CWE-337 Variant Draft

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 time or a process ID, to begin its sequence.

Definition

What is CWE-337?

This vulnerability occurs when a Pseudo-Random Number Generator (PRNG) uses an easily guessable starting value, like the current system time or a process ID, to begin its sequence.
A PRNG's security depends heavily on the secrecy and unpredictability of its initial seed. When developers use common, low-entropy sources—such as timestamps, process IDs, or other publicly available system values—they dramatically shrink the pool of possible starting points. An attacker can often deduce or narrow down these seeds with minimal effort, compromising the entire random sequence that follows. In practice, this means that cryptographic operations, session tokens, random identifiers, or any security mechanism relying on this PRNG become predictable. Instead of facing a vast, unguessable number of possibilities, an attacker can run a small, feasible set of seed guesses to replicate the generator's output, bypassing protections that were assumed to be random.
Real-world impact

Real-world CVEs caused by CWE-337

  • CVE-2020-7010

    Cloud application on Kubernetes generates passwords using a weak random number generator based on deployment time.

  • CVE-2019-11495

    server uses erlang:now() to seed the PRNG, which results in a small search space for potential random seeds

  • CVE-2008-0166

    The removal of a couple lines of code caused Debian's OpenSSL Package to only use the current process ID for seeding a PRNG

  • CVE-2016-10180

    Router's PIN generation is based on rand(time(0)) seeding.

  • CVE-2018-9057

    cloud provider product uses a non-cryptographically secure PRNG and seeds it with the current time

How attackers exploit it

Step-by-step attacker path

  1. 1

    Identify a code path that handles untrusted input without validation.

  2. 2

    Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.

  3. 3

    Deliver the payload through a normal request and observe the application's reaction.

  4. 4

    Iterate until the response leaks data, executes attacker code, or escalates privileges.

Vulnerable code example

Vulnerable Java

Both of these examples use a statistical PRNG seeded with the current value of the system clock to generate a random number:

Vulnerable Java 
Random random = new Random(System.currentTimeMillis());
  int accountID = 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-337

  • Use non-predictable inputs for seed generation.
  • 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.
  • Implementation Use a PRNG that periodically re-seeds itself using input from high-quality sources, such as hardware devices with high entropy. However, do not re-seed too frequently, or else the entropy source might block.
Detection signals

How to detect CWE-337

SAST High

Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.

DAST Moderate

Run dynamic application security testing against the live endpoint.

Runtime Moderate

Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.

Code review Moderate

Code review: flag any new code that handles input from this surface without using the validated framework helpers.

Plexicus auto-fix

Plexicus auto-detects CWE-337 and opens a fix PR in under 60 seconds.

Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.

Get a demo Try Plexicus free
Frequently asked questions

Frequently asked questions

What is CWE-337?

This vulnerability occurs when a Pseudo-Random Number Generator (PRNG) uses an easily guessable starting value, like the current system time or a process ID, to begin its sequence.

How serious is CWE-337?

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-337?

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

How can I prevent CWE-337?

Use non-predictable inputs for seed generation. 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-337?

Plexicus's SAST engine matches the data-flow signature for CWE-337 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-337?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/337.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-337

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-336 Sibling

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.

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

Ready when you are

Don't Let Security
Weigh You Down.

Stop choosing between AI velocity and security debt. Plexicus is the only platform that runs Vibe Coding Security and ASPM in parallel — one workflow, every codebase.

Get started free Book a demo