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Cross-site scripting in Bleach

Moderate severity GitHub Reviewed Published Feb 1, 2021 in mozilla/bleach • Updated Sep 7, 2023

Package

pip bleach (pip)

Affected versions

< 3.3.0

Patched versions

3.3.0

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

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:

References

@g-k g-k published to mozilla/bleach Feb 1, 2021
Reviewed Feb 2, 2021
Published to the GitHub Advisory Database Feb 2, 2021
Published by the National Vulnerability Database Feb 16, 2023
Last updated Sep 7, 2023

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
Required
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

EPSS score

0.087%
(37th percentile)

Weaknesses

CVE ID

CVE-2021-23980

GHSA ID

GHSA-vv2x-vrpj-qqpq

Source code

No known source code
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