CVE-2020-6816

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

• 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]

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

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2020-6816 has a CVSS score of 6.1 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.1.2); upgrading removes the vulnerable code path.

Kodem intelligence

Users are encouraged to upgrade to bleach v3.1.2 or greater.

1. What is CVE-2020-6816? CVE-2020-6816 is a medium-severity cross-site scripting (XSS) vulnerability in bleach (pip), affecting versions < 3.1.2. It is fixed in 3.1.2. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2020-6816? CVE-2020-6816 has a CVSS score of 6.1 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of bleach are affected by CVE-2020-6816? bleach (pip) versions < 3.1.2 is affected.
4. Is there a fix for CVE-2020-6816? Yes. CVE-2020-6816 is fixed in 3.1.2. Upgrade to this version or later.
5. Is CVE-2020-6816 exploitable, and should I be worried? Whether CVE-2020-6816 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2020-6816 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2020-6816? Upgrade bleach to 3.1.2 or later.

Other vulnerabilities in bleach