Cross-site scripting in Bleach
Impact
A mutation XSS affects users calling bleach.clean with all of:
svgormathin the allowed tagsporbrin allowed tagsstyle,title,noscript,script,textarea,noframes,iframe, orxmpin 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.cleancalls to at least one of:- not allow the
style,title,noscript,script,textarea,noframes,iframe, orxmptag - not allow
svgormathtags - not allow
porbrtags - set
strip_comments=True
- not allow the
-
A strong Content-Security-Policy without
unsafe-inlineandunsafe-evalscript-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
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | bleach | < 3.3.0 | 3.3.0 |
References
- https://github.com/mozilla/bleach/security/advisories/GHSA-vv2x-vrpj-qqpq
- https://advisory.checkmarx.net/advisory/CX-2021-4303
- https://github.com/mozilla/bleach/commit/79b7a3c5e56a09d1d323a5006afa59b56162eb13
- https://nvd.nist.gov/vuln/detail/CVE-2021-23980
- https://bugzilla.mozilla.org/show_bug.cgi?id=1689399
- https://bugzilla.mozilla.org/show_bug.cgi?id=CVE-2021-23980
- https://cure53.de/fp170.pdf
- https://github.com/mozilla/bleach
- https://github.com/mozilla/bleach/blob/79b7a3c5e56a09d1d323a5006afa59b56162eb13/CHANGES#L4
- https://github.com/pypa/advisory-database/tree/main/vulns/bleach/PYSEC-2021-865.yaml
- https://pypi.org/project/bleach
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N- Weakness (CWE)
CWE-79- Platform
- Python (PyPI)
- Package
bleach- Disclosed
- 02.02.2021
- Updated
- 11.10.2026 13:19
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2021-865 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-vv2x-vrpj-qqpq - CC BY 4.0
Vulnerability history: bleach
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 8
- First → latest
- 03.2018 06.2026
- Without a fix
- 1
- Exploited in attacks
- 0
Severity breakdown
- Critical 1
- High 1
- Medium 5
- Low 1
Disclosures per year
- 2018: 1
- 2019: 0
- 2020: 3
- 2021: 1
- 2022: 0
- 2023: 0
- 2024: 0
- 2025: 0
- 2026: 3
-
Cross-site scripting in Bleach this entry
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