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GHSA-vv2x-vrpj-qqpq

MediumCVSS 6.1 / 10
Published Feb 2, 2021·Last modified Mar 13, 2026
Affected Components(0)

No affected components available

Description

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:

Risk Scores
Base Score
6.1

The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. The attacker needs the user to perform some action, like clicking a link. The vulnerability can affect other systems as well, not just the initial system. There is a low impact on the confidentiality of the information. There is a low impact on the integrity of the data.

Threat Intelligence
5.6

Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.

EPSS
0.48%

The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.

Exploit
Not available

We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.

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