The session prediction attack focuses on predicting session ID values that permit an attacker to bypass the authentication schema of an application. By analyzing and understanding the session ID generation process, an attacker can predict a valid session ID value and get access to the application.
In the first step, the attacker needs to collect some valid session ID values that are used to identify authenticated users. Then, he must understand the structure of session ID, the information that is used to create it, and the encryption or hash algorithm used by application to protect it. Some bad implementations use sessions IDs composed by username or other predictable information, like timestamp or client IP address. In the worst case, this information is used in clear text or coded using some weak algorithm like base64 encoding.
In addition, the attacker can implement a brute force technique to generate and test different values of session ID until he successfully gets access to the application.
The session ID information for a certain application is normally composed by a string of fixed width, which randomness is very important to avoid its prediction. Looking at the example in Figure 1, the session ID variable is represented by JSESSIONID and its value is “user01”, that correspond to the username. By trying new values for it, like “user02”, it could be possible to get inside the application without prior authentication.
Figure 1. Predictable cookie