Reference page — cumulative record through Monday, October 19, 2026 UTC. Reference pages update as the archive grows; only dated daily editions are immutable pages of record.

CVE-2021-28363

CVE-2021-28363MEDIUM

n/a n/a — The urllib3 library 1.26.x before 1.26.4 for Python omits SSL certificate validation in some cases involving HTTPS to HTTPS proxies.

  AV  AC  PR  UI  S  C  I  A   CVSS    EPSS   %ile   KEV
   N   L   N   N  U  L  L  N    6.5   .0217   81.9     —

AFFECTED
  Product  Versions  Fixed
  n/a      n/a –     —

TIMELINE
  Mar 13  Reserved by mitre
  Mar 15  Published (CNA: mitre)
  Oct 14  ENRICHED — CVE-2021-28363. Received CVSS 6.5 and CPE data from NVD.

CWE-295 · CNA: mitre · CVSS v3.1 · 10 references · NVD status: Modified

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.

Lifecycle

Complete event history — 3 events, chronological
Date	Event	Detail
March 13, 2021	Reserved	Reserved by mitre
March 15, 2021	Published	Published (CNA: mitre)
October 14, 2026	ENRICHED	ENRICHED — CVE-2021-28363. Received CVSS 6.5 and CPE data from NVD.

Affected

Affected products and packages — 1 row
Vendor	Product / Package	Ecosystem	Version introduced	Fixed
n/a	n/a	—	n/a	—

Weaknesses

CWE-295

References (10)

https://cert-portal.siemens.com/productcert/html/ssa-398330.html
https://github.com/urllib3/urllib3/commit/8d65ea1ecf6e2cdc27d42124e587c1b83a3118b0 [Patch, Third Party Advisory]
https://github.com/urllib3/urllib3/commits/main [Patch, Third Party Advisory]
https://github.com/urllib3/urllib3/security/advisories/GHSA-5phf-pp7p-vc2r [Mitigation, Third Party Advisory]
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/4S65ZQVZ2ODGB52IC7VJDBUK4M5INCXL/
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 [Patch, Third Party Advisory]

Authoritative record: CVE-2021-28363 at cve.org

Weaknesses: CWE-295

About this page

This is a reference page, not a dated page of record. It assembles the complete lifecycle of CVE-2021-28363 from the CVE Program record, NVD enrichment, the CISA KEV catalog, EPSS, and OSV advisories. The box score's numbers (CVSS, EPSS, KEV status) are current as of Monday, October 19, 2026 UTC and are re-derived as the archive grows; only dated daily editions are immutable pages of record. The authoritative source for this identifier is cve.org.