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.
https://www.checkmarx.com/blog/vulnerabilities-discovered-in-mozilla-bleach
https://github.com/mozilla/bleach/security/advisories/GHSA-m6xf-fq7q-8743