Over half a million Apple Macintosh computers have been infected with a variation of the Mac Flashback Trojan.
According to Ars Technica, the malware targets a known Java vulnerability that had not been patched by Apple until last week.
Startlingly, 274 of the machines caught in the botnet are believed to be based in Cupertino, California, where Apple is based.
Get daily business news.
The latest stories, funding information, and expert advice. Free to sign up.
Users are advised to immediately update Mac OSX to receive the patch.