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CRITICAL severity

Policies not properly enforced in bluemondayGHSA-x95h-979x-cf3j

CRITICALFix: microcosm-cc/bluemonday@c788a2a

GHSA-x95h-979x-cf3j is a critical-severity (CVSS 9.8) vulnerability in github.com/microcosm-cc/bluemonday. 1 public exploit reference exists, so weaponization risk is real. A fix is available for github.com/microcosm-cc/bluemonday — see the affected versions and patch details below.

Also known asCVE-2021-42576GO-2022-0588PYSEC-2021-849
Published
Updated
Affected
2 pkgs
Patched
2 / 2
Exploits
1 known
Exploitation data as of Oct 11, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
1.6%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs75th percentile — riskier than 75% of all scored CVEsHighest risk
0.00%0.69%1.38%2.06%0.3%1.6%Apr 26Aug 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

GHSA-x95h-979x-cf3j by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

2 pkgs affected
🐹github.com/microcosm-cc/bluemonday🐍pybluemonday

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go, PyPI packages — download data is not available via public APIs for these ecosystems.

Description

The bluemonday sanitizer before 1.0.16 for Go, and before 0.0.8 for Python (in pybluemonday), does not properly enforce policies associated with the SELECT, STYLE, and OPTION elements.

Affected Packages

2 total 2 fixed
EcosystemPackageVulnerable rangeFix
🐹Gogithub.com/microcosm-cc/bluemondayall versions1.0.16go get github.com/microcosm-cc/bluemonday@v1.0.16
🐍PyPIpybluemondayall versions0.0.8pip install --upgrade 'pybluemonday==0.0.8'

Affected Products

2 products · 2 configurations
Application
bluemondaymicroco
< 1.0.16
range
Application
pybluemondaypython
< 0.0.8
range
Exploits & PoCs
1

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

The bluemonday sanitizer before 1.0.16 for Go, and before 0.0.8 for Pyth…

docs.google.comNVDOct 2021

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/microcosm-cc/bluemonday, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update github.com/microcosm-cc/bluemonday to 1.0.16 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-x95h-979x-cf3j is resolved across your whole dependency graph.

  3. 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.

Frequently Asked Questions

The bluemonday sanitizer before 1.0.16 for Go, and before 0.0.8 for Python (in pybluemonday), does not properly enforce policies associated with the SELECT, STYLE, and OPTION elements.
O3 Security · Impact-Aware SCA

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Policies not properly enforced in bluemonday (Critical 9.8)