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Vulnerability record · CVE-2021-33503 · published 29 June 2021

CVE-2021-33503: Python urllib3 uncontrolled resource consumption vulnerability

Python · Urllib3

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 were passed as a parameter or redirected to via an HTTP redirect.

7.5 CVSS 3.1 High EPSS 3.3% · top 12.0% CWE-400 · Uncontrolled resource consumption
7.5CVSS 3.1 base score, v2 5.0
3.3%EPSS exploitation probability, 30 days
NoNot in CISA KEV
5Affected product versions listed by NVD
12References
17 Jun 2026Last modified by NVD

Description

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 were passed as a parameter or redirected to via an HTTP redirect.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected products

5 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://github.com/advisories/GHSA-q2q7-5pp4-w6pg Third Party Advisory
https://github.com/urllib3/urllib3/commit/2d4a3fee6de2fa45eb82169361918f759269b4ec PatchThird Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/6SCV7ZNAHS3E6PBFLJGENCD
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/FMUGWEAUYGGHTPPXT6YBD53
https://security.gentoo.org/glsa/202107-36 Third Party Advisory
https://www.oracle.com/security-alerts/cpuoct2021.html PatchThird Party Advisory
https://github.com/advisories/GHSA-q2q7-5pp4-w6pg Third Party Advisory
https://github.com/urllib3/urllib3/commit/2d4a3fee6de2fa45eb82169361918f759269b4ec PatchThird Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/6SCV7ZNAHS3E6PBFLJGENCD
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/FMUGWEAUYGGHTPPXT6YBD53
https://security.gentoo.org/glsa/202107-36 Third Party Advisory
https://www.oracle.com/security-alerts/cpuoct2021.html PatchThird Party Advisory

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Source: NIST National Vulnerability Database (record CVE-2021-33503), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.