D2, MD4, MD5, SHA1 signature algorithms are known to be vulnerable to collision attacks. Attackers can exploit this to generate another certificate with the same digital signature, allowing them to masquerade as the affected service.

A hash function takes a variable-length digital input and coverts it into a fixed-length random hash value.

Hasing algorthems like MD5 and SHA-1 are vulnerable to collision attacks. In a collision attack an attacker finds two messages with the same hashed output and sends the incorrect one to the receiver.

It is recommended to use safer alternatives, such as SHA-256, SHA-512, SHA-3.

Bad practice

import hashlib
import Crypto

m1 = hashlib.md5() # Insecure, Use of MD5
m2 = hashlib.sha1() # Insecure, Use of SHA1
m3 = Crypto.Hash.MD5.new() # Insecure, Use of MD5

Recommended

It is recommended to either use stronger hash algorithms:

import hashlib
import Crypto

m1 = hashlib.sha512()
m2 = hashlib.sha256()
m3 = Crypto.Hash.SHA256.new()

OR, if the use-case is not security sensitive, set the usedforsecurity argument to False as a way to signal that:

import hashlib

hasher = hashlib.md5(usedforsecurity=False)  # issue will no longer be raised.

References:

• OWASP Top 10 2021 Category A02 - Cryptographic Failures
• OWASP Top 10 2021 Category A06 - Vulnerable and Outdated Components
• CWE 327 - Use of a Broken or Risky Cryptographic Algorithm
• Breaking MD5 and other hash functions