# CVE-2021-28363: Using default SSLContext for HTTPS requests in an HTTPS proxy doesn't verify certificate hostname for proxy connection

Source: https://junglewise.ai/threats/cve-2021-28363-using-default-sslcontext-for-https-requests-in-an-https-proxy  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2021-03-15  
License: CC BY 4.0 with a link back

CVE-2021-28363 · Severity: low · CVSS 3.1 · Published 2021-03-15

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.

Technologies: [urllib3 (PyPI)](https://junglewise.ai/threats/technologies/urllib3). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2021-28363 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2021-28363 being exploited in the wild.

**How severe is CVE-2021-28363?** Low severity, with a CVSS score of 3.1, published 15 March 2021.

**Which products and versions are affected?** urllib3 (PyPI) ECOSYSTEM:introduced=1.26.0,fixed=1.26.4, GIT:introduced=0,fixed=8d65ea1ecf6e2cdc27d42124e587c1b83a3118b0|ECOSYSTEM:introduced=1.26.0,fixed=1.26.4.

**Is there a fix for CVE-2021-28363?** Yes. urllib3 (PyPI): upgrade to 1.26.4 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**Am I affected by CVE-2021-28363?** Check whether you run urllib3 (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- urllib3 (PyPI): upgrade to 1.26.4 or later

  ```sh
  pip install "urllib3>=1.26.4"
  ```


## Affected products

- urllib3 (PyPI) ECOSYSTEM:introduced=1.26.0,fixed=1.26.4, GIT:introduced=0,fixed=8d65ea1ecf6e2cdc27d42124e587c1b83a3118b0|ECOSYSTEM:introduced=1.26.0,fixed=1.26.4

## Identifiers

- CVE-2021-28363
- GHSA-5phf-pp7p-vc2r
- PYSEC-2021-59

## Related threats

- [urllib3 HTTP client unbounded memory allocation in chunked transfer](https://junglewise.ai/threats/cve-2026-97689-urllib3-is-an-http-client-library-for-python-from-1-10-3-until-2)
- [urllib3 infinite loop in Deflate decoder](https://junglewise.ai/threats/cve-2026-97688-urllib3-is-an-http-client-library-for-python-from-2-6-2-until-2-8)
- [urllib3 HTTPS proxy TLS settings isolation](https://junglewise.ai/threats/cve-2026-97687-urllib3-is-an-http-client-library-for-python-from-1-26-0-until-2)
- [PYSEC-2026-1997 - urllib3 does not control redirects in browsers and Node.js](https://junglewise.ai/threats/cve-2025-50182-urllib3-does-not-control-redirects-in-browsers-and-node-js)
- [PYSEC-2026-1999 - urllib3 redirects are not disabled when retries are disabled on PoolManager instantiation](https://junglewise.ai/threats/cve-2025-50181-urllib3-redirects-are-not-disabled-when-retries-are-disabled-on)