CWE-334 Base Draft

Small Space of Random Values

This vulnerability occurs when a system uses a random number generator that produces too few possible values. Attackers can easily predict or guess these values through brute force attacks.

Definition

What is CWE-334?

This vulnerability occurs when a system uses a random number generator that produces too few possible values. Attackers can easily predict or guess these values through brute force attacks.
When an application relies on random values for security—like generating session tokens, encryption keys, or password reset codes—using a random source with a small output space is a critical flaw. For example, a random number generator that only produces 65,536 possible values (16 bits) is trivial for an automated script to exhaustively try all combinations. This directly undermines security mechanisms designed to be unpredictable. To prevent this, developers must ensure their random value generators have an output space large enough to resist brute-force attempts for the required lifespan of the secret. This typically means using cryptographically secure pseudo-random number generators (CSPRNGs) that produce values with at least 128 bits of entropy. Always match the randomness strength to the sensitivity of the operation it protects.
Real-world impact

Real-world CVEs caused by CWE-334

  • CVE-2002-0583

    Product uses 5 alphanumeric characters for filenames of expense claim reports, stored under web root.

  • CVE-2002-0903

    Product uses small number of random numbers for a code to approve an action, and also uses predictable new user IDs, allowing attackers to hijack new accounts.

  • CVE-2003-1230

    SYN cookies implementation only uses 32-bit keys, making it easier to brute force ISN.

  • CVE-2004-0230

    Complex predictability / randomness (reduced space).

How attackers exploit it

Step-by-step attacker path

  1. 1

    The following XML example code is a deployment descriptor for a Java web application deployed on a Sun Java Application Server. This deployment descriptor includes a session configuration property for configuring the session ID length.

  2. 2

    This deployment descriptor has set the session ID length for this Java web application to 8 bytes (or 64 bits). The session ID length for Java web applications should be set to 16 bytes (128 bits) to prevent attackers from guessing and/or stealing a session ID and taking over a user's session.

  3. 3

    Note for most application servers including the Sun Java Application Server the session ID length is by default set to 128 bits and should not be changed. And for many application servers the session ID length cannot be changed from this default setting. Check your application server documentation for the session ID length default setting and configuration options to ensure that the session ID length is set to 128 bits.

Vulnerable code example

Vulnerable XML

The following XML example code is a deployment descriptor for a Java web application deployed on a Sun Java Application Server. This deployment descriptor includes a session configuration property for configuring the session ID length.

Vulnerable XML 
<sun-web-app>
  		...
  		<session-config>
  				<session-properties>
  					<property name="idLengthBytes" value="8">
  						<description>The number of bytes in this web module's session ID.</description>
  					</property>
  				</session-properties>
  		</session-config>
  		...
  </sun-web-app>
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-334

  • Architecture and Design / Requirements Use products or modules that conform to FIPS 140-2 [REF-267] to avoid obvious entropy problems. Consult FIPS 140-2 Annex C ("Approved Random Number Generators").
Detection signals

How to detect CWE-334

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-334 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-334?

This vulnerability occurs when a system uses a random number generator that produces too few possible values. Attackers can easily predict or guess these values through brute force attacks.

How serious is CWE-334?

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-334?

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

How can I prevent CWE-334?

Use products or modules that conform to FIPS 140-2 [REF-267] to avoid obvious entropy problems. Consult FIPS 140-2 Annex C ("Approved Random Number Generators").

How does Plexicus detect and fix CWE-334?

Plexicus's SAST engine matches the data-flow signature for CWE-334 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-334?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/334.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-334

CWE-330 Parent

Use of Insufficiently Random Values

This vulnerability occurs when an application uses random values that are not sufficiently unpredictable in security-sensitive operations,…

CWE-1204 Sibling

Generation of Weak Initialization Vector (IV)

This vulnerability occurs when software uses a weak or predictable Initialization Vector (IV) for cryptographic operations. Many…

CWE-1241 Sibling

Use of Predictable Algorithm in Random Number Generator

This vulnerability occurs when a device or application relies on a predictable algorithm to generate pseudo-random numbers, making the…

CWE-331 Sibling

Insufficient Entropy

This vulnerability occurs when a system's random number generator or algorithm lacks sufficient unpredictability, creating patterns or…

CWE-335 Sibling

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-338 Sibling

Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)

This vulnerability occurs when software uses a pseudo-random number generator (PRNG) that is not cryptographically strong for…

CWE-340 Sibling

Generation of Predictable Numbers or Identifiers

This vulnerability occurs when a system creates numbers or identifiers that are too easy to guess, undermining security mechanisms that…

CWE-344 Sibling

Use of Invariant Value in Dynamically Changing Context

This vulnerability occurs when code uses a fixed, unchanging value (like a hardcoded string, number, or reference) in a situation where…

CWE-6 Child

J2EE Misconfiguration: Insufficient Session-ID Length

This vulnerability occurs when a J2EE application uses session identifiers that are too short, making them easier for attackers to predict…

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Further reading

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