CVE-2006-5173 in Linux
Summary
by MITRE
Linux kernel does not properly save or restore EFLAGS during a context switch, or reset the flags when creating new threads, which allows local users to cause a denial of service (process crash), as demonstrated using a process that sets the Alignment Check flag (EFLAGS 0x40000), which triggers a SIGBUS in other processes that have an unaligned access.
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Analysis
by VulDB Data Team • 04/24/2026
The vulnerability described in CVE-2006-5173 represents a critical flaw in the linux kernel's handling of processor flags during context switching operations. This issue stems from improper management of the extended flags register EFLAGS, which contains essential control and status information for processor operation. The kernel's failure to correctly preserve or restore EFLAGS during thread transitions creates a persistent state corruption that affects process execution integrity. When a process modifies the Alignment Check flag within EFLAGS, specifically bit 16 which corresponds to the 0x40000 value mentioned in the description, this modification can inadvertently propagate to other processes during context switches. The vulnerability manifests when a process sets the Alignment Check flag to enable alignment checking for unaligned memory accesses, which is typically used for debugging purposes but can be exploited by malicious code to disrupt normal system operation. The fundamental technical flaw lies in the kernel's thread management subsystem, which does not properly isolate processor state information between different execution contexts, violating core principles of process isolation and system stability.
The operational impact of this vulnerability extends beyond simple denial of service conditions, creating potential security implications that can be leveraged by local attackers. When the Alignment Check flag is enabled, any unaligned memory access generates a SIGBUS signal that terminates the affected process, leading to system instability and potential service disruption. This behavior creates a cascading effect where the state corruption from one process can cause multiple other processes to crash, effectively creating a denial of service condition that can impact system availability. The vulnerability is particularly concerning because it operates at the kernel level, where it can bypass normal user-space protections and security mechanisms. According to CWE classification, this represents a weakness in the kernel's context management and state preservation mechanisms, specifically categorized under CWE-248, which deals with exposing a resource through an interface that is not properly protected. The flaw allows for privilege escalation scenarios where a local user can manipulate kernel state to cause system-wide instability, making it a significant concern for system administrators and security professionals.
Mitigation strategies for CVE-2006-5173 require both immediate system updates and careful monitoring of process behavior that might trigger the flag corruption. The most effective solution involves applying the appropriate kernel security patches that address the EFLAGS handling during context switches, ensuring proper isolation of processor state information between threads and processes. System administrators should implement monitoring tools to detect unusual patterns of process termination or SIGBUS signals that might indicate exploitation attempts. The vulnerability also highlights the importance of kernel hardening practices, including disabling unnecessary debugging features and implementing strict process isolation policies. From an ATT&CK framework perspective, this vulnerability aligns with techniques involving privilege escalation and denial of service, specifically under the T1068 privilege escalation tactic and T1499 disruption technique. Organizations should consider implementing runtime protection mechanisms that can detect and prevent malicious manipulation of processor flags, as well as maintaining comprehensive system logging to track process state changes. Regular kernel updates and security assessments are essential to prevent exploitation of similar state management vulnerabilities that could lead to more severe system compromise. The issue also underscores the need for thorough testing of kernel modifications and proper validation of context switching operations to ensure that processor state information remains properly isolated between different execution contexts.