": "WEB",
      "url": "https://pypi.org/project/bleach"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    },
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Cross-site scripting in Bleach"
}


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Sightings

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  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.



Detection rules are retrieved from Rulezet.