Catastrophic backtracking in URL authority parser when passed URL containing many @ characters
Impact
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.
Patches
The issue has been fixed in urllib3 v1.26.5.
References
For more information
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Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | urllib3 | >= 1.25.4, < 1.26.5 | 1.26.5 |
References
- https://github.com/advisories/GHSA-q2q7-5pp4-w6pg
- https://github.com/urllib3/urllib3/commit/2d4a3fee6de2fa45eb82169361918f759269b4ec
- https://github.com/urllib3/urllib3/security/advisories/GHSA-q2q7-5pp4-w6pg
- https://nvd.nist.gov/vuln/detail/CVE-2021-33503
- https://github.com/urllib3/urllib3/commit/5b047b645f5f93900d5e2fc31230848c25eb1f5f#diff-52026d639119bf1e0364836b4e8a18bd9ed3c95c6ba39b26534a5057a65e35bbR65
- https://github.com/pypa/advisory-database/tree/main/vulns/urllib3/PYSEC-2021-108.yaml
- https://github.com/urllib3/urllib3
- https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/6SCV7ZNAHS3E6PBFLJGENCDRDRWRZZ6W
- https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/FMUGWEAUYGGHTPPXT6YBD53WYXQGVV73
- https://security.gentoo.org/glsa/202107-36
- https://www.oracle.com/security-alerts/cpuoct2021.html
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H- Weakness (CWE)
CWE-400- Platform
- Python (PyPI)
- Package
urllib3- Disclosed
- 01.06.2021
- Updated
- 11.10.2026 13:19
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2021-108 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-q2q7-5pp4-w6pg - CC BY 4.0
Vulnerability history: urllib3
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 22
- First → latest
- 01.2017 09.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Critical 1
- High 11
- Medium 10
Disclosures per year
- 2017: 1
- 2018: 1
- 2019: 2
- 2020: 2
- 2021: 2
- 2022: 0
- 2023: 3
- 2024: 1
- 2025: 4
- 2026: 6
-
Catastrophic backtracking in URL authority parser when passed URL containing many @ characters this entry
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