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Insecure randomness

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

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Using a cryptographically weak pseudo-random number generator to generate a security-sensitive value, such as a password, makes it easier for an attacker to predict the value.

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

Use a cryptographically secure pseudo-random number generator if the output is to be used in a security sensitive context. As a rule of thumb, 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.

For C#, RNGCryptoServiceProvider provides a cryptographically secure pseudo-random number generator. Random is not cryptographically secure, and should be avoided in security contexts. For contexts which are not security sensitive, Random may be preferable as it has a more convenient interface, and is likely to be faster.

Example

The following examples show different ways of generating a password.

In the first case, we generate a fresh password 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 password.

In the second example, a cryptographically secure random number generator is used for the same purpose. In this case, it is much harder to predict the generated integers.

In the final example, the password is generated using the Membership.GeneratePassword library method, which generates a password with a bias, therefore should be avoided.

usingSystem.Security.Cryptography;usingSystem.Web.Security;stringGeneratePassword(){// BAD: Password is generated using a cryptographically insecure RNGRandomgen=newRandom();stringpassword="mypassword"+gen.Next();// GOOD: Password is generated using a cryptographically secure RNGusing(RNGCryptoServiceProvidercrypto=newRNGCryptoServiceProvider()){byte[]randomBytes=newbyte[sizeof(int)];crypto.GetBytes(randomBytes);password="mypassword"+BitConverter.ToInt32(randomBytes);}// BAD: Membership.GeneratePassword generates a password with a biaspassword=Membership.GeneratePassword(12,3);returnpassword;}

References

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