{"id":"CVE-2021-33503","aliases":["GHSA-q2q7-5pp4-w6pg","PYSEC-2021-108"],"title":"Catastrophic backtracking in URL authority parser when passed URL containing many @ characters","summary":"Catastrophic backtracking in URL authority parser when passed URL containing many @ characters","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":"2021-06-01","updated":"2026-07-08","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/GHSA-q2q7-5pp4-w6pg","references":[{"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/2d4a3fee6de2fa45eb82169361918f759269b4ec"},{"url":"https://github.com/urllib3/urllib3/commit/5b047b645f5f93900d5e2fc31230848c25eb1f5f#diff-52026d639119bf1e0364836b4e8a18bd9ed3c95c6ba39b26534a5057a65e35bbR65"},{"url":"https://github.com/advisories/GHSA-q2q7-5pp4-w6pg"},{"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":0.87871,"ingestedAt":"2026-07-08T18:25:52.064Z","slug":"CVE-2021-33503","body":"## Overview\n\n### Impact\n\nWhen 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\n### Patches\n\nThe issue has been fixed in urllib3 v1.26.5.\n\n### References\n\n- [CVE-2021-33503](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-33503)\n- [JVNVU#92413403 (English)](https://jvn.jp/en/vu/JVNVU92413403/)\n- [JVNVU#92413403 (Japanese)](https://jvn.jp/vu/JVNVU92413403/)\n- [urllib3 v1.26.5](https://github.com/urllib3/urllib3/releases/tag/1.26.5)\n\n### For more information\nIf you have any questions or comments about this advisory:\n* Ask in our [community Discord](https://discord.gg/urllib3)\n* Email [sethmichaellarson@gmail.com](mailto:sethmichaellarson@gmail.com)\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":[]}