---
id: CVE-2021-28363
aliases:
  - GHSA-5phf-pp7p-vc2r
  - PYSEC-2021-59
title: >-
  Using default SSLContext for HTTPS requests in an HTTPS proxy doesn't verify
  certificate hostname for proxy connection
summary: >-
  Using default SSLContext for HTTPS requests in an HTTPS proxy doesn't verify
  certificate hostname for proxy connection
severity: medium
cvss: 6.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N'
vendor: urllib3
product: urllib3
ecosystem: pip
affected:
  - 'urllib3 >= 1.26.0, < 1.26.4'
patched:
  - urllib3 1.26.4
published: '2021-03-19'
updated: '2026-07-08'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-5phf-pp7p-vc2r'
references:
  - url: 'https://github.com/urllib3/urllib3/security/advisories/GHSA-5phf-pp7p-vc2r'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2021-28363'
  - url: >-
      https://github.com/urllib3/urllib3/commit/8d65ea1ecf6e2cdc27d42124e587c1b83a3118b0
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/urllib3/PYSEC-2021-59.yaml
  - url: >-
      https://github.com/pypa/advisory-db/tree/main/vulns/urllib3/PYSEC-2021-59.yaml
  - url: 'https://github.com/urllib3/urllib3'
  - url: 'https://github.com/urllib3/urllib3/blob/main/CHANGES.rst#1264-2021-03-15'
  - url: 'https://github.com/urllib3/urllib3/commits/main'
  - url: 'https://github.com/urllib3/urllib3/releases/tag/1.26.4'
  - url: >-
      https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/4S65ZQVZ2ODGB52IC7VJDBUK4M5INCXL
  - url: >-
      https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/4S65ZQVZ2ODGB52IC7VJDBUK4M5INCXL
  - url: 'https://pypi.org/project/urllib3/1.26.4'
  - url: 'https://security.gentoo.org/glsa/202107-36'
  - url: 'https://security.gentoo.org/glsa/202305-02'
  - url: 'https://security.netapp.com/advisory/ntap-20240621-0007'
  - url: 'https://www.oracle.com/security-alerts/cpuoct2021.html'
tags:
  - osv
  - pip
epss: 0.02109
epssPercentile: 0.81003
ingestedAt: '2026-07-08T18:25:46.043Z'
---

## Overview

### 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 default `SSLContext`

### For more information
If you have any questions or comments about this advisory:
* Email us at [sethmichaellarson@gmail.com](mailto:sethmichaellarson@gmail.com)

## Affected packages

- `urllib3 >= 1.26.0, < 1.26.4`

## Remediation

Upgrade to a patched release:

- `urllib3 1.26.4`