The Flashback Trojan continues to compromise Macs, according to a report by security firm Dr. Web. The Trojan can infect Macs with or without a user's permissions and is dominating Snow Leopard systems. The attacks seem to be directed via Twitter.

The original version of the virus - which emerged around April 6 - was unsophisticated in comparison to its current variant/form. In its analysis, Dr. Web found that the malware is using an exploit in Java, which is configured with a list of servers through which it receives commands and configuration updates. The malware asks a user for permission, though installs regardless in the home directory.

"The download malignant program application interacts with two types of control servers. Servers belonging to the first category intercept web search traffic and redirect the user to malicious sites controlled by criminals. The second group issues commands to bots to perform backdoor tasks in the compromised system. Doctor Web analysts managed to take over control server domain names known to Backdoor. Flashback payload malware and analysed requests sent to bots by servers," Dr. Web reported.

Additionally, if the malware doesn't get a correct response from a server on its internally generated lists it will search Twitter for posts containing a string of text using the current date. The string serves as a hash tag in a search, using https://mobile.twitter.com/searches?q=.

"For example, some Trojans generated a string of the "rgdgkpshxeoa" format for the date 04.13.2012 ... If the Trojan manages to find a Twitter message containing bumpbegin and endbump tags enclosing a control server address, it will be used as a domain name," the firm added. Dr. Web took over domains of this category on April 13 to try and halt the hacker, but its Twitter account was blocked the following day.

63.4 percent of the infected Macs are running Snow Leopard, not the most recent version of OS X - Lion. Snow Leopard is the newest version of Apple's operating system that comes with Java. Snow Leopard also maintained the largest share of OS X last month, according to Net Applications.

Leopard is the second most-used Flashback-infected operating system, Dr. Web said, installed on 25.5 percent of the 90,000 infected Macs. OS X 10.5 released in Oct 2007.

(reported by Jonathan Charles, edited by Dave Clark)