CVE & CISA-KEV Catalog

CVE-2021-28363

MEDIUMEPSS 81th pctl
6.5
CVSS v3
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.

CWE-295Improper Certificate Validation

How to fix

Remediation Available
python-pip-wheelAmazon
Fixed in:0:20.2.2-1.amzn2.0.3
python2-pipAmazon
Fixed in:0:20.2.2-1.amzn2.0.3
python3-pipAmazon
Fixed in:0:20.2.2-1.amzn2.0.3
python-urllib3Debian
Fixed in:1.26.4-1CVE-2021-28363
Fixed in:1.26.4-1CVE-2021-28363
Fixed in:1.26.4-1CVE-2021-28363
urllib3PyPI (Python)
Fixed in:1.26.4GHSA-5phf-pp7p-vc2r

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Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork
Attack ComplexityLow
Privileges RequiredNone
User InteractionNone
ScopeUnchanged

Impact

ConfidentialityLow
IntegrityLow
AvailabilityNone

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

Exploit Intelligence

2.11%probability of exploitation in 30 days
81stpercentile

Elevated risk: more likely to be exploited than 81% of all known CVEs.

References

Mitigation1
Third-Party Advisory2

Related Vulnerabilities

Other CWE-295 (Improper Certificate Validation) vulnerabilities, ordered by exploit likelihood. View all

CVESeverityCVSSEPSSExploitedFix
CVE-2015-4000Low3.7100%-Fix
CVE-2020-0601High8.189%KEVFix
CVE-2009-3555Critical9.887%-Fix
CVE-2022-26923High8.884%KEVFix
CVE-2023-27823Critical9.854%--
CVE-2022-42979High8.824%--

Common questions

How do I fix CVE-2021-28363?

Published advisories record a fix for 5 affected products. The "How to fix" section on this page lists the fixed version and source advisory for each one, so apply the entry matching what you actually run.

Is CVE-2021-28363 being actively exploited?

Not that we know of. CVE-2021-28363 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 2.1% is the estimated probability that it will be exploited in the next 30 days. That is higher than 81% of all scored CVEs.

How severe is CVE-2021-28363?

CVE-2021-28363 has a CVSS v3 base score of 6.5, rated medium. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.

What does CVE-2021-28363 affect?

Published advisories record a fix for python-pip-wheel (Amazon), python2-pip (Amazon), python3-pip (Amazon), python-urllib3 (Debian), and 1 more. Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-06-17.