{"id":"CVE-2021-33503","aliases":["PYSEC-2021-108","GHSA-q2q7-5pp4-w6pg"],"title":"An issue was discovered in urllib3 before 1.26.5. When provided with a URL containing many @ characters in the authority component, the a…","summary":"An issue was discovered in urllib3 before 1.26.5. When provided with a URL containing many @ characters in the authority component, the authority regular expression exhibits catastrophic backtracking, causing a denial of service if a URL…","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":"urllib3","product":"urllib3","ecosystem":"pip","affected":["urllib3 >= 1.25.4, < 1.26.5"],"patched":["urllib3 1.26.5"],"published":"2021-06-29","updated":"2026-10-13","sourceUpdated":"2026-10-13T09:30:02.842062535Z","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/PYSEC-2021-108","references":[{"url":"https://github.com/advisories/GHSA-q2q7-5pp4-w6pg"},{"url":"https://github.com/urllib3/urllib3/commit/2d4a3fee6de2fa45eb82169361918f759269b4ec"},{"url":"https://github.com/urllib3/urllib3/security/advisories/GHSA-q2q7-5pp4-w6pg"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2021-33503"},{"url":"https://github.com/urllib3/urllib3/commit/5b047b645f5f93900d5e2fc31230848c25eb1f5f#diff-52026d639119bf1e0364836b4e8a18bd9ed3c95c6ba39b26534a5057a65e35bbR65"},{"url":"https://github.com/pypa/advisory-database/tree/main/vulns/urllib3/PYSEC-2021-108.yaml"},{"url":"https://github.com/urllib3/urllib3"},{"url":"https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/6SCV7ZNAHS3E6PBFLJGENCDRDRWRZZ6W"},{"url":"https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/FMUGWEAUYGGHTPPXT6YBD53WYXQGVV73"},{"url":"https://security.gentoo.org/glsa/202107-36"},{"url":"https://www.oracle.com/security-alerts/cpuoct2021.html"}],"tags":["osv","pip"],"epss":0.03273,"epssPercentile":0.8835,"cvssSource":"osv","ingestedAt":"2026-07-08T18:25:52.064Z","slug":"CVE-2021-33503","body":"## Overview\n\nAn issue was discovered in urllib3 before 1.26.5. When provided with a URL containing many @ characters in the authority component, the authority regular expression exhibits catastrophic backtracking, causing a denial of service if a URL were passed as a parameter or redirected to via an HTTP redirect.\n\n## Affected packages\n\n- `urllib3 >= 1.25.4, < 1.26.5`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `urllib3 1.26.5`","depth":"twilight","depthScore":42,"depthScoreParts":{"impact":41.3,"likelihood":0.7,"exploitation":0,"ransomware":0},"changes":[]}