Citing data from its own analysis of the largest Mac botnet to date, Dr. Web notes that around 650,000 computers are still affected, which is stark contradiction to the 30,000 number provided by well-known security companies Symantec and Kaspersky.
Analysts from the Russian firm researched the discrepancy and found that the raw data coming in from the larger companies' servers were likely inaccurate due to Flashback's use of complex domain name creation techniques and a unique TCP connection operation that effectively masks bots from command and control servers.
"BackDoor.Flashback.39 uses a sophisticated routine to generate control server names: a larger part of the domain names is generated using parameters embedded in the malware resources, others are created using the current date. The Trojan sends consecutive queries to servers according to its pre-defined priorities."
When the malware was first discovered in early April, Dr. Web registered for the main domains used as Flashback command servers while other security firms most likely use "hijacked servers" that are in this case less reliable. The report explains that Flashback's mode of operation allows its network of bots to go largely unnoticed by the hijacked servers which could be the reason for the precipitous drop reported this week that saw the number of affected machines fall from 140,000 to 30,000.
Source: Dr. Web
"On April 16th additional domains whose names are generated using the current date were registered. Since these domain names are used by all BackDoor.Flashback.39 variants, registration of additional control server names has allowed to more accurately calculate the number of bots on the malicious network, which is indicated on the graph."
Dr. Web notes that the trojan send requests to a server run by an unidentified third party, which in turn communicates with the bots but fails to close the TCP connection. This action is critical to researchers as it puts the bots in standby mode which means they do not communicate with other command servers monitored by information security specialists.
Code illustrating how an open TCP connection to the command server causes a bot to freeze. | Source: Dr. Web
There has been no response by Symantec or Kaspersky Labs and their respective website still reflect a "Very Low" threat level from the Flashback trojan.
The first iteration of the malware appeared in 2011 disguised as an Adobe Installer, and later morphed into the current self-installing version that was seen on 600,000 Macs worldwide. Following installation, Flashback harvests sensitive data like user IDs, passwords and web browsing history and sends the information to an off-site server.
Apple has responded to the malware by releasing a number of software updates, including a specially-designed Flashback removal tool, over the past two weeks.