No exploitation reportedFix available

CVE-2021-28363

Q: Is CVE-2021-28363 actively exploited?

Active exploitation of CVE-2021-28363 has not been confirmed. Its EPSS score was 2.2% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

Q: What is the CVSS score for CVE-2021-28363?

CVE-2021-28363 has a CVSS v3 base score of 6.5 (MEDIUM severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N.

No exploitation reported by the sources we read.

Published2021-03-15
WeaknessCWE-295

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

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.

Timeline

dates in UTC

2021-03-1518:15CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 21:37 UTC

Indicators tagged with this CVE, per day0 indicators over 7 days

Block these indicators

Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2021-28363?format=txt
CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-28363?format=csv
TAXII collection of exploited CVEs
The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD · OSV

Fedoraproject fedora

Branch	Affected	Fixed in	Source
34.x	= 34	no fix published	NVD

Oracle peoplesoft enterprise peopletools

Branch	Affected	Fixed in	Source
8.59	= 8.59	no fix published	NVD

Python urllib3

Branch	Affected	Fixed in	Source
1.26	≥ 1.26.0, < 1.26.4	1.26.4	NVD · OSV

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Fix

Advisories and patches

Detection

Nuclei template: none so far
Exploit-DB: no entry
Proof-of-concept repositories: none listed

Sources

NVDCVSS 3.16.5read 2026-10-23
NVDCVSS 2.06.4read 2026-10-23
NVDweakness (CWE)CWE-295read 2026-10-23
FIRST EPSSEPSS score2.2 %2026-10-22
OSVaffected packages—

Technical details

CVSS v3 Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
Published: 2021-03-15
Last Modified: 2026-10-14
NVD status: Modified

Frequently asked questions

What is CVE-2021-28363?

Is CVE-2021-28363 actively exploited?

Active exploitation of CVE-2021-28363 has not been confirmed. Its EPSS score was 2.2% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-28363?

CVE-2021-28363 has a CVSS v3 base score of 6.5 (MEDIUM severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N.

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