To track user activity from page to page within an application, developers create and assign unique session token values to each user. Most session token mechanisms include session IDs, hidden form fields, or cookies. Cookies are placed within the user's browser on the client-side.
These session tokens should be examined by a penetration tester to ensure their uniqueness, randomness, and cryptographic strength, to prevent information leakage.
If a session token value is easily guessable or remains unchanged after login, an attacker could apply (or fixate) a pre-known token value to a user. This is known as a session fixation attack. Generally speaking, the purpose of the attack is to harvest sensitive data in the user's account, since the session token is known to the attacker.