CWE-331 Base Draft

Insufficient Entropy

This vulnerability occurs when a system's random number generator or algorithm lacks sufficient unpredictability, creating patterns or predictable outputs that are easier for attackers to guess.

Definition

What is CWE-331?

This vulnerability occurs when a system's random number generator or algorithm lacks sufficient unpredictability, creating patterns or predictable outputs that are easier for attackers to guess.
Insufficient entropy means your random values aren't random enough. This often stems from using weak algorithms (like `rand()`), seeding generators with predictable values (like the current time), or drawing from a source with limited possible outcomes. Attackers can exploit these patterns to predict security-critical values like session tokens, cryptographic keys, or password reset codes, effectively bypassing protections that rely on randomness. To prevent this, developers should use cryptographically secure pseudorandom number generators (CSPRNGs) provided by the operating system or language's security libraries. Always ensure your entropy source is robust and unpredictable, especially for initialization vectors, nonces, and key generation. For high-stakes systems, consider using hardware random number generators or dedicated security services to guarantee the quality of your randomness.
Auswirkungen in der Praxis

Real-world CVEs caused by CWE-331

  • 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.

  • 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)

Wie Angreifer es ausnutzen

Angreiferpfad Schritt für Schritt

  1. 1

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

  2. 2

    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.

  3. 3

    This example also exhibits a Small Seed Space (CWE-339).

  4. 4

    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.

  5. 5

    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.

Verwundbares Codebeispiel

Vulnerable PHP

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

Verwundbar PHP 
function generateSessionID($userID){
  	srand($userID);
  	return rand();
  }
Sicheres Codebeispiel

Secure pseudo

Sicher 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.
Präventions-Checkliste

How to prevent CWE-331

  • Implementation 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.
Erkennungssignale

How to detect CWE-331

SAST High

Führe statische Analyse (SAST) auf der Codebasis aus und suche im Datenfluss nach dem unsicheren Muster.

DAST Moderate

Führe dynamische Application-Security-Tests gegen den Live-Endpoint aus.

Runtime Moderate

Beobachte Runtime-Logs auf ungewöhnliche Exception-Traces, fehlerhafte Eingaben oder Versuche, Autorisierung zu umgehen.

Code review Moderate

Code Review: Markiere jeden neuen Code, der Eingaben von dieser Oberfläche ohne validierte Framework-Helper verarbeitet.

Plexicus Auto-Fix

Plexicus erkennt CWE-331 automatisch und öffnet in unter 60 Sekunden einen Fix-PR.

Codex Remedium scannt jeden Commit, identifiziert genau diese Schwachstelle und liefert einen reviewer-ready Pull Request mit dem Patch. Keine Tickets. Keine Hand-offs.

Demo anfordern Plexicus kostenlos testen
Häufig gestellte Fragen

Frequently asked questions

Was ist CWE-331?

This vulnerability occurs when a system's random number generator or algorithm lacks sufficient unpredictability, creating patterns or predictable outputs that are easier for attackers to guess.

Wie gravierend ist CWE-331?

MITRE hat für diese Schwachstelle keine Exploit-Wahrscheinlichkeit veröffentlicht. Behandle sie als mittlere Auswirkung, bis dein Threat Model anderes belegt.

Welche Sprachen oder Plattformen sind von CWE-331 betroffen?

MITRE hat für diese CWE keine betroffenen Plattformen spezifiziert — sie kann in den meisten Anwendungs-Stacks auftreten.

Wie kann ich CWE-331 verhindern?

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.

Wie erkennt und behebt Plexicus CWE-331?

Die SAST-Engine von Plexicus erkennt die Datenfluss-Signatur von CWE-331 bei jedem Commit. Bei einem Treffer öffnet unser Codex-Remedium-Agent einen Fix-PR mit korrigiertem Code, Tests und einer einzeiligen Zusammenfassung für den Reviewer.

Wo erfahre ich mehr über CWE-331?

MITRE veröffentlicht die kanonische Definition unter https://cwe.mitre.org/data/definitions/331.html. Für ergänzende Hinweise kannst du auch die OWASP- und NIST-Dokumentation heranziehen.

Verwandte Schwachstellen

Weaknesses related to CWE-331

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-334 Sibling

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…

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-332 Child

Insufficient Entropy in PRNG

This vulnerability occurs when a Pseudo-Random Number Generator (PRNG) doesn't have enough randomness (entropy) to start with, or isn't…

Ressourcen

Further reading

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