Pre-OS Boot: Bootkit

Adversaries may use bootkits to persist on systems. Bootkits reside at a layer below the operating system and may make it difficult to perform full remediation unless an organization suspects one was used and can act accordingly.

A bootkit is a malware variant that modifies the boot sectors of a hard drive, including the Master Boot Record (MBR) and Volume Boot Record (VBR). [1] The MBR is the section of disk that is first loaded after completing hardware initialization by the BIOS. It is the location of the boot loader. An adversary who has raw access to the boot drive may overwrite this area, diverting execution during startup from the normal boot loader to adversary code. [2]

The MBR passes control of the boot process to the VBR. Similar to the case of MBR, an adversary who has raw access to the boot drive may overwrite the VBR to divert execution during startup to adversary code.

ID: T1542.003
Sub-technique of:  T1542
Tactics: Persistence, Defense Evasion
Platforms: Linux, Windows
Permissions Required: Administrator, SYSTEM
Data Sources: API monitoring, MBR, VBR
Defense Bypassed: Anti-virus, File monitoring, Host intrusion prevention systems
Version: 1.0
Created: 19 December 2019
Last Modified: 07 May 2020

Procedure Examples

Name Description
APT28

APT28 has deployed a bootkit along with Downdelph to ensure its persistence on the victim. The bootkit shares code with some variants of BlackEnergy.[11]

APT41

APT41 deployed Master Boot Record bootkits on Windows systems to hide their malware and maintain persistence on victim systems.[12]

BOOTRASH

BOOTRASH is a Volume Boot Record (VBR) bootkit that uses the VBR to maintain persistence.[1][5][6]

FinFisher

Some FinFisher variants incorporate an MBR rootkit.[7][8]

Lazarus Group

Lazarus Group malware WhiskeyAlfa-Three modifies sector 0 of the Master Boot Record (MBR) to ensure that the malware will persist even if a victim machine shuts down.[9][10]

ROCKBOOT

ROCKBOOT is a Master Boot Record (MBR) bootkit that uses the MBR to establish persistence.[5]

Mitigations

Mitigation Description
Boot Integrity

Use Trusted Platform Module technology and a secure or trusted boot process to prevent system integrity from being compromised. [3] [4]

Privileged Account Management

Ensure proper permissions are in place to help prevent adversary access to privileged accounts necessary to install a bootkit.

Detection

Perform integrity checking on MBR and VBR. Take snapshots of MBR and VBR and compare against known good samples. Report changes to MBR and VBR as they occur for indicators of suspicious activity and further analysis.

References