Improper validation of certificate with host mismatch in urllib3 - CVE-2021-28363

Detailed vulnerability description

The vulnerability allows a remote attacker to perform MitM attack.

The vulnerability exists due to urllib3 library 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. A remote attacker can supply a valid SSL certificate for a different hostname and perform Man-in-the-Middle (MitM) attack.

How to mitigate CVE-2021-28363

Sources

• https://github.com/urllib3/urllib3/commit/8d65ea1ecf6e2cdc27d42124e587c1b83a3118b0
• https://github.com/urllib3/urllib3/commits/main
• https://github.com/urllib3/urllib3/security/advisories/GHSA-5phf-pp7p-vc2r
• https://pypi.org/project/urllib3/1.26.4/