{"id":"CVE-2020-6817","aliases":["GHSA-vqhp-cxgc-6wmm","PYSEC-2020-340","SNYK-PYTHON-BLEACH-561754"],"title":"regular expression denial-of-service (ReDoS) in Bleach","summary":"regular expression denial-of-service (ReDoS) in Bleach","severity":"high","cvss":7.5,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","vendor":"bleach","product":"bleach","ecosystem":"pip","affected":["bleach < 3.1.4"],"patched":["bleach 3.1.4"],"published":"2020-03-30","updated":"2026-09-10","sourceUpdated":"2026-09-10T03:48:29.583864458Z","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/GHSA-vqhp-cxgc-6wmm","references":[{"url":"https://github.com/mozilla/bleach/security/advisories/GHSA-vqhp-cxgc-6wmm"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2020-6817"},{"url":"https://bugzilla.mozilla.org/show_bug.cgi?id=1623633"},{"url":"https://github.com/mozilla/bleach"},{"url":"https://github.com/mozilla/bleach/releases/tag/v3.1.4"},{"url":"https://github.com/pypa/advisory-database/tree/main/vulns/bleach/PYSEC-2020-340.yaml"},{"url":"https://snyk.io/vuln/SNYK-PYTHON-BLEACH-561754"}],"tags":["osv","pip"],"epss":0.00718,"epssPercentile":0.52411,"ingestedAt":"2026-09-12T03:13:01.731Z","slug":"CVE-2020-6817","body":"## Overview\n\n### Impact\n\n`bleach.clean` behavior parsing style attributes could result in a regular expression denial of service (ReDoS).\n\nCalls to ``bleach.clean`` with an allowed tag with an allowed ``style`` attribute are vulnerable to ReDoS. For example, ``bleach.clean(..., attributes={'a': ['style']})``.\n\n### Patches\n\n3.1.4\n\n### Workarounds\n\n* do not whitelist the style attribute in `bleach.clean` calls\n\n* limit input string length\n\n### References\n\n* https://bugzilla.mozilla.org/show_bug.cgi?id=1623633\n* https://www.regular-expressions.info/redos.html\n* https://blog.r2c.dev/posts/finding-python-redos-bugs-at-scale-using-dlint-and-r2c/\n* https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-6817\n\n### Credits\n\n* Reported by schwag09 of r2c\n\n### For more information\nIf you have any questions or comments about this advisory:\n\n* Open an issue at https://github.com/mozilla/bleach/issues\n* Email us at security@mozilla.org\n\n## Affected packages\n\n- `bleach < 3.1.4`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `bleach 3.1.4`","depth":"twilight","depthScore":41,"depthScoreParts":{"impact":41.3,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}