CVE-2002-0518 in FreeBSD
Summary
by MITRE
The SYN cache (syncache) and SYN cookie (syncookie) mechanism in FreeBSD 4.5 and earlier allows remote attackers to cause a denial of service (crash) (1) via a SYN packet that is accepted using syncookies that causes a null pointer to be referenced for the socket s TCP options, or (2) by killing and restarting a process that listens on the same socket, which does not properly clear the old inpcb pointer on restart.
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Analysis
by VulDB Data Team • 02/11/2017
The vulnerability described in CVE-2002-0518 targets the TCP connection handling mechanisms within FreeBSD operating systems version 4.5 and earlier. This issue specifically affects the SYN cache and SYN cookie implementations that are fundamental components in managing TCP three-way handshakes and preventing SYN flood attacks. The SYN cache mechanism maintains temporary connection state information for incoming SYN packets, while SYN cookies provide an alternative approach to handle connection requests without storing state information in memory. Both mechanisms serve as critical defenses against denial of service attacks by preventing resource exhaustion through malicious SYN packet flooding.
The technical flaw manifests in two distinct scenarios that can lead to system crashes and denial of service conditions. The first vulnerability occurs when the system accepts a SYN packet using the syncookie mechanism and subsequently references a null pointer during socket TCP option processing. This represents a classic null pointer dereference vulnerability that can cause kernel crashes and system instability. The second scenario involves process restart operations on listening sockets where the system fails to properly clear old inpcb (internet protocol control block) pointers during the restart process. This memory management oversight creates dangling pointers that can lead to kernel panics when the system attempts to access freed memory locations.
From an operational perspective, this vulnerability presents a significant risk to FreeBSD systems deployed in production environments where network availability is critical. Attackers can exploit these weaknesses to cause system crashes without requiring authentication or elevated privileges, making them particularly dangerous in environments where unauthorized access is possible. The impact extends beyond simple service disruption as kernel crashes can result in complete system unavailability, data loss, and potential compromise of the underlying system integrity. The vulnerability affects the core networking stack of the operating system, meaning that any service relying on TCP connectivity could be impacted by these crashes.
The security implications of CVE-2002-0518 align with CWE-476 which identifies null pointer dereference as a critical weakness in software systems. This vulnerability also maps to ATT&CK technique T1499.004 which covers network denial of service attacks. The attack vectors leverage fundamental TCP/IP stack mechanisms that are essential for network communications, making this a particularly effective method for causing widespread disruption. Organizations running affected FreeBSD versions face significant risk as the vulnerability can be exploited remotely without requiring special privileges or extensive knowledge of the target system. The impact is amplified by the fact that these mechanisms are active by default and cannot be easily disabled without affecting network functionality.
Mitigation strategies for this vulnerability include immediate upgrading to FreeBSD versions that contain patches addressing these issues, typically FreeBSD 4.6 and later releases. System administrators should also implement network-level protections such as SYN flood detection and mitigation mechanisms, rate limiting for incoming connections, and connection tracking rules that can help detect and prevent exploitation attempts. Additionally, monitoring for unusual patterns in system crashes or network connectivity issues can help identify exploitation attempts. The patching approach addresses both the null pointer dereference issue in the syncookie mechanism and the improper memory cleanup during process restarts. Organizations should also consider implementing redundant systems or failover mechanisms to maintain service availability during the patching process, as the vulnerability can cause complete system crashes that may require manual intervention to restore normal operations.