Weak crypto
Use of cryptographic algorithms that are broken or should-be-avoided: MD5 (collision attacks 2004), SHA-1 (collision 2017), DES (key too short), RC4 (statistical biases), single DES, ECB mode for block ciphers.
Migration list: MD5/SHA-1 → SHA-256+, DES/3DES → AES, RSA-1024 → RSA-3072 or ECC, hashed passwords with MD5/SHA → bcrypt/Argon2. PKI CAs stopped issuing SHA-1 certs 2017. SSH clients refuse old algorithms (DSA, RSA-1024, ssh-rsa with SHA-1) since OpenSSH 9.0. TLS 1.3 (2018) dropped weak ciphers (RC4, 3DES, CBC, MD5). Real-world: SHAttered attack (Google 2017) demonstrated SHA-1 PDF collision under a research budget. Modern guideline: NIST SP 800-131A, Mozilla's Crypto-recommendations.