“From our sinkhole data, we have estimated that the number of computers infected with this threat in the last 24 hours is in the region of 270,000, down from 380,000,” Symantec said in its report.
Flashback is the biggest malware epidemic to hit the Mac platform, ever. It relies on a vulnerability in Java, a vulnerability that Apple has been criticized for moving to slowly to patch.
The problem is related to a trojan that was first discovered in September of 2011. As noted in the comments below, earlier in 2012, the bad guys found a way to exploit a Java vulnerability that allowed them to remotely install the same malware without user intervention.
It was that vulnerability for which Apple released a patch for Snow Leopard and Lion on April 4th. The company said on Tuesday that it was also developing an app to remove the infection on Macs already infested.
In the meanwhile, The Mac Observer published instructions for detecting and removing the malware manually.
The decrease in the number of infected Macs tracks with the release of Apple’s patches last week and increased awareness of the problem brought by Dr. Web’s initial report of 600,000 Macs.