CVE-2020-14372 in grub2
Summary
by MITRE • 03/04/2021
A flaw was found in grub2 in versions prior to 2.06, where it incorrectly enables the usage of the ACPI command when Secure Boot is enabled. This flaw allows an attacker with privileged access to craft a Secondary System Description Table (SSDT) containing code to overwrite the Linux kernel lockdown variable content directly into memory. The table is further loaded and executed by the kernel, defeating its Secure Boot lockdown and allowing the attacker to load unsigned code. The highest threat from this vulnerability is to data confidentiality and integrity, as well as system availability.
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Analysis
by VulDB Data Team • 10/13/2024
The vulnerability identified as CVE-2020-14372 represents a critical flaw in the GRUB2 bootloader affecting versions prior to 2.06, fundamentally undermining the security assurances provided by Secure Boot mechanisms. This vulnerability resides in the bootloader's handling of ACPI commands when Secure Boot is active, creating an unintended execution path that bypasses critical kernel security protections. The flaw specifically enables privileged attackers to manipulate the Secondary System Description Table (SSDT) structure, which is typically used for hardware-specific firmware interfaces. When the kernel processes these manipulated tables, it executes code that directly modifies the Linux kernel's lockdown variable, effectively neutralizing the Secure Boot protection that should prevent execution of unsigned code. This represents a sophisticated attack vector that exploits the trust relationship between the bootloader and kernel, leveraging legitimate ACPI functionality to achieve malicious code execution.
The technical exploitation of this vulnerability follows a multi-stage attack pattern that begins with an attacker gaining privileged access to the system, typically through local administrative rights or physical access. The attacker crafts a malicious SSDT table containing shellcode designed to overwrite kernel memory locations that control the lockdown mechanism. When the kernel loads and executes this table, it effectively disables the Secure Boot lockdown, creating a persistent backdoor for code injection. This vulnerability directly maps to CWE-119 Improper Access Control and CWE-264 Permissions, Privileges, and Access Controls, as it represents a privilege escalation flaw that allows unauthorized modification of kernel memory. The attack leverages the legitimate ACPI table loading mechanism, making it particularly difficult to detect through traditional security monitoring approaches.
The operational impact of CVE-2020-14372 extends beyond simple code execution to encompass comprehensive data integrity and confidentiality breaches. Once the kernel lockdown is defeated, attackers can load unsigned kernel modules, modify running kernel memory, and potentially escalate privileges to root access. This vulnerability affects system availability through potential denial of service conditions when legitimate kernel protections are bypassed, and it compromises data integrity by enabling persistent backdoors that can remain undetected for extended periods. The threat level is particularly high for enterprise environments where physical security controls may be insufficient, as the vulnerability can be exploited through local administrative access or physical presence. Organizations using affected GRUB2 versions face significant risk of advanced persistent threats that can establish long-term system compromise, as demonstrated by ATT&CK technique T1068 Privilege Escalation and T1014 Rootkit.
Mitigation strategies for CVE-2020-14372 primarily focus on upgrading to GRUB2 version 2.06 or later, which contains the necessary patches to prevent the improper enabling of ACPI commands during Secure Boot execution. System administrators should implement comprehensive patch management procedures to ensure all affected systems receive updates promptly. Additional protective measures include disabling unnecessary ACPI features when Secure Boot is active, implementing kernel lockdown configurations that prevent modification of critical kernel variables, and deploying enhanced monitoring solutions that can detect anomalous SSDT table loading activities. Organizations should also consider implementing hardware security modules and trusted platform modules to strengthen the overall security posture beyond the bootloader level. The vulnerability highlights the importance of maintaining current firmware and bootloader versions, as well as implementing defense-in-depth strategies that protect against multiple attack vectors simultaneously.