Password hashing algorithms are one-way functions and it is very difficult to get back the original message through reversing the process. Knowing this however, attackers explore other techniques to gain unauthorized access to systems and thereby to hash values. The best approaches for the attackers is to look for and steal precomputed password hash values. Further, attackers make use of these values for password guessing purposes and steal credentials.
In Rainbow Table attack, crackers make best guess of password and compute the hash value for the guessed password. Finally, they will compare it with the hash values from the Rainbow Tables to find out if it matches. Through these repetitive procedures, attackers try finding hash value matches and simulate login procedures to gain unauthorized access to resources.