CVE-2003-0187 in IPTables
Summary
by MITRE
The connection tracking core of Netfilter for Linux 2.4.20, with CONFIG_IP_NF_CONNTRACK enabled or the ip_conntrack module loaded, allows remote attackers to cause a denial of service (resource consumption) due to an inconsistency with Linux 2.4.20's support of linked lists, which causes Netfilter to fail to identify connections with an UNCONFIRMED status and use large timeouts.
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Analysis
by VulDB Data Team • 03/08/2021
The vulnerability described in CVE-2003-0187 represents a critical flaw in the Linux kernel's netfilter framework that affects systems running kernel version 2.4.20 and potentially other versions in the 2.4.x series. This issue specifically targets the connection tracking subsystem which is responsible for maintaining state information about network connections passing through the system. The vulnerability manifests when the ip_conntrack module is either compiled into the kernel with CONFIG_IP_NF_CONNTRACK enabled or dynamically loaded, creating a scenario where the system's ability to properly track and manage network connections becomes compromised. The flaw exists in the interaction between the connection tracking core and the kernel's linked list implementation, which is a fundamental data structure used throughout the operating system for organizing and managing various resources. This particular inconsistency in the kernel's handling of linked lists creates a condition where the system cannot properly identify connections that have not yet been confirmed, leading to improper resource management and ultimately system instability.
The technical root cause of this vulnerability lies in how the netfilter subsystem handles connections in an UNCONFIRMED state within the context of the kernel's linked list operations. When a connection enters the UNCONFIRMED state, it should be subject to specific timeout mechanisms that ensure these connections do not consume system resources indefinitely. However, due to the linked list inconsistency in kernel 2.4.20, the connection tracking code fails to properly recognize these unconfirmed connections, causing the system to maintain them indefinitely with large timeout values. This results in a gradual consumption of system resources as the number of unconfirmed connections grows without proper cleanup. The flaw specifically impacts the kernel's ability to perform efficient resource management because the connection tracking subsystem cannot distinguish between legitimate active connections and those that have become stale or invalid due to network conditions. This mismanagement creates a resource exhaustion scenario where the system's memory and connection tracking tables become filled with stale entries that should have been cleared according to normal connection tracking protocols.
The operational impact of CVE-2003-0187 is severe and directly translates to a denial of service condition that can render affected systems unusable. Attackers can exploit this vulnerability by sending specially crafted network packets that trigger the creation of unconfirmed connections, causing the system to allocate resources for tracking these connections without proper cleanup mechanisms. The large timeouts associated with these unconfirmed connections mean that even after the initial attack, the system continues to consume resources for extended periods, potentially leading to complete system exhaustion. This vulnerability is particularly dangerous because it affects the core networking functionality of the Linux kernel, which is fundamental to most server operations and network infrastructure. The impact extends beyond simple resource consumption to potentially affect system responsiveness, network availability, and overall system stability. Organizations relying on Linux-based systems for network services, firewalls, or routing functions would be particularly vulnerable to this attack vector, as it can effectively disable critical network infrastructure without requiring privileged access or complex exploitation techniques.
Mitigation strategies for CVE-2003-0187 focus on both immediate defensive measures and long-term system hardening approaches. The most direct solution involves upgrading to a patched kernel version that addresses the linked list inconsistency in the connection tracking subsystem, which would typically be available in kernel versions subsequent to 2.4.20. System administrators should also consider implementing connection tracking limits through kernel parameters that restrict the maximum number of connections that can be tracked simultaneously, effectively preventing resource exhaustion even if the underlying vulnerability exists. Additionally, network administrators can implement rate limiting and connection tracking timeouts at the network level to reduce the impact of potential attacks. The vulnerability aligns with CWE-129, which describes improper handling of buffer sizes and resource limits, and can be mapped to ATT&CK technique T1499.004 for resource exhaustion attacks. Organizations should also implement monitoring systems to detect unusual connection tracking behavior and establish automated alerts when connection tracking tables approach their limits, providing early warning of potential exploitation attempts. Given the nature of the vulnerability, it is essential to maintain up-to-date kernel versions and regularly review system configurations to ensure that connection tracking parameters are appropriately tuned for the specific network environment and traffic patterns.