How severe is CVE-2021-23980?

CVE-2021-23980 has a CVSS score of 6.1/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-23980?

CVE-2021-23980 affects the following packages: bleach (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2021-23980?

Update bleach to 3.3.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-23980 is resolved across your whole dependency graph.

How do I detect CVE-2021-23980 in my PyPI dependencies?

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

How do I mitigate CVE-2021-23980 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is CVE-2021-23980 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2021-23980, 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-2021-23980?

CVE-2021-23980 has an EPSS (Exploit Prediction Scoring System) score of 0.5%, placing it in the 42th 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.

What type of vulnerability is CVE-2021-23980?

CVE-2021-23980 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2021-23980 published, and has it been updated?

CVE-2021-23980 was published on February 16, 2023 and was last updated on October 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

MEDIUM severity

CVE-2021-23980 — bleach

MEDIUMFix: mozilla/bleach@79b7a3c

CVE-2021-23980 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in bleach. 1 public exploit or proof-of-concept reference is indexed, so weaponization risk is real. A fix is available for bleach — see the affected versions and patch details below.

Cross-site scripting in Bleach

Also known asGHSA-vv2x-vrpj-qqpqPYSEC-2021-865

Published

Feb 16, 2023

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

1 known

Exploitation data as of Oct 29, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

Proof-of-concept exploit code exists

CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.

Exploitation and automatability from CISA’s SSVC triage for CVE-2021-23980.

EPSS Exploitation Probability

via FIRST.org ↗

0.5%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs42th percentile — riskier than 42% of all scored CVEsHighest risk

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

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected

🐍bleach

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

Description

Impact

A mutation XSS affects users calling bleach.clean with all of:

svg or math in the allowed tags
p or br in allowed tags
style, title, noscript, script, textarea, noframes, iframe, or xmp in 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.clean calls to at least one of:
- not allow the style, title, noscript, script, textarea, noframes, iframe, or xmp tag
- not allow svg or math tags
- not allow p or br tags
- set strip_comments=True
A strong Content-Security-Policy without unsafe-inline and unsafe-eval script-srcs) will also help mitigate the risk.

References

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 [email protected]

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`bleach`	all versions	3.3.0`pip install --upgrade 'bleach==3.3.0'`

Affected Products

1 product · 1 configurations

Application

bleachmozilla

< 3.3.0

range

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.

A mutation XSS affects users calling bleach.clean with all of: svg or ma…

bugzilla.mozilla.orgNVDFeb 2023

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for bleach, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update bleach to 3.3.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-23980 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

Product	Fixed in	Advisory
Red Hat Automation Hub 4.2 for RHEL 7	`automation-hub-0:4.2.2-1.el7pc`	RHSA-2021:0781

UbuntuMEDIUM

USN-8077-1Ubuntu

Frequently Asked Questions

### Impact A [mutation XSS](https://cure53.de/fp170.pdf) affects users calling `bleach.clean` with all of: * `svg` or `math` in the allowed tags * `p` or `br` in allowed tags * `style`, `title`, `noscript`, `script`, `textarea`, `noframes`, `iframe`, or `xmp` in allowed tags * the keyword argument `strip_comments=False` Note: none of the above tags ar