Session fixation

Session fixation is when someone tricks you into using a session (a temporary login) that they control instead of creating a new one just for you. Imagine you're playing with blocks and someone asks you to use the green block they've been holding onto instead of getting a new one. They may have messed with the green block already, so now they know everything you do with it. This can happen on websites when someone sends you a link that logs you in with their session, letting them see everything you do while you're logged in. It's important to always make sure you're starting with a fresh login to keep your information safe.