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CVE-2020-6816: mutation XSS via whitelisted math or svg and RCDATA tag with strip=False

In Mozilla Bleach before 3.12, a mutation XSS in bleach.clean when RCDATA and either svg or math tags are whitelisted and the keyword argument strip=False.

CVE
#xss#vulnerability#git#pdf

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

  • 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]

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