CWE-342 Base Draft

Predictable Exact Value from Previous Values

This vulnerability occurs when a system uses a predictable sequence for generating values, allowing an attacker to accurately guess future numbers by analyzing past ones.

Definition

What is CWE-342?

This vulnerability occurs when a system uses a predictable sequence for generating values, allowing an attacker to accurately guess future numbers by analyzing past ones.
Many applications rely on random or unique values for critical tasks like generating session tokens, encryption keys, or transaction IDs. When the algorithm producing these values is not truly random—perhaps using a simple counter, a weak pseudo-random number generator (PRNG), or a time-based seed—it creates a pattern. An attacker who observes a few previous values can often deduce the algorithm and predict the next value with high accuracy, completely bypassing the intended security. This flaw is particularly dangerous in security contexts. For instance, predictable session IDs can lead to account hijacking, and guessable initial sequence numbers (ISNs) in TCP connections can enable session spoofing. To prevent this, developers must use cryptographically secure random number generators (CSPRNGs) that are explicitly designed to be unpredictable, even when previous outputs are known, and ensure seeds have sufficient entropy from a reliable system source.
Real-world impact

Real-world CVEs caused by CWE-342

  • CVE-2002-1463

    Firewall generates easily predictable initial sequence numbers (ISN), which allows remote attackers to spoof connections.

  • CVE-1999-0074

    Listening TCP ports are sequentially allocated, allowing spoofing attacks.

  • CVE-1999-0077

    Predictable TCP sequence numbers allow spoofing.

  • CVE-2000-0335

    DNS resolver uses predictable IDs, allowing a local user to spoof DNS query results.

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 pseudo

MITRE has not published a code example for this CWE. The pattern below is illustrative — see Resources for canonical references.

Vulnerable pseudo 
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
  // Untrusted input flows directly into the sensitive sink.
  return executeUnsafe(input);
}
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-342

  • Increase the entropy used to seed a PRNG.
  • 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").
  • Implementation Use a PRNG that periodically re-seeds itself using input from high-quality sources, such as hardware devices with high entropy. However, do not re-seed too frequently, or else the entropy source might block.
Detection signals

How to detect CWE-342

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

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Frequently asked questions

Frequently asked questions

What is CWE-342?

This vulnerability occurs when a system uses a predictable sequence for generating values, allowing an attacker to accurately guess future numbers by analyzing past ones.

How serious is CWE-342?

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-342?

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

How can I prevent CWE-342?

Increase the entropy used to seed a PRNG. 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-342?

Plexicus's SAST engine matches the data-flow signature for CWE-342 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-342?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/342.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-342

CWE-340 Parent

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-341 Sibling

Predictable from Observable State

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

CWE-343 Sibling

Predictable Value Range from Previous Values

This vulnerability occurs when a system's random number generator produces values that, after observing previous outputs, allow an…

Resources

Further reading

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