Insufficient Entropy

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

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

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

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 Two

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.

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

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.

See Also