The Emergence of FiveSys, a Malicious Signed Rootkit

Bitdefender security researchers have identified a new Microsoft-signed rootkit, named FiveSys, that somehow got through the verification process and ended up in the wild.

Rootkits used to be all the rage more than a decade ago as a form of malware designed to provide attackers access at a low level in infected operating systems. Some changes by Microsoft, starting with Windows Vista, made the propagation of rootkits much more difficult. Security solutions have also become much more efficient, using new technologies that were only a dream 10 years ago.

Typically, new drivers go through a rigorous process of validation before Microsoft digitally signs them. Complicating matters further, new changes to this validation system should make the attackers' attempts to circumvent the process a lot less likely to succeed.

This new requirement ensures that all drivers are validated and signed by the operating system vendor rather than the original developer and, as such, digital signatures offer no indication of the identity of the real developer. FiveSys' developers somehow managed to trick the validation process.

The purpose of the rootkit is straightforward: it aims to redirect the internet traffic in the infected machines through a custom proxy, which is drawn from a built-in list of 300 domains. The redirection works for both HTTP and HTTPS; the rootkit installs a custom root certificate for HTTPS redirection to work. In this way, the browser doesn't warn of the unknown identity of the proxy server.

The rootkit also uses various strategies to protect itself, like blocking the ability to edit the registry and stopping the installation of other rootkits and malware from different groups.

We have reached out to Microsoft to flag this abuse of digital trust. Microsoft revoked the signature shortly after. For more technical details and a full list of indicators of compromise, please read the "Digitally-Signed Rootkits are Back – A Look at FiveSys and Companions" whitepaper.