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:

• Open an issue at https://github.com/mozilla/bleach/issues
• Email us at security@mozilla.org