A length extension attack is a type of attack that hackers can use to try and gain access to website information. It is a tricky type of attack that takes advantage of the way computers store and process information. When a computer sets up a secure connection (like when you're logging into a website), it uses something called a “hash” to make sure that the information is secure and that it is coming from the correct place. When a hacker does a length extension attack, they try to guess parts of the hash. If they guess correctly, then they can trick the computer into thinking that the hacker is the correct person - even when they're not. This can give them access to information that they shouldn't have access to.