CVE-2019-20908 in Linux
Summary
by MITRE
An issue was discovered in drivers/firmware/efi/efi.c in the Linux kernel before 5.4. Incorrect access permissions for the efivar_ssdt ACPI variable could be used by attackers to bypass lockdown or secure boot restrictions, aka CID-1957a85b0032.
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Analysis
by VulDB Data Team • 07/16/2020
The vulnerability identified as CVE-2019-20908 resides within the Linux kernel's EFI firmware interface implementation, specifically in the drivers/firmware/efi/efi.c file. This flaw represents a critical security weakness that affects Linux systems running kernel versions prior to 5.4. The issue manifests through improper access controls for the efivar_ssdt ACPI variable, which serves as a critical interface point for EFI firmware operations. The vulnerability allows unauthorized modification of system firmware variables that should normally be protected from user-space manipulation, creating a potential pathway for privilege escalation and system compromise.
The technical nature of this vulnerability stems from inadequate permission checking mechanisms within the EFI variable handling code. The efivar_ssdt variable is designed to store ACPI tables that define system hardware configuration and power management behaviors. When access permissions are incorrectly configured, attackers can manipulate this variable to inject malicious ACPI code or modify existing firmware behavior. This flaw directly impacts the kernel's lockdown mechanism, which is intended to prevent modification of critical system components during boot and runtime operations. The vulnerability creates a direct bypass path for secure boot restrictions that are fundamental to preventing unauthorized firmware modifications and maintaining system integrity.
From an operational perspective, this vulnerability poses significant risks to system security and integrity. Attackers with local access can exploit this weakness to bypass kernel lockdown protections that are designed to prevent unauthorized modification of kernel memory and firmware interfaces. The impact extends beyond simple privilege escalation to potentially enable complete system compromise through firmware-level attacks. This vulnerability aligns with CWE-284, which addresses improper access control, and represents a classic example of how insufficient access controls in kernel space can lead to privilege escalation and system compromise. The flaw particularly affects systems that rely on secure boot implementations and kernel lockdown features for protection against malicious firmware modifications.
The exploitation of this vulnerability demonstrates the importance of proper access control implementation in kernel-level firmware interfaces. The issue connects to ATT&CK technique T1068, which covers local privilege escalation through kernel exploits, and T1542.001, which involves system binary proxy execution. Organizations should prioritize immediate patching to kernel versions 5.4 and later, where the access control mechanisms have been properly implemented. Additional mitigations include disabling unnecessary EFI variables, implementing proper system hardening measures, and monitoring for unauthorized modifications to EFI firmware variables. The vulnerability underscores the critical need for robust access control verification in kernel space, particularly for interfaces that interact with low-level system firmware components that are essential for maintaining system security posture.