CSP-D311: Use of a Weak Pseudo-Random Number Generator (PRNG)¶

Vulnerability Category: Cryptography

Severity: LOW

Description¶

This rule flags the use of Python's random module for generating random numbers in a security-sensitive context. The random module implements a pseudo-random number generator (specifically, the Mersenne Twister), which is perfectly suitable for modeling, simulation, and other non-cryptographic tasks.

However, it is not cryptographically secure. Its output is predictable; if an attacker can learn the internal state of the generator, they can predict all future (and past) random numbers. This makes it unsuitable for any security purpose, such as: - Generating passwords, session tokens, or other credentials. - Generating cryptographic keys or nonces. - Creating salts for password hashing. - Any part of an authentication or authorization mechanism.

Vulnerable Code Example¶

import random
import string

def generate_reset_token(length=16):
    # Using random.choice is not secure for generating tokens.
    # An attacker could potentially predict the token.
    chars = string.ascii_letters + string.digits
    return ''.join(random.choice(chars) for _ in range(length))

print(f"Insecure reset token: {generate_reset_token()}")

Safe Code Example¶

For any security-related need for randomness, use the secrets module, which was introduced in Python 3.6. The secrets module uses the operating system's most secure source of randomness (os.urandom() on most systems) and is designed specifically for cryptographic use.

import secrets
import string

def generate_secure_reset_token(length=16):
    # secrets.choice is cryptographically secure.
    chars = string.ascii_letters + string.digits
    return ''.join(secrets.choice(chars) for _ in range(length))

print(f"Secure reset token: {generate_secure_reset_token()}")

# For generating a URL-safe text string (e.g., for tokens)
secure_token = secrets.token_urlsafe(16) # Creates a 16-byte random token
print(f"URL-safe token: {secure_token}")

For generating raw random bytes (e.g., for cryptographic keys), you can use os.urandom() or secrets.token_bytes().

import os
import secrets

# Both of these are secure ways to get random bytes.
key1 = os.urandom(16)
key2 = secrets.token_bytes(16)

How to Suppress a Finding¶

You should only suppress this finding if you are confident the use of the random module is for a non-security purpose.

import random

# This is for a simulation, not for security.
# ignore
random_value = random.randint(1, 100)

Or, for this specific rule:

# ignore: CSP-D311
random_value = random.random()