The Gawker hack: how a million passwords were lost

Their system had many holes, one of which was the salt length. I did not mean that the salt length was the largest hole, nor that it was the one that was exploited. Just that it is a hole which could have been exploited if the others were absent.

The salt's only purpose is to prevent a password hash result being used more than once. The normal attack is to pre-compute and store a table of passwords and hash results by running lots of common passwords through the algorithm in advance. The salt needs to be large enough to prevent anybody storing a pre-computed table which has a chance of matching a result in your database. The salt also needs to be large enough that your own salts do not collide because this would reduce the work needed to crack the ones with salt collisions.