CVE-2021-3116: 
Python vulnerability analysis and mitigation

CVE-2021-3116 is a security vulnerability discovered in proxy.py version 2.3.0, a feature-rich HTTP proxy server written in Python. The vulnerability was discovered on January 9, 2021, and affects the basic authentication mechanism of the proxy server. The issue was disclosed privately to the developers and subsequently fixed in version 2.3.1 (Cardaci Advisory).

The vulnerability exists in the before_upstream_connection function within proxy/http/proxy/auth.py. The issue stems from a logic bug where incorrect boolean operators (and instead of or) were used in the authentication validation code. This implementation error allows bypass of the proxy authentication mechanism because the condition check uses AND operators instead of OR operators, meaning that if any one of the conditions is false, the authentication check would pass (Cardaci Advisory, GitHub Fix).

The vulnerability allows attackers to bypass the proxy authentication mechanism completely. Any valid header with any credentials would work, effectively nullifying the authentication protection. This means unauthorized users could access the proxy server regardless of the configured credentials (Cardaci Advisory).

The vulnerability is easily exploitable by sending any credentials in the Proxy-Authorization header. A proof of concept demonstrates that while requests without credentials are correctly rejected with a 407 Proxy Authentication Required error, requests with any arbitrary credentials (e.g., 'x:x') are allowed through (Cardaci Advisory).

The vulnerability was fixed in proxy.py version 2.3.1. The fix involved changing the logical operators from 'and' to 'or' in the authentication validation code. Users should upgrade to version 2.3.1 or later to address this security issue (GitHub Fix).