Hash Collision.

Two distinct inputs that produce the same cryptographic hash value, breaking integrity, uniqueness, and signature guarantees that depend on the hash function. It belongs to the Cryptography category of cybersecurity.

Two distinct inputs that produce the same cryptographic hash value, breaking integrity, uniqueness, and signature guarantees that depend on the hash function.

A hash collision is a pair of different inputs that map to identical hash outputs. Because hash functions compress arbitrary data to a fixed size, collisions exist mathematically, but a secure hash makes them computationally infeasible to find. When a function is broken, attackers can forge certificates, manipulate signed documents, or substitute one binary for another. MD5 has been practically collidable since 2004 (Wang et al.) and was used in the Flame malware to forge a Microsoft code-signing certificate. SHA-1 was broken by the SHAttered attack from Stevens et al. in 2017. Modern systems should use SHA-256, SHA-3, or BLAKE2/BLAKE3 for collision-resistant hashing.

Defences for Hash Collision typically combine technical controls and operational practices, as detailed in the full definition above.