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John von Neumann's quote is widely known:

 "Anyone who considers arithmetical methods of producing random digits is, of course, in a state of sin."

Pseudorandom number generators (PRNGs) use deterministic mathematical algorithms  algorithms to produce a sequence of numbers with good statistical properties. However, but the sequences of numbers produced are not genuinely randomfail to achieve true randomness. PRNGs usually start with an arithmetic seed value. The algorithm uses this seed in order to generate an output value and a new seed as well, which is used to generate the next value, and so on.

The Java API provides a PRNG, the java.util.Random class. This PRNG is portable and repeatable. As a consequence of thatConsequently, if two Random instances instances of the java.util.Random class that are created using the same seed , they will generate identical sequences of numbers in all Java implementations. Seed values are often reused on application initialization or after every system reboot. In other cases, the seed is derived from the current time obtained from the system clock. An attacker can learn the value of the seed by performing some reconnaissance on the vulnerable target and can then build a lookup table for estimating future seed values.

Consequently, the java.util.Random class must not be used either for security-critical applications or for protecting sensitive data. Use a more secure random number generator, such as the java.security.SecureRandom class.

Noncompliant Code Example

If you use the same seed value, you will always get the same sequence of numbers; thus they will not be "random."This noncompliant code example uses the insecure java.util.Random class. This class produces an identical sequence of numbers for each given seed value; consequently, the sequence of numbers is predictable.

import java.util.Random;
// ...

Random number = new Random(123L);
//...
for (int i = 0; i i<20< 20; i++)
 {
   // generateGenerate another random integer in the range [0, 20]
   int n = number.nextInt(21);
   System.out.println(n);
}

...

Compliant Solution

Using a null seed value may prevent such problems. Java's default seed uses the system's time in milliseconds.This compliant solution uses the java.security.SecureRandom class to produce high-quality random numbers:

import java.security.SecureRandom;
import java.utilsecurity.RandomNoSuchAlgorithmException;
// ...

Randompublic numberstatic =void newmain Random();
int n;
//...
for (int i=0; i<20; i++)
String args[]) {
   // re-seed generator
  SecureRandom number = new RandomSecureRandom();
   // Generate 20 generate another random integer in the range [0,20]
   n = number.nextInt(21);
integers 0..20
  for (int i = 0; i < 20; i++) {
    System.out.println(n(number.nextInt(21));
  }
}

For noncritical cases, such as adding some randomness to a game, the Random class is considered fine. However, it is not random enough to be used by more serious applications, such as cryptography.

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Compliant Solution (Java 8)

This compliant solution uses the java.security.SecureRandom class in order to produce high quality random numbersSecureRandom.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.getInstance getInstanceStrong("SHA1PRNG");
      // ...
      // generateGenerate 20 integers 0..20
      for (int i = 0; i < i<2020; 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.

import java.util.Random;
// ...

Random number = new Random(); // Used only for demo  catch (NoSuchAlgorithmException nsae) {}
}

Risk Assessment

purposes
int n;
//...
for (int i = 0; i < 20; i++) {
  // Reseed generator
  number = new Random();
  // Generate another random integer in the range [0, 20]
  n = number.nextInt(21);
  System.out.println(n);
}

For noncritical cases, such as adding some randomness to a game or unit testing, the use of class Random is acceptable. However, it is worth reiterating that the resulting low-entropy random numbers are insufficiently random to be used for more security-critical applications, such as cryptography.

MSC02-J-EX1: Predictable sequences of pseudorandom numbers are required in some cases, such as when running regression tests of program behavior. Use of the insecure java.util.Random class is permitted in such cases. However, security-related applications may invoke this exception only for testing purposes; this exception may not be applied in a production context.

Risk Assessment

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

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Other Languages

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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

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References

\[[API 06|https://www.securecoding.cert.org/confluence/display/java/AA.+Java+References#AA.JavaReferences-API06]\] [Class Random|http://java.sun.com/javase/6/docs/api/java/util/Random.html]
\[[API 06|https://www.securecoding.cert.org/confluence/display/java/AA.+Java+References#AA.JavaReferences-API06]\] [Class SecureRandom|http://java.sun.com/javase/6/docs/api/java/security/SecureRandom.html]
\[[Find Bugs 08|https://www.securecoding.cert.org/confluence/display/java/AA.+Java+References#AA.JavaReferences-FindBugs08]\] BC: Random objects created and used only once
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 327 |http://cwe.mitre.org/data/definitions/327.html], "Use of a Broken or Risky Cryptographic Algorithm," [CWE ID 330|http://cwe.mitre.org/data/definitions/330.html], "Use of Insufficiently Random Values", [CWE ID 333| http://cwe.mitre.org/data/definitions/333.html] "Failure to Handle Insufficient Entropy in TRNG", [CWE ID 332|http://cwe.mitre.org/data/definitions/332.html] "Insufficient Entropy in PRNG", [CWE ID 337|http://cwe.mitre.org/data/definitions/337.html] "Predictable Seed in PRNG"

Bibliography

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Image Added Image Added Image AddedMSC05-J. Make sensitive classes noncloneable      11. Miscellaneous (MSC)      99. The Void (VOID)