Bleach vulnerable to mutation XSS via whitelisted math or svg and raw tag
Impact
A mutation XSS affects users calling bleach.clean with all of:
- the
svgormathin the allowed/whitelisted tags - an RCDATA tag (see below) in the allowed/whitelisted tags
- the keyword argument
strip=False
Patches
Users are encouraged to upgrade to bleach v3.1.2 or greater.
Workarounds
- modify
bleach.cleancalls to usestrip=True, or not whitelistmathorsvgtags and one or more of the following tags:
script
noscript
style
noframes
xmp
noembed
iframe- 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=1621692
- https://cure53.de/fp170.pdf
- https://nvd.nist.gov/vuln/detail/CVE-2020-6816
- https://www.checkmarx.com/blog/vulnerabilities-discovered-in-mozilla-bleach
Credits
- Reported by Yaniv Nizry from the CxSCA AppSec group at Checkmarx
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 | < 3.1.2 | 3.1.2 |
References
- https://github.com/mozilla/bleach/security/advisories/GHSA-m6xf-fq7q-8743
- https://www.checkmarx.com/blog/vulnerabilities-discovered-in-mozilla-bleach
- https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/EDQU2SZLZMSSACCBUBJ6NOSRNNBDYFW5/
- https://advisory.checkmarx.net/advisory/CX-2020-4277
- https://nvd.nist.gov/vuln/detail/CVE-2020-6816
- https://github.com/mozilla/bleach
- https://github.com/mozilla/bleach/releases/tag/v3.1.2
- https://github.com/pypa/advisory-database/tree/main/vulns/bleach/PYSEC-2020-28.yaml
- https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/EDQU2SZLZMSSACCBUBJ6NOSRNNBDYFW5
- 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
- 24.03.2020
- Updated
- 11.10.2026 13:19
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2020-28 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-m6xf-fq7q-8743 - 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
-
Bleach vulnerable to mutation XSS via whitelisted math or svg and raw tag this entry
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