CVE-2018-18653 in Linux
Summary
by MITRE
The Linux kernel, as used in Ubuntu 18.10 and when booted with UEFI Secure Boot enabled, allows privileged local users to bypass intended Secure Boot restrictions and execute untrusted code by loading arbitrary kernel modules. This occurs because a modified kernel/module.c, in conjunction with certain configuration options, leads to mishandling of the result of signature verification.
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Analysis
by VulDB Data Team • 06/03/2023
The vulnerability described in CVE-2018-18653 represents a critical security flaw in the Linux kernel implementation of UEFI Secure Boot mechanisms, specifically affecting Ubuntu 18.10 systems. This issue demonstrates how kernel-level security controls can be circumvented through subtle modifications to core kernel components, creating a pathway for privilege escalation and unauthorized code execution. The vulnerability exploits the interaction between modified kernel/module.c code and specific kernel configuration options, fundamentally undermining the trust model that Secure Boot is designed to establish.
The technical root cause of this vulnerability lies in the improper handling of signature verification results within the kernel's module loading mechanism. When UEFI Secure Boot is enabled, the system should enforce strict validation of all kernel modules before loading them into memory. However, the modified kernel/module.c code fails to properly validate the outcomes of signature verification checks, allowing maliciously crafted modules to bypass these security measures. This flaw operates at the kernel level, where the security boundary between trusted and untrusted code becomes compromised, effectively rendering the Secure Boot enforcement mechanism ineffective.
From an operational perspective, this vulnerability creates a severe risk for systems that rely on UEFI Secure Boot as a primary security control. Privileged local users who can modify kernel components or access system resources can exploit this flaw to load arbitrary kernel modules, potentially gaining root-level access to the system. The impact extends beyond simple privilege escalation, as attackers could install persistent backdoors, modify system behavior, or exfiltrate sensitive data. The vulnerability is particularly dangerous because it operates at the kernel level, making it difficult to detect through conventional security monitoring approaches and allowing for deep system compromise.
The exploitation of this vulnerability aligns with several ATT&CK techniques including privilege escalation through kernel modules and persistence mechanisms. It represents a sophisticated attack vector that leverages the legitimate kernel module loading infrastructure to bypass security controls. Organizations should consider implementing additional security controls such as kernel lockdown modes, module signature enforcement, and comprehensive monitoring of kernel module loading activities. The vulnerability also highlights the importance of maintaining secure kernel configurations and regularly updating system components to address known security flaws.
This issue demonstrates the critical importance of proper code review and security testing of kernel modifications, particularly in security-sensitive components. The vulnerability is classified under CWE-284 which relates to improper access control in kernel modules, emphasizing the need for robust validation of security checks. System administrators should ensure that all kernel components are properly signed and verified, and that configuration options related to module loading are carefully controlled. The incident underscores the necessity of maintaining secure boot environments through proper kernel management and regular security assessments to prevent similar vulnerabilities from being exploited in production systems.