CVE-2021-28363 is a medium-severity (CVSS 6.5) CWE-295 vulnerability in urllib3. O3 Security confirms whether CVE-2021-28363 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Using default SSLContext for HTTPS requests in an HTTPS proxy doesn't verify certificate hostname for proxy connection
Real-World Exposure
urllib3Real-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
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | urllib3 | ≥ 1.26.0&&< 1.26.4 | 1.26.4 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for urllib3. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
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.
How O3 protects you
O3 pinpoints whether CVE-2021-28363 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2021-28363. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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 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…
Frequently Asked Questions
Is CVE-2021-28363 in your dependencies?
O3 detects CVE-2021-28363 across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.