A group of security researchers Wednesday said they have infiltrated one of the world's biggest botnets and can snatch control of compromised machines from the hackers.

But while 3Com's TippingPoint researchers say they have the ability to disinfect the systems by eradicating the malware installed on the hijacked PCs, the company has decided against the move, citing liability issues.

Pedram Amini, who leads TippingPoint's security research group, and Cody Pierce, a security researcher who is also part of that team, collaborated on a weeklong project that started with the idea of verifying the size of the "Kraken" botnet but ended with an ethical quandary.

Pierce created a fake Kraken command-and-control server by reverse engineering the list of domain names found in a captured sample of the bot, and then registered some of the sub-domains Kraken looks for. The server essentially acted as a command-and-control honeypot that waited for connections from PCs infected with the bot.

"Stated simply, Kraken infected systems worldwide start to connect to a server we control," Amini said in a post to a company blog.

The two researchers monitored the incoming communications from Kraken bots for seven days, Pierce said. "We listened and collected statistics for a week, and filtered out [for] the IP addresses and then the systems," he said on the telephone Wednesday." He was able to identify each infected machine by using the malware's encryption key, which was unique across the entire botnet.

The total count for the week: about 25,000 infected machines.

Others have estimated Kraken's size at between 185,000 and 600,000 compromised PCs. SecureWorks' Joe Stewart, who uses the moniker "Bobax" rather than Kraken for the botnet, pegged it at the lower number earlier this month based on an in-depth traffic analysis and bot-fingerprinting project.

In other words, TippingPoint had identified between 4 percent and 14 percent of the total Kraken botnet.

But the company's research didn't stop there. Pierce wrote code that would let him redirect infected PCs, or better yet, use the bot's built-in update mechanism - something most malware includes - to remove Kraken.

There, however, things got sticky. "This is where we got into the ethical discussion," Pierce said. He and Amini wanted to use that capability to clean out Kraken-infected systems. Their boss, David Endler, the director of TippingPoint's DVLabs, disagreed.

"From our point of view, if someone doesn't do something about bots, they'll just continue on and on," Pierce said. "If you have the opportunity to do something, take it."

But Endler had the last word. In a comment attached to Amini's initial blog post, Endler put it plain. "Cleansing the systems would probably help 99 percent of the infected user base, it's just the 1 percent of corner cases that scares me from a corporate liability standpoint," he said.

"That's the other side," Pierce said. "It's not our property, and it's not up to us" to disinfect bot-infected machines. When asked who it was up to, he answered quickly: "I don't know. I wouldn't know the answer to that."

Corporate liability is the stumbling block, he agreed. "I think most people have the same opinion [as Amini and I do]," he said. 'You have to reduce the number of bots out there, whether that's infiltration or by the operating system or at the ISP. Something needs to be done.

"But corporate liability, everybody agrees on that. Cleaning the bots would be opening up a pretty large can of worms."

Most of the TippingPoint blog readers who logged comments took Pierce's side. "Clean them. If you don't, a rival bot net owner will," said one anonymous user.

Others, however, agreed with Endler. "You not only face a moral dilemma, but updating a computer without authorisation is illegal in the US," said a user identified as Roan. "I fall on the side of pro-active patching, but there is more than just the moral decision to decide upon before taking action."

In the US, the Computer Fraud and Abuse Act prohibits unauthorised access to others' PCs; also, state anti-spyware laws have been regularly used to prosecute people who have accessed machines without permission.

Pierce has posted a video (Flash file) of the fake Kraken server connecting with, then cleaning, an in-the-lab system infected with the bot.