CWE-331: Insufficient Entropy

Code Examples & CVEs

Demonstrative Examples

This code generates a unique random identifier for a user's session.

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.

BadPHP

function generateSessionID($userID){srand($userID);return rand();}

The following code uses a statistical PRNG to create a URL for a receipt that remains active for some period of time after a purchase.

This code uses the Random.nextInt() function to generate "unique" identifiers for the receipt pages it generates. Because Random.nextInt() is a statistical PRNG, it is easy for an attacker to guess the strings it generates. Although the underlying design of the receipt system is also faulty, it would be more secure if it used a random number generator that did not produce predictable receipt identifiers, such as a cryptographic PRNG.

BadJava

String GenerateReceiptURL(String baseUrl) {Random ranGen = new Random();ranGen.setSeed((new Date()).getTime());return(baseUrl + ranGen.nextInt(400000000) + ".html");}

Observed CVE Examples (2)

CVE-2001-0950

Insufficiently random data used to generate session tokens using C rand(). Also, for certificate/key generation, uses a source that does not block when entropy is low.

View Details

CVE-2008-2108

Chain: insufficient precision (CWE-1339) in random-number generator causes some zero bits to be reliably generated, reducing the amount of entropy (CWE-331)

View Details

Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Verify controls for CWE-331 with SSDF evidence

Use NIST SSDF verification and vulnerability-response practices to detect CWE-331, Insufficient Entropy, throughout the product lifecycle. Derive review questions, static or dynamic checks, and negative tests from the CWE's causal behavior; define the components and lifecycle stages each check covers; and retain findings with enough evidence to distinguish the root cause from symptoms and impacts. Track escapes and false negatives, then improve the verification plan after every confirmed occurrence.

NIST SP 800-218 Secure Software Development Framework — National Institute of Standards and Technology

Address Insufficient Entropy during Implementation

MITRE associates mitigation with Implementation; documented detection approaches include Automated Static Analysis; recorded impacts include Bypass Protection Mechanism, and Other. Use these source-defined anchors to turn CWE-331 into implementation, review, and verification checks for the affected component.

CWE-331: Insufficient Entropy — MITRE CWE

Triage CWE-331 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-331, Insufficient Entropy, has evidence of exploitation in the wild. Confirm the CVE-to-CWE root-cause mapping independently before attaching the example, then capture the affected product, required action, and remediation deadline. A missing KEV match is not evidence that the weakness is unexploited, and a KEV entry must not be generalized to every occurrence of this CWE.

Known Exploited Vulnerabilities Catalog — Cybersecurity and Infrastructure Security Agency

Apply precise root-cause mapping to CWE-331

Apply MITRE's full root-cause mapping guidance when using CWE-331, Insufficient Entropy. Separate weakness language from attacker prerequisites and technical impact, check the entry's abstraction and vulnerability-mapping notes, and prefer the most specific Base or Variant supported by the evidence. Record the rejected alternatives and require an independent review before the mapping is used for remediation trends or program metrics.

CVE to CWE Root Cause Mapping Guidance — MITRE CWE

Validate CWE-331 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-331, Insufficient Entropy. Confirm the finding describes the causal weakness rather than an impact or attack pattern, compare the abstraction and mapping notes with plausible alternatives, and have a second reviewer challenge the selection. Preserve the evidence and reasoning so recurring defects can be measured against one consistent identifier.

CVE to CWE Root Cause Mapping Quick Tips — MITRE CWE

Frequently Asked Questions

What is CWE-331: Insufficient Entropy?+

CWE-331: Insufficient Entropy is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The product uses an algorithm or scheme that produces insufficient entropy, leaving patterns or clusters of values that are more likely to occur than others.

What are the security consequences of Insufficient Entropy?+

If exploited, CWE-331 (Insufficient Entropy) it can compromise Access Control and Other, leading to outcomes such as Bypass Protection Mechanism and Other.

How do you prevent or mitigate Insufficient Entropy?+

Recommended mitigations for CWE-331 include: Determine the necessary entropy to adequately provide for randomness and predictability. This can be achieved by increasing the number of bits of objects such as keys and seeds.

Which programming languages are affected by Insufficient Entropy?+

CWE-331 commonly affects Not Language-Specific. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

What are real-world examples of Insufficient Entropy?+

MITRE documents real CVEs mapped to CWE-331, including CVE-2001-0950 and CVE-2008-2108. You can look up the full details of each CVE, including CVSS scores and remediation guidance, on our CVE Lookup tool.

What is the difference between a CWE and a CVE?+

A CWE (Common Weakness Enumeration) like CWE-331 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.

CWE-331: Insufficient Entropy

Technical Details

Applicable To

Security Consequences

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Impact

Mitigation Strategies

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Description

Detection Methods