{"id":"CVE-2021-23980","aliases":["GHSA-vv2x-vrpj-qqpq","PYSEC-2021-865"],"url":"https://o3.security/vulnerability/CVE-2021-23980","summary":"Cross-site scripting in Bleach","details":"### Impact                                                                                                                                                                                                                                                    \n\nA [mutation XSS](https://cure53.de/fp170.pdf) affects users calling `bleach.clean` with all of:\n\n* `svg` or `math` in the allowed tags\n* `p` or `br` in allowed tags\n* `style`, `title`, `noscript`, `script`, `textarea`, `noframes`, `iframe`, or `xmp` in allowed tags\n* the keyword argument `strip_comments=False`\n\nNote: none of the above tags are in the default allowed tags and `strip_comments` defaults to `True`.\n\n### Patches\n\nUsers are encouraged to upgrade to bleach v3.3.0 or greater.\n\nNote: bleach v3.3.0 introduces a breaking change to escape HTML comments by default.\n\n### Workarounds\n\n* modify `bleach.clean` calls to at least one of:\n  * not allow the `style`, `title`, `noscript`, `script`, `textarea`, `noframes`, `iframe`, or `xmp` tag\n  * not allow `svg` or `math` tags\n  * not allow `p` or `br` tags\n  * set `strip_comments=True`\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=1689399\n* https://advisory.checkmarx.net/advisory/CX-2021-4303\n* https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-23980\n* https://cure53.de/fp170.pdf\n\n### Credits\n\n* Reported by [Yaniv Nizry](https://twitter.com/ynizry) from the CxSCA AppSec group at Checkmarx\n* Additional eject tags not mentioned in the original advisory and the CSP mitigation line being truncated in the revised advisory reported by [Michał Bentkowski](https://twitter.com/SecurityMB) at Securitum\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":"2023-02-16T22:15:10.713Z","modified":"2026-10-20T10:54:32.586930689Z","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":{"score":0.00479,"percentile":0.39481,"asOf":"2026-08-23"},"cisaKev":null,"exploitsKnown":1,"affectedPackages":[{"ecosystem":"PyPI","name":"bleach","fixedVersion":"3.3.0"}],"fix":{"url":"https://github.com/mozilla/bleach/commit/79b7a3c5e56a09d1d323a5006afa59b56162eb13","label":"mozilla/bleach@79b7a3c"},"references":[{"type":"ADVISORY","url":"https://github.com/mozilla/bleach/security/advisories/GHSA-vv2x-vrpj-qqpq"},{"type":"REPORT","url":"https://bugzilla.mozilla.org/show_bug.cgi?id=CVE-2021-23980"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-23980"},{"type":"WEB","url":"https://github.com/mozilla/bleach/commit/79b7a3c5e56a09d1d323a5006afa59b56162eb13"},{"type":"WEB","url":"https://advisory.checkmarx.net/advisory/CX-2021-4303"},{"type":"WEB","url":"https://bugzilla.mozilla.org/show_bug.cgi?id=1689399"},{"type":"WEB","url":"https://cure53.de/fp170.pdf"},{"type":"PACKAGE","url":"https://github.com/mozilla/bleach"},{"type":"WEB","url":"https://github.com/mozilla/bleach/blob/79b7a3c5e56a09d1d323a5006afa59b56162eb13/CHANGES#L4"},{"type":"WEB","url":"https://github.com/pypa/advisory-database/tree/main/vulns/bleach/PYSEC-2021-865.yaml"},{"type":"WEB","url":"https://pypi.org/project/bleach"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-20T10:54:32.586930689Z"}}