The Bitly oauth2_proxy in version 2.1 and earlier was affected by an open redirect vulnerability during the start and termination of the 2-legged OAuth flow. This issue was caused by improper input validation and a violation of RFC-6819
https://github.com/advisories/GHSA-jm34-xm8m-w958
https://tools.ietf.org/html/rfc6819#section-5.2.3.5