According to AP, the hackers are using previously-unknown security holes in Microsoft Office. Apparently it was attacks like this that forced Microsoft to issue large numbers of patches between 2006 and 2007. Mikko Hyppönen, the chief research officer for software security vendor F-Secure, told RSA conference most of the attacks originated in China.

They appear to be part of a cyber attack strategy when the attackers started sending e-mails to victims with booby-trapped Word documents and Excel spreadsheets attached.

The Pentagon and pro-Tibet groups have previously acknowledged the intrusions, but Hyppönen is the first to claim that a particular rash of Volish patches were linked to the attacks.

The hacks were highly successful. One multi-billion-dollar defence contractor who went to F-Secure for help found that a single compromised Windows box had been secretly siphoning information to a server in mainland China for 18 months. It is not certain how much intelligence about pro-Tibetan groups was handed over to the Chinese using similar techniques.