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USB devices are still the second biggest vector for malware despite Microsoft's best efforts to close down this avenue for hackers by disabling the Autorun feature in Windows, according to Czech security firm Avast.
The firm's senior virus analyst, Jiri Sejtko, revealed the surprising news during a press event at Avast headquarters in Prague.
"We have seen no decline and I don't know why except there are probably a lot of pirated systems out there that didn't have automatic updates from Microsoft," he explained.
He said the prevalence of the "remove Windows Genuine Advantage" program on the web indicates there are still a substantial number of piracy issues for Redmond.
In February Microsoft announced the update, which limits how Autorun behaves, after realising how often it was used to propagate malware.
The update applies to all applies to all Windows releases with the exception of Windows 7 and Windows Server 2008 R2.
Sejtko also explained the frustrations that Avast's security team has had in trying to co-operate with ad networks in order to stamp out malvertising.
"We don't really know how they work. We've had many discussions with the ad companies but they've not been able to explain why they allow such content," he said. "They asked us for a lot but we've gotten nothing back in reply."