Malware authors might be able to subvert components of Windows Update to distribute viruses, security researchers at Symantec warn.
Analysis by the security firm reveals that a recent Trojan distributed by email at the end of March 2007 used a Windows component named "BITS" (Background Intelligent Transfer Service) to download files.
BITS is a Windows component that allows transfer of files between machines. The service is used by Windows Update, Windows Server Update Services, and Systems Management Server to deliver software updates to clients. It's also used by Microsoft instant messenging products to facilitate file transfer.
The asynchronous download service is an elegant way to download files while consuming a minimum of network bandwidth. But what's sauce for the goose is sauce for the gander and the service can also be misused to download malicious files, offering certain advantages for hackers.
"Using BITS to download malicious files is a clever trick because it bypasses local firewalls, as the download is performed by Windows itself, and does not require suspicious actions for process injection," Symantec researcher Elia Florio explains in a blog posting.
The approach might be misused to download additional spyware components onto already infected machines without alerting users anything is amiss (a handy feature for bot masters), kill personal firewalls, inject malware into trusted components of the OS, or perform other malevolent actions.
The BITS download method has been known as a way to kill firewalls since the end of 2006. However, the German Trojan represents a refinement of the technique as a means to download malware components onto systems whose firewall defences might have been already subverted.
In the case of the German Trojan, the malicious downloader sample gets access to the BITS component via the COM interface with "CoCreateInsance(), and it uses CreateJob() and AddFile() methods to configure the file to download and the destination path", he adds.
Worse still, there is no immediate workaround to guard against this type of attack. "It's not easy to check what BITS should download and not download," he said, adding that the misuse of BITS technology is an example of "good technologies used for bad purposes".
Florio said Microsoft needs to modify its software with a patch to guard against attack. "Probably the BITS interface should be designed to be accessible only with a higher level of privilege, or the download jobs created with BITS should be restricted to only trusted URLs," he writes. ®