{"id":"CVE-2020-6816","aliases":["GHSA-m6xf-fq7q-8743","PYSEC-2020-28"],"url":"https://o3.security/vulnerability/CVE-2020-6816","summary":"Bleach vulnerable to mutation XSS via whitelisted math or svg and raw tag","details":"### Impact\n\nA [mutation XSS](https://cure53.de/fp170.pdf) affects users calling `bleach.clean` with all of:\n\n* the `svg` or `math` in the allowed/whitelisted tags\n* an RCDATA tag (see below) in the allowed/whitelisted tags\n* the keyword argument `strip=False`\n\n### Patches\n\nUsers are encouraged to upgrade to bleach v3.1.2 or greater.\n\n### Workarounds\n\n* modify `bleach.clean` calls to use `strip=True`, or not whitelist `math` or `svg` tags and one or more of the following tags:\n\n```\nscript\nnoscript\nstyle\nnoframes\nxmp\nnoembed\niframe\n```\n\n* A strong [Content-Security-Policy](https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP) without `unsafe-inline` and `unsafe-eval` [`script-src`s](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src)) will also help mitigate the risk.\n\n### References\n\n* https://bugzilla.mozilla.org/show_bug.cgi?id=1621692\n* https://cure53.de/fp170.pdf\n* https://nvd.nist.gov/vuln/detail/CVE-2020-6816\n* https://www.checkmarx.com/blog/vulnerabilities-discovered-in-mozilla-bleach\n\n### Credits\n\n* Reported by [Yaniv Nizry](https://twitter.com/ynizry) from the CxSCA AppSec group at Checkmarx\n\n### For more information\n\nIf you have any questions or comments about this advisory:\n\n* Open an issue at [https://github.com/mozilla/bleach/issues](https://github.com/mozilla/bleach/issues)\n* Email us at [security@mozilla.org](mailto:security@mozilla.org)","published":"2020-03-24T22:15:12.657Z","modified":"2026-10-20T10:53:29.593166215Z","cvss":{"score":6.1,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":2,"affectedPackages":[{"ecosystem":"PyPI","name":"bleach","fixedVersion":"3.1.2"}],"fix":null,"references":[{"type":"WEB","url":"https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/EDQU2SZLZMSSACCBUBJ6NOSRNNBDYFW5/"},{"type":"ADVISORY","url":"https://github.com/mozilla/bleach/security/advisories/GHSA-m6xf-fq7q-8743"},{"type":"EVIDENCE","url":"https://advisory.checkmarx.net/advisory/CX-2020-4277"},{"type":"EVIDENCE","url":"https://www.checkmarx.com/blog/vulnerabilities-discovered-in-mozilla-bleach"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2020-6816"},{"type":"PACKAGE","url":"https://github.com/mozilla/bleach"},{"type":"WEB","url":"https://github.com/mozilla/bleach/releases/tag/v3.1.2"},{"type":"WEB","url":"https://github.com/pypa/advisory-database/tree/main/vulns/bleach/PYSEC-2020-28.yaml"},{"type":"WEB","url":"https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/EDQU2SZLZMSSACCBUBJ6NOSRNNBDYFW5"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-20T10:53:29.593166215Z"}}