A large distributed brute force attack against WordPress sites is understood to be occurring. A large botnet with more than 90,000 servers is attempting to log in by cycling through different usernames and passwords.
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http://www.siliconrepublic.com/strategy/item/32269-major-brute-force-attack/
http://thehackernews.com/2013/04/massive-brute-force-attack-targets.html