Most people whose computers have been turned into bots and linked to a botnet have no idea that their machines have been commandeered by cybercriminals. Their PCs send spam, steal information, and participate in denial-of-service attacks without any obvious sign.

But new software, funded by a grant from the U.S. Army Research Office and developed by SRI International, promises to provide users with more insight into what their computers are doing.

BotHunter, announced on Monday, is a free malware-detection application for Mac OS X, Linux/Unix, and Windows that monitors network activity. Unlike intrusion detection system (IDS) tools that scan only incoming data, BotHunter looks for patterns that indicate malware activity in both incoming and outgoing data.

"We do a lot of inbound egress monitoring," said Phillip Porras, SRI program director of enterprise and infrastructure security and lead developer of the BotHunter project. "BotHunter really flips that paradigm around."

As an automated network-flow analysis tool, BotHunter uses IDS routines to scan inbound and outbound network packet headers and payloads. It does so without revealing packet payload contents, which is necessary to protect privacy and make it usable in government environments. The machine profiles it sends to the BotHunter repository are anonymized to remove local network identification data.

The software has been downloaded some 35,000 times to date and several thousand instances are running in the U.S. military. So far, about 250 users have reported finding that their PCs have been turned into bots, said Porras.

Though the software is aimed at technically savvy users, specifically network administrators, the Windows version should install easily and should be usable by those without deep networking expertise. The Mac version requires the target machine to have Apple's developer tools installed to function.