LAST June, one of the world's most advanced hacker groups hit a problem. The US defence contractor whose systems it wanted to access only allowed a small set of trusted IP addresses to connect to their network. In an unusual move – hackers typically go for the low-hanging fruit – the group hacked the company that provided the IP whitelisting service, enabling it to forge access certificates. This group, which calls itself Hidden Lynx, was given a vague face last week when antivirus software-maker Symantec released a report profiling it. Believed to be based in China, the group is known only through traces of malicious software bearing its mark found in the compromised computers of some of the world's largest companies. One of the best known exploits of the so-called Hidden Lynx group was the devastating compromise of security firm Bit9 in 2012. The Waltham, Massachusetts, company provides an "application whitelisting" service that allows custo