Flame malware authors have ordered infected computers still under their control to download and execute a component designed to remove all traces of the cyber-espionage malware and prevent forensic analysis.
Security researchers from Symantec said that Flame has a built-in feature called SUICIDE that can be used to uninstall the malware from infected computers. However, late last week, Flame's creators decided to distribute a different self-removal module to infected computers that connected to servers still under their control, Symantec's security response team said.
The module is called browse32.ocx and its most recent version was created on May 9, 2012. "It is unknown why the malware authors decided not to use the SUICIDE functionality, and instead make Flamer perform explicit actions based on a new module," the Symantec researchers said.
However, even though it is similar in functionality to the SUICIDE feature - both being able to delete a large number of files associated with the malware - the new module goes a step further.
"It locates every Flame file on disk, removes it, and subsequently overwrites the disk with random characters to prevent anyone from obtaining information about the infection," the Symantec researchers said. "This component contains a routine to generate random characters to use in the overwriting operation. It tries to leave no traces of the infection behind."
Deleting a file in Windows does not remove its actual data from the physical hard disk. It only flags the hard disk sectors occupied by that file as available for the operating system to rewrite.
However, since there is no way to predict when the operating system will actually overwrite those sectors, the deleted file, or portions of it, can be recovered with special data recovery tools - at least for a limited period of time.
According to Aleks Gostev, chief security expert with Kaspersky Lab's global research & analysis team, the overwriting of file data with meaningless characters happens before the Flame files get deleted by browse32.ocx, not after as Symantec suggested. However, the goal is the same - eliminating all traces of the malware and making forensic analysis harder, he said.
Last week, Kaspersky's researchers said that they discovered Flame while investigating a series of data loss incidents in Iran that could have been caused by a piece of malware. However, no evidence that links Flame to those attacks has been found yet.
Kaspersky's researchers didn't exclude the possibility that a yet-to-be-identified Flame component was responsible for the data destruction in Iran, but if such a component exists, it's probably not browse32.ocx.
"Browse32 does not overwrite the hard disk the way Wiper the mystery malware did it," Gostev said. "It wipes only files related to Flame."