GHSA-m6xf-fq7q-8743
MEDIUMGHSA-m6xf-fq7q-8743 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in bleach. 2 public exploit references exist, so weaponization risk is real. O3 Security confirms whether GHSA-m6xf-fq7q-8743 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Bleach vulnerable to mutation XSS via whitelisted math or svg and raw tag
Blast Radius
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Description
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 [email protected]
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | bleach | all versions | 3.1.2 |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for bleach. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update bleach to 3.1.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-m6xf-fq7q-8743 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-m6xf-fq7q-8743 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-m6xf-fq7q-8743. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-m6xf-fq7q-8743 in your dependencies?
O3 detects GHSA-m6xf-fq7q-8743 across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.