Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.
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
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Cloud application on Kubernetes generates passwords using a weak random number generator based on deployment time.
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server uses erlang:now() to seed the PRNG, which results in a small search space for potential random seeds
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The removal of a couple lines of code caused Debian's OpenSSL Package to only use the current process ID for seeding a PRNG
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Router's PIN generation is based on rand(time(0)) seeding.
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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
Identify a code path that handles untrusted input without validation.
- 2
Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.
- 3
Deliver the payload through a normal request and observe the application's reaction.
- 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:
Random random = new Random(System.currentTimeMillis());
int accountID = random.nextInt(); Secure code example
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
Run dynamic application security testing against the live endpoint.
Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.
Code review: flag any new code that handles input from this surface without using the validated framework helpers.
CWE-337
Don't catalog this weakness. Prove it's reachable.
Plexicus turns CWE catalogs into evidence: every CWE-pattern is matched against your real code graph, reach is proven on a sandbox clone, and verified findings ship as reviewed PRs.
Frequently asked questions What is CWE-337?
How serious is CWE-337?
What languages or platforms are affected by CWE-337?
How can I prevent CWE-337?
How does Plexicus detect and fix CWE-337?
Where can I learn more about CWE-337?
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
- MITRE — official CWE-337 https://cwe.mitre.org/data/definitions/337.html
- FIPS PUB 140-2: SECURITY REQUIREMENTS FOR CRYPTOGRAPHIC MODULES https://csrc.nist.gov/files/pubs/fips/140-2/upd2/final/docs/fips1402.pdf
- FIPS PUB 140-3: SECURITY REQUIREMENTS FOR CRYPTOGRAPHIC MODULES https://csrc.nist.gov/publications/detail/fips/140/3/final
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