Open redirect vulnerability in F5 BIG-IP APM 11.2.1, 11.4.x, 11.5.x, and 11.6.x before 11.6.0 HF6 and Edge Gateway 11.2.1, when using multi-domain single sign-on (SSO), allows remote attackers to redirect users to arbitrary web sites and conduct phishing attacks via a base64-encoded URL in the SSO_ORIG_URI parameter.