CVE-1999-1463 in Windows
Summary
by MITRE
Windows NT 4.0 before SP3 allows remote attackers to bypass firewall restrictions or cause a denial of service (crash) by sending improperly fragmented IP packets without the first fragment, which the TCP/IP stack incorrectly reassembles into a valid session.
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Analysis
by VulDB Data Team • 04/16/2026
This vulnerability in Windows NT 4.0 before Service Pack 3 represents a critical flaw in the TCP/IP stack implementation that fundamentally undermines network security and system stability. The issue stems from how the operating system handles IP packet reassembly, specifically when fragments arrive without the initial segment of a packet sequence. This weakness creates a pathway for malicious actors to either circumvent firewall protections or deliberately crash systems through carefully crafted network traffic patterns.
The technical root cause lies in the improper handling of fragmented IP packets within the Windows NT 4.0 TCP/IP protocol stack. When network packets are fragmented across multiple segments for transmission, the operating system is designed to reassemble them into their original form before processing. However, the vulnerability occurs when attackers send IP fragments that omit the first fragment in the sequence, a scenario that the system's reassembly logic fails to properly validate or handle. This flaw falls under the CWE-129 weakness category, which encompasses issues related to improper validation of input data during processing, specifically in the context of network protocol implementations.
The operational impact of this vulnerability extends beyond simple denial of service scenarios to encompass serious security implications. Attackers can exploit this weakness to bypass firewall restrictions by crafting fragmented packets that, when reassembled, appear to originate from trusted sources or contain legitimate traffic patterns. This capability directly violates fundamental network security principles and can enable more sophisticated attacks such as port scanning, service exploitation, or even privilege escalation attempts. The vulnerability also creates a reliable means for denial of service attacks that can crash the target system, making it particularly dangerous in production environments where system availability is critical.
From a cybersecurity perspective, this vulnerability demonstrates the importance of robust input validation and proper error handling in network protocol implementations. The flaw aligns with several ATT&CK techniques including T1071.004 for application layer protocol and T1499.004 for network disruption, highlighting how basic protocol implementation issues can be weaponized for both reconnaissance and attack execution. The vulnerability also reflects broader concerns about the security of legacy systems and the critical importance of timely security updates, as Windows NT 4.0 SP3 was specifically designed to address this and similar issues.
Organizations affected by this vulnerability should implement immediate mitigations including applying the appropriate service pack, configuring network firewalls to drop fragmented packets, and implementing intrusion detection systems that can identify suspicious packet patterns. The recommended approach involves both reactive measures to address the immediate threat and proactive strategies to prevent similar issues in other network infrastructure components. This vulnerability serves as a stark reminder of the critical importance of maintaining up-to-date security patches and the potential consequences of running unsupported legacy systems in modern network environments.