How severe is CVE-2020-6816?

CVE-2020-6816 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-2020-6816?

CVE-2020-6816 affects the following packages: bleach (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2020-6816?

Update bleach to 3.1.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-6816 is resolved across your whole dependency graph.

How do I detect CVE-2020-6816 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-2020-6816 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-2020-6816 actively exploited in the wild?

Yes. There are 2 known exploit references for CVE-2020-6816, including 2 in-the-wild exploitations 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-2020-6816?

CVE-2020-6816 has an EPSS (Exploit Prediction Scoring System) score of 1.3%, 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.

What type of vulnerability is CVE-2020-6816?

CVE-2020-6816 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-2020-6816 published, and has it been updated?

CVE-2020-6816 was published on March 24, 2020 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-2020-6816 — bleach

MEDIUM

CVE-2020-6816 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in bleach. 2 public exploit references exist, so weaponization risk is real. A fix is available for bleach — see the affected versions and patch details below.

Bleach vulnerable to mutation XSS via whitelisted math or svg and raw tag

Also known asGHSA-m6xf-fq7q-8743PYSEC-2020-28

Published

Mar 24, 2020

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

2 known

Exploitation data as of Oct 28, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.3%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs71th percentile — riskier than 71% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2020-6816 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 397,508 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

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:

the svg or math in 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.clean calls to use strip=True, or not whitelist math or svg tags and one or more of the following tags:

script
noscript
style
noframes
xmp
noembed
iframe

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

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	3.1.2`pip install --upgrade 'bleach==3.1.2'`

Affected Products

2 products · 2 configurations

fedorafedoraproject

1 version

Application

bleachmozilla

< 3.1.2

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.

In Mozilla Bleach before 3.12, a mutation XSS in bleach.clean when RCDAT…

checkmarx.comNVDMar 2021

In Mozilla Bleach before 3.12, a mutation XSS in bleach.clean when RCDAT…

advisory.checkmarx.netNVDOct 2022

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 3.1.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-6816 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.

UbuntuMEDIUM

USN-8077-1Ubuntu

Frequently Asked Questions

### Impact A [mutation XSS](https://cure53.de/fp170.pdf) affects users calling `bleach.clean` with all of: * the `svg` or `math` in 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.clean` calls to use `strip=True`, or not whitelist `math` or `svg` tags and one or more of the following tags: ``` script noscript style noframes xmp noembed iframe ``` * A strong [Content-Security-Policy](https://developer.mozi