CWE-339 Variant Draft

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 attacker to guess through brute force.

Definition

What is CWE-339?

This vulnerability occurs when a Pseudo-Random Number Generator (PRNG) uses a seed that has too few possible values, making it easy for an attacker to guess through brute force.
A PRNG's output is not truly random; it's a predictable sequence determined entirely by its initial seed. If an attacker can observe some of the generator's output, they can launch a brute-force attack by systematically trying every possible seed value until they find the one that produces the matching sequence. Once the seed is discovered, the attacker can predict every future number the PRNG will generate, completely breaking its security. This risk is directly tied to the size of the seed space. A small seed space—like one using a 16-bit integer (65,536 possibilities) instead of a cryptographically secure 256-bit seed—drastically reduces the number of guesses an attacker needs to make. Modern computing power can exhaust such small spaces quickly, rendering the PRNG useless for security-sensitive operations like generating encryption keys, session tokens, or authentication codes.
Real-world impact

Real-world CVEs caused by CWE-339

  • CVE-2019-10908

    product generates passwords via org.apache.commons.lang.RandomStringUtils, which uses java.util.Random internally. This PRNG has only a 48-bit seed.

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 Python

This code grabs some random bytes and uses them for a seed in a PRNG, in order to generate a new cryptographic key.

Vulnerable Python 
```
# getting 2 bytes of randomness for the seeding the PRNG * 
   seed = os.urandom(2)
   random.seed(a=seed)
   key = random.getrandbits(128)
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-339

  • Architecture and Design Use well vetted pseudo-random number generating algorithms with adequate length seeds. Pseudo-random number generators can produce predictable numbers if the generator is known and the seed can be guessed. A 256-bit seed is a good starting point for producing a "random enough" number.
  • 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-339

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-339 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.

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Frequently asked questions

Frequently asked questions

What is CWE-339?

This vulnerability occurs when a Pseudo-Random Number Generator (PRNG) uses a seed that has too few possible values, making it easy for an attacker to guess through brute force.

How serious is CWE-339?

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-339?

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

How can I prevent CWE-339?

Use well vetted pseudo-random number generating algorithms with adequate length seeds. Pseudo-random number generators can produce predictable numbers if the generator is known and the seed can be guessed. A 256-bit seed is a good starting point for producing a "random enough" number. 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-339?

Plexicus's SAST engine matches the data-flow signature for CWE-339 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-339?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/339.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-339

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-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-341 Peer

Predictable from Observable State

This vulnerability occurs when an attacker can guess or deduce sensitive values, like random numbers or identifiers, by observing…

Resources

Further reading

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