The vulnerability stems from improper validation of redirect URLs after OpenID authentication. While exact code isn't available, the advisory explicitly states the plugin fails to properly verify redirect targets. In OpenID implementations, this typically occurs in the session handling component that processes authentication responses and manages redirects. The confidence is high because: 1) The CWE-601 classification specifically points to URL redirection issues 2) Multiple sources confirm the improper validation behavior 3) The vulnerability pattern matches known OpenID/OAuth implementation flaws where redirect URIs aren't properly validated against allowed domains.