Those cracking applications use word lists compiled from other password breaches in so-called dictionary attacks, which seek to match already computed hashes with those on the new list. Another method is a brute-force attack in which the programs rapidly try different password combinations in the hope of finding a matching hash. Brute-force attacks are more time consuming for longer passwords that contain a mix of capital letters and symbols.

Robert David Graham, CEO of the security consultancy Errata Security, that each letter of a password has 100 possible combinations composed of either upper or lower case, digits or symbols. A five-letter password would have 10 billion possible combinations and could be cracked in five seconds using a top-of-the-line Radeon HD 7970 graphics processor.

A six-letter password would take a little over seven seconds, but a seven-letter password would take 13 hours, Graham wrote. Eight characters pushes the time up to 57 days, with a nine-character password taking up to 15 years.

"In other words, if your password was seven letters, the hacker has already cracked it, but if it's nine letters, it's too difficult to crack with brute force," Graham wrote.

Many of the hashes in the dump have five zeros as the first five characters of the hash. Graham wrote that some people "think that this means that the hacker has already cracked any passwords that have been zeroed out this way."