Once installed, FiveSys rootkit redirects internet traffic to a proxy server, which it does by installing a custom root certificate so that the browser won't warn about the unknown identity of the proxy. It's uncertain how FiveSys is actually distributed, but researchers believe that it's bundled with cracked software downloads. While the digital signing requirements detect and stop most of the rootkits, they are not foolproof,' Bogdan Botezatu, director of threat research and reporting at Bitdefender, told ZDNet. 'Chances is that it was submitted for validation and somehow it got through the checks. SEE: A winning strategy for cybersecurity (ZDNet special report) It's a still a mystery how cyber criminals were able to get hold of a valid certificate. It's known for cyber criminals to use stolen digital certificates, but in this case, they've managed to acquire a valid one.