Update: 10/20/2016 - MBRFilter has been intentionally made difficult to remove to prevent malware from simply disabling or removing this protection during the infection process. Test thoroughly before deploying within production environments.
Ransomware has become increasingly prevalent in the industry, and in many cases, unless there is a publicly released decryptor available, there is often not an easy means of retrieving encrypted files once a system has been infected. In addition to the creation and maintenance of regular system backups, it is increasingly important to focus on a multi-tiered defense-in-depth network architecture in an effort to prevent initial endpoint infection. This is often difficult in an evolving threat landscape where new ransomware families are being developed and deployed seemingly every day by threat actors of varying levels of sophistication.
While many ransomware families focus on the encryption of all or portions of a target system’s files others, such as Petya, rely on overwriting the contents of the Master Boot Record (MBR) to force a system reboot then only encrypt the Master File Table (MFT) of the hard drive on infected systems as a way to coerce users into paying the threat actors to retrieve the encryption keys required to decrypt their files.
To help combat ransomware that attempts to modify the MBR, Talos has released a new tool to the open source community, MBRFilter, a driver that allows the MBR to be placed into a read-only mode, preventing malicious software from writing to or modifying the contents of this section of the storage device.