What is Use of Cryptographically Weak (CWE-338)?

TL;DR: A weak PRNG can lead to authentication and authorization bypasses, allowing attackers to access sensitive data or systems.

At-a-Glance Table

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

Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) (CWE-338) is a type of cryptographic failure that occurs when an application uses a pseudo-random number generator that is not cryptographically strong. As defined by the MITRE Corporation under CWE-338, and classified by the OWASP Foundation under A04:2025 - Cryptographic Failures…

Quick Summary

Using a weak PRNG can lead to authentication and authorization bypasses, allowing attackers to access sensitive data or systems. This is a critical vulnerability that affects many systems, including Java, Python, C++, Node.js, and PHP applications.

Jump to: Quick Summary · Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) Overview · How Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) Works · Business Impact of Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) · Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) Attack Scenario · How to Detect Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) · How to Fix Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) · Framework-Specific Fixes for Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) · How to Ask AI to Check Your Code for Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) · Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) Best Practices Checklist · Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) FAQ · Vulnerabilities Related to Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) · References · Scan Your Own Site

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

What: A weak PRNG is a pseudo-random number generator that does not meet the necessary cryptographic standards.

Why it matters: Using a weak PRNG can lead to authentication and authorization bypasses, allowing attackers to access sensitive data or systems.

Where it occurs: CWE-338 vulnerabilities can occur in any application that uses a pseudo-random number generator.

Who is affected: Any user of an application that uses a weak PRNG is at risk of being affected by this vulnerability.

Who is NOT affected: Users of applications that do not use pseudo-random number generators are not affected by this vulnerability.

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

Root Cause

A weak PRNG is a pseudo-random number generator that does not meet the necessary cryptographic standards.

Attack Flow

1. An attacker attempts to guess or predict the output of the weak PRNG.
2. The attacker uses the predicted output to bypass authentication and authorization mechanisms.
3. The attacker gains access to sensitive data or systems.

Prerequisites to Exploit

• The application must use a pseudo-random number generator that is not cryptographically strong.
• The attacker must be able to guess or predict the output of the weak PRNG.

Vulnerable Code

import java.security.SecureRandom;

public class WeakPRNG {
  public static void main(String[] args) {
    SecureRandom random = new SecureRandom();
    int randomNumber = random.nextInt(100);
    System.out.println("Random Number: " + randomNumber);
  }
}

This code uses the SecureRandom class, which is not cryptographically strong.

Secure Code

import java.security.SecureRandom;

public class SecurePRNG {
  public static void main(String[] args) {
    SecureRandom random = new SecureRandom();
    int randomNumber = random.nextInt(100);
    System.out.println("Random Number: " + randomNumber);
  }
}

This code uses the SecureRandom class, which is cryptographically strong.

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

Confidentiality: The use of a weak PRNG can lead to unauthorized access to sensitive data or systems.

Integrity: The use of a weak PRNG can allow attackers to modify sensitive data or systems.

Availability: The use of a weak PRNG can disrupt the availability of sensitive data or systems.

Real-World Consequences

• Financial loss due to unauthorized access to sensitive data or systems.
• Compliance issues due to failure to protect sensitive data or systems.
• Reputation damage due to public disclosure of a CWE-338 vulnerability.

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

1. An attacker attempts to guess or predict the output of the weak PRNG.
2. The attacker uses the predicted output to bypass authentication and authorization mechanisms.
3. The attacker gains access to sensitive data or systems.

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

Manual Testing

• Review code for use of pseudo-random number generators.
• Test code with different inputs to ensure that the PRNG generates unpredictable numbers.

Automated Scanners (SAST/DAST)

Automated scanners can detect CWE-338 vulnerabilities by analyzing code and identifying potential weaknesses. However, dynamic analysis is required to confirm the presence of a weak PRNG.

PenScan Detection

PenScan’s automated scanners can detect CWE-338 vulnerabilities in your applications.

False Positive Guidance

• Be cautious when reviewing scanner results, as some patterns may look risky but are actually safe due to context.
• Use manual testing and code review to verify the presence of a weak PRNG.

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

• Use a cryptographically secure pseudo-random number generator, such as the one provided by the OpenSSL library.
• Ensure that the PRNG is properly seeded with a high-quality source of randomness.

Framework-Specific Fixes for Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)

Java

Use the SecureRandom class to generate cryptographically strong random numbers.

import java.security.SecureRandom;

public class SecurePRNG {
  public static void main(String[] args) {
    SecureRandom random = new SecureRandom();
    int randomNumber = random.nextInt(100);
    System.out.println("Random Number: " + randomNumber);
  }
}

Node.js

Use the crypto module to generate cryptographically strong random numbers.

const crypto = require('crypto');

function generateRandomNumber() {
  const randomNumber = crypto.randomBytes(4).readUInt32BE(0);
  return randomNumber;
}

console.log(generateRandomNumber());

Python/Django

Use the secrets module to generate cryptographically strong random numbers.

import secrets

def generate_random_number():
    randomNumber = secrets.randbelow(100)
    return randomNumber

print(generate_random_number())

PHP

Use the random_bytes function to generate cryptographically strong random numbers.

function generateRandomNumber() {
  $randomNumber = random_bytes(4);
  $randomNumber = unpack('N', $randomNumber)[1];
  return $randomNumber;
}

print(generateRandomNumber());

How to Ask AI to Check Your Code for Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)

You can use a code review tool or an AI-powered coding assistant to identify potential CWE-338 vulnerabilities in your code.

import java.security.SecureRandom;

public class SecurePRNG {
  public static void main(String[] args) {
    SecureRandom random = new SecureRandom();
    int randomNumber = random.nextInt(100);
    System.out.println("Random Number: " + randomNumber);
  }
}

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

✅ Use a cryptographically secure pseudo-random number generator.

✅ Ensure that the PRNG is properly seeded with a high-quality source of randomness.

✅ Review code regularly to ensure that it uses a cryptographically strong PRNG.

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

How does a cryptographically weak pseudo-random number generator lead to authentication and authorization bypasses?

A weak PRNG can generate predictable or easily guessable numbers, allowing attackers to bypass authentication and authorization mechanisms.

What are the common consequences of using a cryptographically weak pseudo-random number generator?

The use of a weak PRNG can lead to authentication and authorization bypasses, which can result in unauthorized access to sensitive data or systems.

How can I detect Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) vulnerabilities in my application?

You can use automated scanners like PenScan to detect CWE-338 vulnerabilities in your application.

What are the best practices for preventing Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) vulnerabilities?

Use a cryptographically secure pseudo-random number generator, such as the one provided by the OpenSSL library, and ensure that it is properly seeded with a high-quality source of randomness.

How can I fix Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) vulnerabilities in my application?

You can use a cryptographically secure pseudo-random number generator and ensure that it is properly seeded with a high-quality source of randomness.

What are the framework-specific fixes for Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)?

The specific fix will depend on the programming language and framework being used, but generally involves using a cryptographically secure pseudo-random number generator and ensuring that it is properly seeded with a high-quality source of randomness.

How can I ask AI to check my code for Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) vulnerabilities?

You can use a code review tool or an AI-powered coding assistant to identify potential CWE-338 vulnerabilities in your code.

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

References

• MITRE CWE-338
• OWASP A04:2025 - Cryptographic Failures
• NVD

Scan Your Own Site

Manual code review catches what you know to look for. An automated scan catches what you didn’t. Scan your own website using PenScan to find Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) and other risks before an attacker does.