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.

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 Python, secrets provides a cryptographically secure pseudo-random number generator. random is not cryptographically secure, and should be avoided in security contexts.

The example below uses the random package instead of secrets to generate a password:

Instead, use secrets:

  • Wikipedia. Pseudo-random number generator.
  • OWASP: Insecure Randomness.
  • OWASP: Secure Random Number Generation.
    •