CVE-2020-11053: 
Homebrew vulnerability analysis and mitigation

In OAuth2 Proxy versions before 5.1.1, there exists an open redirect vulnerability. The software allows users to provide a redirect address for the proxy to send authenticated users to at the end of the authentication flow. While this redirect URL is meant to be validated to prevent redirects to malicious sites, the validation could be bypassed using HTML-encoded whitespace characters, potentially allowing redirects to any arbitrary URL (GitHub Advisory, NVD).

The vulnerability exists in the URL validation mechanism of OAuth2 Proxy. The validation check was intended to prevent malicious redirects, but it could be circumvented by crafting a redirect URL containing HTML-encoded whitespace characters (such as %0a, %0b, %09, %0d). The vulnerability has been assigned a CVSS v3.1 base score of 6.1 MEDIUM by NVD and 7.1 HIGH by GitHub, with the vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N (NVD).

If exploited, this vulnerability could allow attackers to redirect users to arbitrary websites, potentially leading to phishing attacks. The redirect would occur after the authentication flow, making it more likely for users to trust the malicious destination since they would have just completed a legitimate authentication process (GitHub Advisory).

The vulnerability is exploitable by crafting a special redirect URL that includes HTML-encoded whitespace characters. The attack requires user interaction as it occurs during the authentication flow, but does not require authentication or special privileges to execute (NVD).

The vulnerability has been patched in OAuth2 Proxy version 5.1.1. The fix involves implementing additional validation checks using regular expressions to detect and prevent redirects containing encoded whitespace characters. There are no workarounds available for unpatched versions (GitHub Advisory).