Overview

An incorrect permissions assignment vulnerability in the amwrtdrv.sys kernel driver, included with AOMEI Backupper 8.4.0, allows an unprivileged local user to perform arbitrary writes to the physical disk. When Secure Boot is disabled, this can be leveraged to execute arbitrary UEFI-level code before the operating system loads. This allows an attacker to bypass OS-level security controls, including HVCI, EDR solutions, and Microsoft Defender. The attack may also enable capture of BitLocker Volume Master Key (VMK) material, depending on the system's BitLocker configuration.

Description

AOMEI Backupper from AOMEI International Network Limited is designed to provide backup and disaster recovery services. It also helps individuals and businesses to create system images, disk clones, and file backups. AOMEI Backupper is available as a Windows application and can be integrated into enterprise backup workflows or directly used by end users.

CVE-2026-12780: An Incorrect Permission Assignment for Critical Resource (CWE-732) vulnerability in the amwrtdrv.sys kernel driver used by AOMEI Backupper 8.4.0 allows an unprivileged local attacker to achieve UEFI-level arbitrary code execution by directly writing to physical disk devices. The driver creates a world-accessible device object without a security descriptor, therefore allowing any user-mode process to open the device and issue unrestricted write requests. Hence, an attacker can modify disk sectors in the pre-partition gap (LBA 34–2047), inject a malicious UEFI payload, and alter the GPT to reference the payload as an EFI System Partition. The payload can then execute during the UEFI Boot Device Selection (BDS) phase, before operating system security mechanisms are loaded.

Impact

An attacker with unprivileged local access to a system running AOMEI Backupper 8.4.0 can exploit this vulnerability by opening the world-accessible \\.\mwrtdrv\DISK0 device object and sending specially crafted write commands to an arbitrary physical disk. When Secure Boot is disabled, a successful exploitation allows the attacker to inject UEFI code that executes before the Windows kernel loads, completely bypassing kernel-mode security features including Hyper-V Code Integrity (HVCI), Endpoint Detection and Response (EDR) solutions, Windows Defender, and Hyper-V isolation. On systems using BitLocker with TPM-only protection, this attack vector enables evil maid attacks whereby VMK credentials can be captured during the pre-boot phase Boot Device Selection (BDS) phase.

Solution

Please see the Vendor Information section for patches provided by AOMEI International Network Limited to address this issue. CERT/CC recommends that AOMEI Backupper users update to a version that includes the corrected amwrtdrv.sys driver and implements appropriate access controls.

Users who cannot immediately apply the available update should consider uninstalling AOMEI Backupper. Alternatively, users may disable the amwrtdrv.sys service by changing its start type from AUTO_START to disabled. Enabling Secure Boot in UEFI firmware settings provides additional defense in depth by requiring signed bootloaders, but it does not address the underlying driver vulnerability.

Acknowledgements

Thank you to SiCk / afflicted.sh for reporting this vulnerability. This document was written by Vijay Sarvepalli.

Vendor Information

References

• https://aomeitech.com
• https://learn.microsoft.com/en-us/windows/security/application-security/application-control/app-control-for-business/design/microsoft-recommended-driver-block-rules
• https://learn.microsoft.com/en-us/windows/win32/secauthz/security-descriptor-definition-language