Same Seed in Pseudo-Random Number Generator (PRNG)

A Pseudo-Random Number Generator (PRNG) uses the same seed each time the product is initialized.


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.


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){
  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

Comprehensive Categorization: Randomness

Weaknesses in this category are related to randomness.

ICS Communications: Frail Security in Protocols

Weaknesses in this category are related to the "Frail Security in Protocols" category from the SEI ETF "Categories of Security Vulnerabilities in ICS" as published in ...

OWASP Top Ten 2021 Category A02:2021 - Cryptographic Failures

Weaknesses in this category are related to the A02 category "Cryptographic Failures" in the OWASP Top Ten 2021.

Comprehensive CWE Dictionary

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