Using default SSLContext for HTTPS requests in an HTTPS proxy doesn't verify certificate hostname for proxy connection
Impact
Users who are using an HTTPS proxy to issue HTTPS requests and haven't configured their own SSLContext via proxy_config.
Only the default SSLContext is impacted.
Patches
urllib3 >=1.26.4 has the issue resolved. urllib3<1.26 is not impacted due to not supporting HTTPS requests via HTTPS proxies.
Workarounds
Upgrading is recommended as this is a minor release and not likely to break current usage.
Configuring an SSLContext with check_hostname=True and passing via proxy_config instead of relying on the default SSLContext
For more information
If you have any questions or comments about this advisory:
- Email us at sethmichaellarson@gmail.com
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | urllib3 | >= 1.26.0, < 1.26.4 | 1.26.4 |
References
- https://github.com/urllib3/urllib3/commits/main
- https://pypi.org/project/urllib3/1.26.4/
- https://github.com/urllib3/urllib3/commit/8d65ea1ecf6e2cdc27d42124e587c1b83a3118b0
- https://github.com/urllib3/urllib3/security/advisories/GHSA-5phf-pp7p-vc2r
- https://nvd.nist.gov/vuln/detail/CVE-2021-28363
- https://github.com/pypa/advisory-database/tree/main/vulns/urllib3/PYSEC-2021-59.yaml
- https://github.com/pypa/advisory-db/tree/main/vulns/urllib3/PYSEC-2021-59.yaml
- https://github.com/urllib3/urllib3
- https://github.com/urllib3/urllib3/blob/main/CHANGES.rst#1264-2021-03-15
- https://github.com/urllib3/urllib3/releases/tag/1.26.4
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/4S65ZQVZ2ODGB52IC7VJDBUK4M5INCXL
- https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/4S65ZQVZ2ODGB52IC7VJDBUK4M5INCXL
- https://pypi.org/project/urllib3/1.26.4
- https://security.gentoo.org/glsa/202107-36
- https://security.gentoo.org/glsa/202305-02
- https://security.netapp.com/advisory/ntap-20240621-0007
- https://www.oracle.com/security-alerts/cpuoct2021.html
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N- Weakness (CWE)
CWE-295- Platform
- Python (PyPI)
- Package
urllib3- Disclosed
- 15.03.2021
- Updated
- 11.10.2026 13:19
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2021-59 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-5phf-pp7p-vc2r - 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
-
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