CWE-340 Class Incomplete

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 rely on unpredictability.

Definition

What is CWE-340?

This vulnerability occurs when a system creates numbers or identifiers that are too easy to guess, undermining security mechanisms that rely on unpredictability.
Predictable identifiers, like session tokens, initial values for cryptographic operations, or password reset codes, act as weak links in your security chain. Attackers can analyze the pattern or sequence to forecast future values, allowing them to hijack user sessions, bypass authentication, or spoof legitimate transactions. This often stems from using weak random number generators, time-based values, or simple incremental counters in security-sensitive contexts. To prevent this, developers should use cryptographically secure pseudo-random number generators (CSPRNGs) provided by the platform's security libraries for all security-critical operations. Always ensure identifiers have sufficient entropy (randomness) and length to resist brute-force guessing attacks. Avoid creating your own algorithms for randomness and regularly audit code that generates any token used for authorization, identification, or uniqueness.
Auswirkungen in der Praxis

Real-world CVEs caused by CWE-340

  • CVE-2022-29330

    Product for administering PBX systems uses predictable identifiers and timestamps for filenames (CWE-340) which allows attackers to access files via direct request (CWE-425).

  • CVE-2001-1141

    PRNG allows attackers to use the output of small PRNG requests to determine the internal state information, which could be used by attackers to predict future pseudo-random numbers.

  • CVE-1999-0074

    Listening TCP ports are sequentially allocated, allowing spoofing attacks.

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).

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-340

  • Architecture Use safe-by-default frameworks and APIs that prevent the unsafe pattern from being expressible.
  • Implementation Validate input at trust boundaries; use allowlists, not denylists.
  • Implementation Apply the principle of least privilege to credentials, file paths, and runtime permissions.
  • Testing Cover this weakness in CI: SAST rules + targeted unit tests for the data flow.
  • Operation Monitor logs for the runtime signals listed in the next section.
Erkennungssignale

How to detect CWE-340

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-340 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-340?

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

Wie gravierend ist CWE-340?

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-340 betroffen?

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

Wie kann ich CWE-340 verhindern?

Use safe-by-default frameworks, validate untrusted input at trust boundaries, and apply the principle of least privilege. Cover the data-flow signature in CI with SAST.

Wie erkennt und behebt Plexicus CWE-340?

Die SAST-Engine von Plexicus erkennt die Datenfluss-Signatur von CWE-340 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-340?

MITRE veröffentlicht die kanonische Definition unter https://cwe.mitre.org/data/definitions/340.html. Für ergänzende Hinweise kannst du auch die OWASP- und NIST-Dokumentation heranziehen.

Verwandte Schwachstellen

Weaknesses related to CWE-340

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-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-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-384 Kann vorausgehen

Session Fixation

Session fixation occurs when an application authenticates a user without first destroying the previous session ID. This allows an attacker…

Ressourcen

Further reading

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