A persistent cookie is a token saved by a browser or application so it remains available after the application closes or the device restarts. Unlike a session-only cookie, it can preserve a login or other session state for a defined period. If the token grants access, anyone who obtains it may be able to act as the associated user until it expires or is invalidated.
In cyber security, persistent cookies matter because they can function as credentials: attackers may steal or misuse them to bypass a password prompt, while defenders should protect them, limit their lifetime, and revoke them when compromise is suspected. In the Cisco Expressway vulnerability context, persistent cookies are described as part of the attack path for certain clustered deployments. Their mention does not establish how an attacker would obtain them or prove that a system was compromised.