CVE-2021-23980

CVE-2021-23980 is a medium-severity cross-site scripting (XSS) vulnerability in bleach (pip), affecting versions < 3.3.0. It is fixed in 3.3.0.

Summary

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:

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.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2021-23980 has a CVSS score of 6.1 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.3.0); upgrading removes the vulnerable code path.

Affected versions

bleach (< 3.3.0)

Security releases

bleach → 3.3.0 (pip)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

See it in your environment

Remediation advice

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.

Frequently Asked Questions

  1. What is CVE-2021-23980? CVE-2021-23980 is a medium-severity cross-site scripting (XSS) vulnerability in bleach (pip), affecting versions < 3.3.0. It is fixed in 3.3.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
  2. How severe is CVE-2021-23980? CVE-2021-23980 has a CVSS score of 6.1 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
  3. Which versions of bleach are affected by CVE-2021-23980? bleach (pip) versions < 3.3.0 is affected.
  4. Is there a fix for CVE-2021-23980? Yes. CVE-2021-23980 is fixed in 3.3.0. Upgrade to this version or later.
  5. Is CVE-2021-23980 exploitable, and should I be worried? Whether CVE-2021-23980 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
  6. What actually determines whether CVE-2021-23980 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
  7. How do I fix CVE-2021-23980? Upgrade bleach to 3.3.0 or later.

Other vulnerabilities in bleach

Other vulnerabilities in bleach

CVE-IDSeverityPackage summary
GHSA-GJ48-438W-JH9VMediumbleach: Bleach clean() / Cleaner() fails to sanitize dangerous URI schemes in allowed formaction attributesGHSA-G75F-G53V-794XMediumbleach: Bleach linkify(parse_email=True) CPU exhaustion via unbounded email regex scanningGHSA-8RFP-98V4-MMR6Lowbleach: Bleach: URI sanitization allows disallowed URI schemes with Unicode > U+00A0 in outputGHSA-5634-RV46-48JFHighbleach: Cross-Site Scripting in bleachCVE-2014-8881Mediumbleach: Regular Expression Denial of Service in bleach

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