Authentication / Predictable Session Token


A predictable session token vulnerability occurs when a web application issues session tokens that follow a discernible pattern or sequence. Session tokens are intended to uniquely identify a user's session and should be random and unpredictable to prevent unauthorized access. If an attacker can predict or determine the session tokens, they can hijack active sessions, gaining unauthorized access to sensitive user data and functionalities. OWASP lists the corresponding attack as Session Prediction.


The risk associated with predictable session tokens is significant. Attackers who can predict session tokens can perform session hijacking attacks. This attack compromises the confidentiality, integrity, and availability of user data and services.


To mitigate the risk of predictable session tokens, the following solutions should be implemented:

  • Use Cryptographically Secure Random Number Generators (CSRNG): Generate session tokens using CSRNG to ensure tokens are random and unpredictable.
  • Session tokens should be long enough so they cannot feasibly be guessed via brute-force attacks.
  • An option could also be the use of a cryptographically signed, short-lived token, such as a JWT.
  • Secure Token Storage and Transmission: Ensure session tokens are transmitted over secure channels (e.g., HTTPS) and are securely stored on both the client and server sides.

Curious? Convinced? Interested?

Arrange a no-obligation consultation with one of our product experts today.