FireEye says it has discovered a type of malware designed to steal payment card data that can be very difficult to detect and remove.
The cybercriminal group behind the malware, which FireEye nicknamed "FIN1," is suspected of being in Russia and has been known to target financial institutions.
The malware, which FIN1 calls Nemesis, infected an organization that processes financial transactions, which FireEye did not identify.
Payment card data is highly sought after by cybercriminals, who have in recent years targeted very large organizations that handle card data. Target, Home Depot and many others have reported large data breaches over the years. Some payment processors were also hit.
Nemesis is a so-called bootkit. It is installed on lower-level operating system components, and even if the operating system is reinstalled, it can remain in place.
"Malware with bootkit functionality can be installed and executed almost completely independent of the Windows operating system," FireEye wrote.
Earlier this year, the cybercriminals started using an utility called Bootrash that modifies a Windows computer's Volume Boot Records (VBR), which are bits of code used in conjunction with the Master Boot Record (MBR).
The MBR is the first sector of a PC's hard drive that the computer looks to before loading the operating system.
Bootrash executes before the OS is loaded, so it avoids any integrity checks done by the OS, FireEye wrote. Since Bootrash's components are stored outside the Windows file system, they're also not scanned by antivirus products.
Those responding to security incidents involving a bootkit "will need tools that can access and search raw disk forensic images for evidence of bootkits," FireEye wrote.
The security firm said it found the bootkit by using a tool from its Mandiant forensics division called Mandiant Intelligent Response (MIR). The tool allows for raw disk access in order to look for persistent malware outside of the OS.
But even if an infection is detected, "re-installing the operating system after a compromise is no longer sufficient."
"System administrators should perform a complete physical wipe of any systems compromised with a bootkit and then reload the operating system," it wrote.