How severe is CVE-2021-42576?

CVE-2021-42576 has a CVSS score of 9.8/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-42576?

CVE-2021-42576 affects the following packages: github.com/microcosm-cc/bluemonday (Go), pybluemonday (PyPI). Ecosystems affected: Go, PyPI.

How do I fix CVE-2021-42576?

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 CVE-2021-42576 is resolved across your whole dependency graph.

How do I detect CVE-2021-42576 in my Go, PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/microcosm-cc/bluemonday. 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.

How do I mitigate CVE-2021-42576 if there is no patch (or I can't update yet)?

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 does O3 Security protect against CVE-2021-42576?

O3 pinpoints whether CVE-2021-42576 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is CVE-2021-42576 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2021-42576, including 1 in-the-wild exploitation observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-42576?

CVE-2021-42576 has an EPSS (Exploit Prediction Scoring System) score of 1.5%, placing it in the 71th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

When was CVE-2021-42576 published, and has it been updated?

CVE-2021-42576 was published on October 18, 2021 and was last updated on March 14, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹 Go🐍 PyPI

CVE-2021-42576

CRITICAL

Policies not properly enforced in bluemonday

Also known asGHSA-x95h-979x-cf3jGO-2022-0588PYSEC-2021-849

Published

Oct 18, 2021

Updated

Mar 14, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

1 known

EPSS Exploitation Probability

via FIRST.org ↗

1.5%probability of exploitation in next 30 days

Lower Risk71th percentile+1.19%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Blast Radius

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

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/microcosm-cc/bluemonday`	all versions	1.0.16
🐍PyPI	`pybluemonday`	all versions	0.0.8

Exploits & PoCs

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

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/microcosm-cc/bluemonday. 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 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 CVE-2021-42576 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 CVE-2021-42576 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 CVE-2021-42576. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

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

Is CVE-2021-42576 in your dependencies?

O3 detects CVE-2021-42576 across Go, PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

Scan my dependencies How O3 SCA works