Cryptography / Small Space of Random Values

Description

Small Space of Random Values (SSRV) is a cryptographic vulnerability which occurs in web and API applications. It is defined in the Common Weakness Enumeration (CWE) directory as CWE-331. SSRV is a problem that occurs when a cryptographic system or protocol uses a small space of random values (SSRV) to generate cryptographic values, such as keys, nonces, and salts, which can easily be guessed. This vulnerability can increase the chances of an attacker being able to guess the values correctly, which can lead to a successful attack against the application. According to the OWASP Testing Guide, SSRV should be tested by checking for weak random values and for an insufficiently sized random value space.

Risk

The risk associated with this vulnerability is that an attacker may be able to guess the values correctly, thereby gaining access to the application. This can lead to a successful attack against the application and the data it contains. It is important to note that this vulnerability can be difficult to detect and exploit, as it requires knowledge of the system and how it is generating the random values.

Solution

The best way to fix this vulnerability is to ensure that the application is using a sufficiently large space of random values. This can be done by ensuring that the application is generating a cryptographically secure random number, such as a cryptographically secure pseudorandom number generator (CSPRNG). Additionally, care should be taken to ensure that the random values generated are long enough to make guessing them difficult.

Example

The following example is taken from the CVE-2020-12644 vulnerability which was identified in the OpenSSL library.

/*
 * Generate a 16-byte random number, using a CSPRNG.
 */
unsigned char rand_bytes[16];
if (!RAND_bytes(rand_bytes, 16)) {
    // Error handling
}