The Hail Mary Cloud was a botnet that slowly and intelligently attempted to bruteforce SSH logins. Why was it dubbed the Hail Mary Cloud though?
closed as too localized by Antony Vennard, Rory Alsop♦ Apr 8 '12 at 9:39
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The name is just a reference to the Hail Mary Pass:
It's been used as a metaphor for anything desparate with little chance for success, which brute force attacks always are. If you read how it works it's sorta clever, for brute force, but each action is very low chance of working, which is why it needs a Cloud to work:
It turns out the page you linked to has the answer
Also, it is worth noting that the third google result for "Hail Mary" is The "Hail Mary" Pass, which is a football term along the same lines. A second, somewhat obvious way that gives you an answer.
This may explain why your question received a down vote.
But then again, "Hindsight is 20-20".