Jenzabar JICS (aka Internet Campus Solution) before 9.0.1 Patch 3, 9.1 before 9.1.2 Patch 2, and 9.2 before 9.2.2 Patch 8 has session cookies that are a deterministic function of the username. There is a hard-coded password to supply a PBKDF feeding into AES to encrypt a username and base64 encode it to a client-side cookie for persistent session authentication. By knowing the key and algorithm, an attacker can select any username, encrypt it, base64 encode it, and save it in their browser with the correct JICSLoginCookie cookie format to impersonate any real user in the JICS database without the need for authenticating (or verifying with MFA if implemented).
Authenticating a user, or otherwise establishing a new user session, without invalidating any existing session identifier gives an attacker the opportunity to steal authenticated sessions.