CVE-1999-1476 in Pentium
Summary
by MITRE
A bug in Intel Pentium processor (MMX and Overdrive) allows local users to cause a denial of service (hang) in Intel-based operating systems such as Windows NT and Windows 95, via an invalid instruction, aka the "Invalid Operand with Locked CMPXCHG8B Instruction" problem.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 04/20/2026
The vulnerability identified as CVE-1999-1476 represents a critical architectural flaw in the Intel Pentium processor family that specifically affects MMX and Overdrive implementations. This issue stems from a fundamental problem in the processor's handling of certain locked instructions, creating a pathway for malicious exploitation that can result in system-wide denial of service conditions. The vulnerability specifically targets the CMPXCHG8B instruction when executed in a locked state, which is a critical component in multi-threaded operations and system synchronization mechanisms.
The technical root cause of this vulnerability lies in the processor's instruction decoding and execution engine where invalid operand combinations with locked CMPXCHG8B instructions trigger an undefined behavior that causes the processor to enter an unrecoverable state. This flaw operates at the hardware level within the processor's microcode implementation, making it particularly dangerous as it cannot be patched through software updates alone. The vulnerability manifests when a local user executes a specially crafted instruction sequence that combines invalid operands with the locked prefix, causing the processor to hang indefinitely and requiring a complete system reboot to restore functionality.
The operational impact of this vulnerability extends beyond simple system crashes to encompass complete service disruption across Intel-based systems running Windows NT and Windows 95 operating environments. This denial of service condition affects critical system operations including network services, file system operations, and application execution, effectively rendering the affected systems unusable until manual intervention occurs. The vulnerability's exploitation requires minimal privileges since it operates at the processor level, making it particularly dangerous in multi-user environments where local access could be gained through various attack vectors.
From a cybersecurity perspective, this vulnerability demonstrates the critical importance of hardware-level security considerations in system design and the potential for fundamental architectural flaws to create widespread impact across entire processor families. The issue aligns with CWE-119, which addresses "Improper Access to Memory" and specifically relates to memory access violations that can occur due to processor instruction mismanagement. Organizations affected by this vulnerability must implement immediate mitigations including system hardening measures, disabling unnecessary processor features, and implementing robust monitoring solutions to detect potential exploitation attempts. The ATT&CK framework categorizes this vulnerability under the T1499.004 technique for "Network Denial of Service" as it can be leveraged to create system-wide availability issues that affect networked environments and service availability.
This vulnerability represents one of the earliest documented cases of hardware-level denial of service attacks that highlighted the need for comprehensive security testing at the processor architecture level. The impact extends beyond immediate system availability concerns to encompass broader implications for system reliability and trust in hardware implementations. Organizations must consider both immediate remediation steps including processor firmware updates where available and longer-term architectural considerations regarding hardware security design principles. The vulnerability underscores the importance of implementing layered security approaches that account for hardware-level weaknesses while maintaining operational continuity through robust system monitoring and incident response procedures.