CVE-2002-0514 in OpenBSD
Summary
by MITRE
PF in OpenBSD 3.0 with the return-rst rule sets the TTL to 128 in the RST packet, which allows remote attackers to determine if a port is being filtered because the TTL is different than the default TTL.
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Analysis
by VulDB Data Team • 06/20/2024
The vulnerability described in CVE-2002-0514 affects the packet filter implementation within OpenBSD version 3.0, specifically when utilizing the return-rst rule configuration. This flaw resides in the network filtering mechanism that governs how the system responds to incoming packets, particularly those attempting to establish connections to closed ports. The packet filter operates as a critical security component that controls network traffic flow and implements various rules for packet handling and response. When the return-rst rule is enabled, it instructs the system to send back reset packets to indicate that connections are being refused or filtered.
The technical implementation of this vulnerability stems from the specific handling of the Time To Live field in reset packets generated by the packet filter. When a remote attacker sends a packet to a closed port on a system running OpenBSD 3.0 with the return-rst rule active, the system responds with a reset packet that has its TTL field explicitly set to the value of 128. This behavior contrasts with the typical default TTL values that systems use when sending packets, creating a distinguishable pattern that reveals information about the network configuration. The inconsistency in TTL values between packets sent from the target system and those sent by the attacker creates a detectable signature that can be used for port scanning and network reconnaissance activities.
The operational impact of this vulnerability extends beyond simple information disclosure, as it provides attackers with valuable reconnaissance capabilities that can be leveraged in subsequent attack phases. Network reconnaissance techniques such as TCP port scanning become significantly more effective when attackers can distinguish between open ports, closed ports, and filtered ports based on TTL values. This information allows for more sophisticated attack planning and can be combined with other reconnaissance methods to build comprehensive network maps of target systems. The vulnerability essentially undermines the effectiveness of network filtering and access control mechanisms by revealing the underlying packet filtering behavior to remote attackers. The ability to determine port filtering status through TTL analysis aligns with techniques described in the attack framework, where initial reconnaissance and information gathering form the foundation for more complex exploitation strategies.
From a cybersecurity perspective, this vulnerability demonstrates the importance of careful protocol implementation and the potential for seemingly minor implementation details to create significant security weaknesses. The flaw relates to the principle of least information disclosure, where systems should not reveal information that could aid attackers in their reconnaissance efforts. The vulnerability also connects to broader concepts in network security where defensive measures can inadvertently provide information that benefits offensive operations. Organizations implementing network filtering solutions must consider not only the primary security functions but also the secondary information leakage that can occur through seemingly innocuous protocol behaviors. The issue highlights the necessity of comprehensive security testing that includes analysis of protocol-level behaviors and their potential for information disclosure.
The mitigation strategies for this vulnerability involve modifying the packet filter configuration to ensure that reset packets generated by the system do not reveal information about the underlying network configuration through their TTL values. System administrators should review their packet filter rules and ensure that return-rst configurations do not explicitly set TTL values that differ from standard defaults. Updates to OpenBSD systems to newer versions that address this specific implementation flaw represent the most effective long-term solution. Security practitioners should also consider implementing additional network monitoring and anomaly detection measures to identify potential exploitation attempts. The vulnerability underscores the importance of maintaining current security patches and configurations, as older versions of operating systems often contain implementation flaws that can be exploited by attackers. This case also emphasizes the need for security professionals to understand the low-level network protocols and their implementation details when assessing system security posture and designing defensive strategies.