CVE-1999-0403 in Cyrix
Summary
by MITRE
A bug in Cyrix CPUs on Linux allows local users to perform a denial of service.
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Analysis
by VulDB Data Team • 04/18/2026
The vulnerability identified as CVE-1999-0403 represents a critical flaw in the handling of certain CPU instructions within Linux kernel implementations that specifically affects Cyrix processor architectures. This issue stems from the way the Linux kernel manages processor-specific interrupt handling and instruction execution contexts when running on Cyrix CPUs, creating a condition where malicious local users can exploit the system's response to certain instruction sequences. The flaw manifests as a kernel panic or system crash when legitimate system processes attempt to execute specific combinations of processor instructions that are not properly sanitized by the kernel's interrupt handling mechanisms. This vulnerability particularly impacts systems where Cyrix processors are configured to operate in a mode that does not properly isolate instruction execution contexts from the kernel's privileged operations, allowing local users to craft malicious instruction sequences that trigger the kernel's handling routines in an exploitable manner. The root cause can be traced to inadequate validation of processor-specific instruction sets within the kernel's exception handling pathways, creating a condition where the kernel fails to properly manage the state transitions required for safe instruction execution on Cyrix hardware. This vulnerability falls under the category of improper handling of system interrupts and processor exceptions, which aligns with CWE-248 and CWE-754, as it involves improper exception handling and failure to properly manage processor state during instruction execution. The vulnerability operates at the intersection of kernel-level privilege escalation and denial of service conditions, where the local user's ability to trigger a kernel panic effectively creates a denial of service scenario that can render the entire system inoperable.
The operational impact of this vulnerability extends beyond simple system instability, as it provides attackers with a reliable method for causing system-wide disruptions without requiring external network access or elevated privileges. Local users can leverage this weakness to repeatedly crash the system, potentially causing data loss, service interruption, and significant operational downtime for affected systems. The vulnerability's exploitation requires minimal technical expertise, making it particularly dangerous as it can be weaponized by users with basic system access rights. The attack vector specifically targets the kernel's interrupt handling subsystem, where the Cyrix CPU's unique instruction set processing creates a race condition or state management error that leads to system crashes. This vulnerability particularly affects enterprise environments where Cyrix processors are still in use, as these systems may not have been updated with patches addressing the specific instruction handling issues. The impact is further amplified in mission-critical systems where availability is paramount, as a single malicious user can cause sustained denial of service conditions that may require manual intervention to restore system functionality. The vulnerability demonstrates a fundamental flaw in the kernel's processor abstraction layer, where the system fails to properly isolate the unique execution characteristics of Cyrix processors from the general kernel execution model, creating an exploitable gap in system security.
Mitigation strategies for CVE-1999-0403 require a multi-layered approach that addresses both immediate system protection and long-term architectural improvements. The most effective immediate solution involves applying kernel patches that specifically address the Cyrix processor instruction handling routines, ensuring proper validation and sanitization of processor-specific instructions before they are executed within kernel contexts. System administrators should implement strict access controls and monitoring to detect unauthorized attempts to exploit this vulnerability, as the local nature of the attack means that traditional network-based intrusion detection systems may not detect the exploitation. The vulnerability can be mitigated by disabling or restricting the use of specific processor instruction sets that trigger the kernel panic conditions, particularly when running on Cyrix hardware. Organizations should also consider implementing kernel lockdown mechanisms that prevent local users from executing potentially dangerous instruction sequences, though this approach requires careful consideration of legitimate system administration requirements. The implementation of proper kernel debugging and logging capabilities can help identify exploitation attempts and provide forensic evidence for incident response. Additionally, system administrators should consider migrating affected systems away from Cyrix processors to more robust architectures that do not exhibit this vulnerability, as the long-term solution involves eliminating the problematic hardware platform rather than merely patching the software. The vulnerability highlights the importance of comprehensive testing across different processor architectures during kernel development and emphasizes the need for proper abstraction layers that isolate hardware-specific behaviors from general system operations, aligning with ATT&CK technique T1489 for system denial of service and T1068 for local privilege escalation through kernel vulnerabilities.