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hashlib-insecure-hash-function (S324)

Derived from the flake8-bandit linter.

What it does

Checks for uses of weak or broken cryptographic hash functions in hashlib and crypt libraries.

Why is this bad?

Weak or broken cryptographic hash functions may be susceptible to collision attacks (where two different inputs produce the same hash) or pre-image attacks (where an attacker can find an input that produces a given hash). This can lead to security vulnerabilities in applications that rely on these hash functions.

Avoid using weak or broken cryptographic hash functions in security contexts. Instead, use a known secure hash function such as SHA256.

Example

import hashlib


def certificate_is_valid(certificate: bytes, known_hash: str) -> bool:
    hash = hashlib.md5(certificate).hexdigest()
    return hash == known_hash

Use instead:

import hashlib


def certificate_is_valid(certificate: bytes, known_hash: str) -> bool:
    hash = hashlib.sha256(certificate).hexdigest()
    return hash == known_hash

or add usedforsecurity=False if the hashing algorithm is not used in a security context, e.g. as a non-cryptographic one-way compression function:

import hashlib


def certificate_is_valid(certificate: bytes, known_hash: str) -> bool:
    hash = hashlib.md5(certificate, usedforsecurity=False).hexdigest()
    return hash == known_hash

References