Do sites store login password with hash? If so, can people can use hash collision to log in?
Databases generally store hashed passwords (as they should), but some store it plaintext.  If someone were to find a hash collision that matched the stored, unsalted, and hashed password, then yes, they would be able to use that collision to login, because the client would send the password to the server, the server would calculate the hash and compare it to the one in the database. Since the two match (different input, same hashed value), login would  succeed. However, I might like to add, it's more computationally feasible that one would bruteforce the password as opposed to finding a collision, especially with the generally small size of a password.