InsecureRandomness.qhelp

<!DOCTYPE qhelp PUBLIC
"-//Semmle//qhelp//EN"
"qhelp.dtd">
<qhelp>
	<overview>
		<p>
			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.
		</p>
		<p>
			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.
		</p>

	</overview>
	<recommendation>
        <p>
           The <code>java.util.Random</code> random number generator is not cryptographically secure. Use a secure random number generator such as <code>java.security.SecureRandom</code> instead.
        </p>
        <p>
		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.
	</p>
	</recommendation>

	<example>

		<p>
			The following examples show different ways of generating a cookie with a random value.
		</p>

        <p>
            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 (<code>Random</code>) is not cryptographically secure,
            so it may be possible for an attacker to predict the generated cookie.
        </p>

        <sample src="examples/InsecureRandomnessCookie.java" />

        <p>
            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 (<code>SecureRandom</code>) is cryptographically secure,
            so it is not possible for an attacker to predict the generated cookie.
        </p>

        <sample src="examples/SecureRandomnessCookie.java" />

	</example>

	<references>
		<li>Wikipedia: <a href="http://en.wikipedia.org/wiki/Pseudorandom_number_generator">Pseudo-random number generator</a>.</li>
        <li>
            Java Docs: <a href="http://docs.oracle.com/javase/8/docs/api/java/util/Random.html">Random</a>.
        </li>
        <li>
            Java Docs: <a href="http://docs.oracle.com/javase/8/docs/api/java/security/SecureRandom.html">SecureRandom</a>.
        </li>
	</references>
</qhelp>