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import java.security.SecureRandom;
import java.security.NoSuchAlgorithmException;
// ...

public static void main (String args[]) {
  SecureRandom number = new SecureRandom();
  // Generate 20 integers 0..20
  for (int i = 0; i < 20; i++) {
    System.out.println(number.nextInt(21));
  }
}

Compliant Solution (Java 8)

This compliant solution uses the SecureRandom.getInstanceStrong() method, introduced in Java 8, to use a strong RNG algorithm, if one is available.

import java.security.SecureRandom;
import java.security.NoSuchAlgorithmException;
// ...

public static void main (String args[]) {
   try {
     SecureRandom number = SecureRandom.getInstancegetInstanceStrong();
     // Generate 20 integers 0..20
     for (int i = 0; i < 20; i++) {
       System.out.println(number.nextInt(21));
     }
   } catch (NoSuchAlgorithmException nsae) { 
     // Forward to handler
   }
}

Exceptions

MSC02-J-EX0: Using the default constructor for java.util.Random applies a seed value that is "very likely to be distinct from any other invocation of this constructor" [API 2014] and may improve security marginally. As a result, it may be used only for noncritical applications operating on nonsensitive data. Java's default seed uses the system's time in milliseconds. When used, explicit documentation of this exception is required.

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Predictable random number sequences can weaken the security of critical applications such as cryptography.

Automated Detection

Related Vulnerabilities

CVE-2006-6969 describes a vulnerability that enables attackers to guess session identifiers, bypass authentication requirements, and conduct cross-site request forgery attacks.

Related Guidelines

Bibliography

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