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Vulnerability record · CVE-2021-28363 · published 15 March 2021

CVE-2021-28363: Python urllib3 improper certificate validation vulnerability

Python · Urllib3

The urllib3 library 1.26.x before 1.26.4 for Python omits SSL certificate validation in some cases involving HTTPS to HTTPS proxies. The initial connection to the HTTPS proxy (if an SSLContext isn't given via proxy_config) doesn't verify the hostname of the certificate. This means certificates for different servers that still validate properly with the default urllib3 SSLContext will be silently accepted.

6.5 CVSS 3.1 Medium EPSS 2.1% · top 19.0% CWE-295 · Improper certificate validation
6.5CVSS 3.1 base score, v2 6.4
2.1%EPSS exploitation probability, 30 days
NoNot in CISA KEV
3Affected product versions listed by NVD
18References
17 Jun 2026Last modified by NVD

Description

The urllib3 library 1.26.x before 1.26.4 for Python omits SSL certificate validation in some cases involving HTTPS to HTTPS proxies. The initial connection to the HTTPS proxy (if an SSLContext isn't given via proxy_config) doesn't verify the hostname of the certificate. This means certificates for different servers that still validate properly with the default urllib3 SSLContext will be silently accepted.

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

Affected products

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

References

LinkTags
https://github.com/urllib3/urllib3/commit/8d65ea1ecf6e2cdc27d42124e587c1b83a3118b0 PatchThird Party Advisory
https://github.com/urllib3/urllib3/commits/main PatchThird Party Advisory
https://github.com/urllib3/urllib3/security/advisories/GHSA-5phf-pp7p-vc2r MitigationThird Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/4S65ZQVZ2ODGB52IC7VJDBU
https://pypi.org/project/urllib3/1.26.4/ Third Party Advisory
https://security.gentoo.org/glsa/202107-36 Third Party Advisory
https://security.gentoo.org/glsa/202305-02
https://security.netapp.com/advisory/ntap-20240621-0007/
https://www.oracle.com/security-alerts/cpuoct2021.html PatchThird Party Advisory
https://github.com/urllib3/urllib3/commit/8d65ea1ecf6e2cdc27d42124e587c1b83a3118b0 PatchThird Party Advisory
https://github.com/urllib3/urllib3/commits/main PatchThird Party Advisory
https://github.com/urllib3/urllib3/security/advisories/GHSA-5phf-pp7p-vc2r MitigationThird Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/4S65ZQVZ2ODGB52IC7VJDBU
https://pypi.org/project/urllib3/1.26.4/ Third Party Advisory
https://security.gentoo.org/glsa/202107-36 Third Party Advisory
https://security.gentoo.org/glsa/202305-02
https://security.netapp.com/advisory/ntap-20240621-0007/
https://www.oracle.com/security-alerts/cpuoct2021.html PatchThird Party Advisory

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Source: NIST National Vulnerability Database (record CVE-2021-28363), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.