CWE-336 Variant Draft

Same Seed in Pseudo-Random Number Generator (PRNG)

This vulnerability occurs when a Pseudo-Random Number Generator (PRNG) is repeatedly initialized with the same starting seed value.

Definition

What is CWE-336?

This vulnerability occurs when a Pseudo-Random Number Generator (PRNG) is repeatedly initialized with the same starting seed value.
PRNGs are deterministic, meaning that if you start them from the same seed, they will produce an identical sequence of 'random' numbers every single time. This predictability breaks the core assumption of randomness that many security features rely on, such as session tokens, cryptographic keys, or password reset tokens. If an attacker discovers or can reasonably guess the seed—for example, if it's a hard-coded value, a simple timestamp, or another predictable source—they can pre-calculate or replicate the entire output sequence. This allows them to anticipate future random values, bypass security controls, impersonate users, or compromise encrypted data that depends on those numbers being truly unpredictable.
Real-world impact

Real-world CVEs caused by CWE-336

  • CVE-2022-39218

    SDK for JavaScript app builder for serverless code uses the same fixed seed for a PRNG, allowing cryptography bypass

How attackers exploit it

Step-by-step attacker path

  1. 1

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

  2. 2

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

  3. 3

    This code attempts to generate a unique random identifier for a user's session.

  4. 4

    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.

  5. 5

    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).

Vulnerable code example

Vulnerable Java

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

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

Secure pseudo

Secure pseudo 
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
  const safe = validateAndEscape(input);
  return executeWithGuards(safe);
}
What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist

How to prevent CWE-336

  • Architecture and Design Do not reuse PRNG seeds. Consider a PRNG that periodically re-seeds itself as needed from a high quality pseudo-random output, such as hardware devices.
  • Architecture and Design / Requirements Use products or modules that conform to FIPS 140-2 [REF-267] to avoid obvious entropy problems, or use the more recent FIPS 140-3 [REF-1192] if possible.
Detection signals

How to detect CWE-336

Automated Static Analysis High

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

Plexicus auto-fix

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Frequently asked questions

Frequently asked questions

What is CWE-336?

This vulnerability occurs when a Pseudo-Random Number Generator (PRNG) is repeatedly initialized with the same starting seed value.

How serious is CWE-336?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-336?

MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.

How can I prevent CWE-336?

Do not reuse PRNG seeds. Consider a PRNG that periodically re-seeds itself as needed from a high quality pseudo-random output, such as hardware devices. Use products or modules that conform to FIPS 140-2 [REF-267] to avoid obvious entropy problems, or use the more recent FIPS 140-3 [REF-1192] if possible.

How does Plexicus detect and fix CWE-336?

Plexicus's SAST engine matches the data-flow signature for CWE-336 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-336?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/336.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-336

CWE-335 Parent

Incorrect Usage of Seeds in Pseudo-Random Number Generator (PRNG)

This vulnerability occurs when a Pseudo-Random Number Generator (PRNG) is used, but its initial seed value is not handled securely or…

CWE-337 Sibling

Predictable Seed in Pseudo-Random Number Generator (PRNG)

This vulnerability occurs when a Pseudo-Random Number Generator (PRNG) uses an easily guessable starting value, like the current system…

CWE-339 Sibling

Small Seed Space in PRNG

This vulnerability occurs when a Pseudo-Random Number Generator (PRNG) uses a seed that has too few possible values, making it easy for an…

Resources

Further reading

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