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CVE-2021-33503: Catastrophic backtracking in URL authority parser when passed URL containing many @ characters

June 1, 2021 (updated June 9, 2026)

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.

References

  • github.com/advisories/GHSA-q2q7-5pp4-w6pg
  • github.com/pypa/advisory-database/tree/main/vulns/urllib3/PYSEC-2021-108.yaml
  • github.com/urllib3/urllib3/commit/2d4a3fee6de2fa45eb82169361918f759269b4ec
  • github.com/urllib3/urllib3/commit/5b047b645f5f93900d5e2fc31230848c25eb1f5f
  • github.com/urllib3/urllib3/security/advisories/GHSA-q2q7-5pp4-w6pg
  • lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/6SCV7ZNAHS3E6PBFLJGENCDRDRWRZZ6W
  • lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/FMUGWEAUYGGHTPPXT6YBD53WYXQGVV73
  • nvd.nist.gov/vuln/detail/CVE-2021-33503
  • security.gentoo.org/glsa/202107-36
  • www.oracle.com/security-alerts/cpuoct2021.html

Code Behaviors & Features

Detect and mitigate CVE-2021-33503 with GitLab Dependency Scanning

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Affected versions

All versions starting from 1.25.4 before 1.26.5

Fixed versions

  • 1.26.5

Solution

Upgrade to version 1.26.5 or above.

Impact 7.5 HIGH

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

Learn more about CVSS

Weakness

  • CWE-400: Uncontrolled Resource Consumption

Source file

pypi/urllib3/CVE-2021-33503.yml

Spotted a mistake? Edit the file on GitLab.

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