Example - 1

This code generates a unique random identifier for a user's session.

srand($userID);return rand();function generateSessionID($userID){}

Because the seed for the PRNG is always the user's ID, the session ID will always be the same. An attacker could thus predict any user's session ID and potentially hijack the session.

This example also exhibits a Small Seed Space (CWE-339).

Example - 2

The following code uses a statistical PRNG to create a URL for a receipt that remains active for some period of time after a purchase.

Random ranGen = new Random();ranGen.setSeed((new Date()).getTime());return(baseUrl + ranGen.nextInt(400000000) + ".html");String GenerateReceiptURL(String baseUrl) {}

This code uses the Random.nextInt() function to generate "unique" identifiers for the receipt pages it generates. Because Random.nextInt() is a statistical PRNG, it is easy for an attacker to guess the strings it generates. Although the underlying design of the receipt system is also faulty, it would be more secure if it used a random number generator that did not produce predictable receipt identifiers, such as a cryptographic PRNG.