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Insecure randomness

ID: java/insecure-randomness
Kind: path-problem
Security severity: 7.8
Severity: warning
Precision: high
Tags:
   - security
   - external/cwe/cwe-330
   - external/cwe/cwe-338
Query suites:
   - java-code-scanning.qls
   - java-security-extended.qls
   - java-security-and-quality.qls

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If you use a cryptographically weak pseudo-random number generator to generate security-sensitive values, such as passwords, attackers can more easily predict those values.

Pseudo-random number generators generate a sequence of numbers that only approximates the properties of random numbers. The sequence is not truly random because it is completely determined by a relatively small set of initial values (the seed). If the random number generator is cryptographically weak, then this sequence may be easily predictable through outside observations.

Recommendation

The java.util.Random random number generator is not cryptographically secure. Use a secure random number generator such as java.security.SecureRandom instead.

Use a cryptographically secure pseudo-random number generator if the output is to be used in a security-sensitive context. As a general rule, a value should be considered “security-sensitive” if predicting it would allow the attacker to perform an action that they would otherwise be unable to perform. For example, if an attacker could predict the random password generated for a new user, they would be able to log in as that new user.

Example

The following examples show different ways of generating a cookie with a random value.

In the first (BAD) case, we generate a fresh cookie by appending a random integer to the end of a static string. The random number generator used (Random) is not cryptographically secure, so it may be possible for an attacker to predict the generated cookie.

Random r = new Random();

byte[] bytes = new byte[16];
r.nextBytes(bytes);

String cookieValue = encode(bytes);

Cookie cookie = new Cookie("name", cookieValue);
response.addCookie(cookie);

In the second (GOOD) case, we generate a fresh cookie by appending a random integer to the end of a static string. The random number generator used (SecureRandom) is cryptographically secure, so it is not possible for an attacker to predict the generated cookie.

SecureRandom r = new SecureRandom();

byte[] bytes = new byte[16];
r.nextBytes(bytes);

String cookieValue = encode(bytes);

Cookie cookie = new Cookie("name", cookieValue);
response.addCookie(cookie);

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