Same Seed in Pseudo-Random Number Generator (PRNG)

Description

Given the deterministic nature of PRNGs, using the same seed for each initialization will lead to the same output in the same order. If an attacker can guess (or knows) the seed, then the attacker may be able to determine the random numbers that will be produced from the PRNG.

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

The following code uses a statistical PRNG to generate account IDs.

private static final long SEED = 1234567890;
public int generateAccountID() {
  Random random = new Random(SEED);
  return random.nextInt();
}

Because the program uses the same seed value for every invocation of the PRNG, its values are predictable, making the system vulnerable to attack.

Example Two

This code attempts to generate 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.

If the user IDs are generated sequentially, or otherwise restricted to a narrow range of values, then this example also exhibits a Small Seed Space (CWE-339).

See Also