Ryan Barnett from Breach Security posted a very detailed distributed brute force attacks against yahoo on his blog . He did a superb job on introducing the problem, analyzing the attacks and proposing defensive takeaways. I took some time to put up a simple perl script to attempt to simulate the attack, the script is very straight forward: it reads username and password files from two separate files.

Read this article:
"Distributed Brute Force Attacks Against Yahoo! Mail", an attempt to reproduce the attack