Researchers have cracked open a botnet that amassed more than 60GB of passwords and other stolen data, even as it cloaked itself using a state-of-the-art technique known as fast flux.

When its command-and-control server was infiltrated, the Mumba botnet had snagged more than 55,000 PCs, according to the researchers from anti-virus provider AVG. The data-stealing operation is the work of the notorious Avalanche Group, a criminal operation that was responsible for two-thirds of all phishing attacks in the second half of 2009, according to a report earlier this year from the Anti-Phishing Working Group.

“These criminals are some of the most sophisticated on the internet, and have perfected a mass-production system for deploying phishing sites and ‘crimeware,’” AVG wrote in a report issued Monday. “This means that mitigating the threat by going after the servers hosting the data using the ‘Mumba’ botnet is now much harder than before.”

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