The Background Intelligent Transfer Service used by Microsoft Corporation"s operating systems to deliver patches via Windows Update, is being used by hackers to sneak malware past firewalls, according to Symantec researchers. BITS, which debuted in Windows XP and is baked into Windows Server 2003 and Windows Vista, is an asynchronous file transfer service with automatic throttling, meaning downloads don"t impact other network chores. It automatically resumes if the connection is broken.
Elia Florio, a researcher with Symantec"s security response team, outlined why some Trojan makers have started to call on BITS to download add-on code to an already compromised computer. "For one simple reason: BITS is part of the operating system, so it"s trusted and bypasses the local firewall while downloading files. It"s not easy to check what BITS should download and not download. 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."
Symantec first caught chatter about BITS on Russian hacker message boards late last year and has been on the lookout for it since. A Trojan spammed in March was one of the first to put the technique into practice. "It"s free and reliable, and they don"t have to write their own download code," said Oliver Friedrichs, director of Symantec"s security response group. Although BITS powers the downloads delivered by Microsoft"s Windows Update service, Friedrichs reassured users that there was no risk to the service itself. "There"s no evidence to suspect that Windows Update can be compromised. If it has a weakness, someone would have found it by now. But this does show how attackers are leveraging components and becoming more and more modular in how they create software."