How Does a Collision in a Hash Function Affect ECDSA Security?

A hash collision occurs when two different inputs produce the same hash output. If an attacker finds a collision, they could potentially replace a legitimate transaction with a malicious one that has the same hash.

Since the ECDSA signature is created on the hash, the valid signature for the legitimate transaction would also be valid for the malicious one. While collision resistance in modern functions like SHA-256 is extremely high, a successful collision attack would fundamentally break the non-repudiation guarantee of ECDSA.