How severe is CVE-2021-28363?

CVE-2021-28363 has a CVSS score of 6.5/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-28363?

CVE-2021-28363 affects the following packages: urllib3 (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2021-28363?

Update urllib3 to 1.26.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-28363 is resolved across your whole dependency graph.

How do I detect CVE-2021-28363 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for urllib3, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-28363 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2021-28363 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-28363 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

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

CVE-2021-28363 has an EPSS (Exploit Prediction Scoring System) score of 2.1%, placing it in the 81th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-28363?

CVE-2021-28363 is classified as CWE-295 (CWE-295). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-28363 published, and has it been updated?

CVE-2021-28363 was published on March 15, 2021 and was last updated on August 7, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

MEDIUM severity

CVE-2021-28363 — urllib3

MEDIUMFix: urllib3/urllib3@8d65ea1

CVE-2021-28363 is a medium-severity (CVSS 6.5) CWE-295 vulnerability in urllib3. A fix is available for urllib3 — see the affected versions and patch details below.

Using default SSLContext for HTTPS requests in an HTTPS proxy doesn't verify certificate hostname for proxy connection

Also known asGHSA-5phf-pp7p-vc2rPYSEC-2021-59

Published

Mar 15, 2021

Updated

Aug 7, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

2.1%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs81th percentile — riskier than 81% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-28363 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

🐍urllib3

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

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 [email protected]

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`urllib3`	≥ 1.26.0&&< 1.26.4	1.26.4`pip install --upgrade 'urllib3==1.26.4'`

Affected Products

3 products · 3 configurations

fedorafedoraproject

1 version

Application

peoplesoft enterprise peopletoolsoracle

1 version

8.59

Application

urllib3python

≥ 1.26.0 && < 1.26.4

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for urllib3, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update urllib3 to 1.26.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-28363 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

* Red Hat OpenShift Container Platform (OCP) 4 delivers the python-urllib3 package which includes a vulnerable version of urllib3 module, however from OCP 4.6 the python-urllib3 package is no longer shipped. OCP 4.5 is out of support scope for Moderate and Low impact vulnerabilities, hence is marked Out Of Support…

Product	Fixed in	Advisory
Red Hat OpenShift Container Platform 3.11	`python-urllib3-1:1.24.3-2.el7`	RHBA-2021:1147

Frequently Asked Questions

### 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](https://github.com/urllib3/urllib3/releases/tag/1.26.4). 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 defa