CVE-2004-0626 in Linux
Summary
by MITRE
The tcp_find_option function of the netfilter subsystem in Linux kernel 2.6, when using iptables and TCP options rules, allows remote attackers to cause a denial of service (CPU consumption by infinite loop) via a large option length that produces a negative integer after a casting operation to the char type.
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Analysis
by VulDB Data Team • 03/09/2021
The vulnerability described in CVE-2004-0626 represents a critical flaw in the Linux kernel's netfilter subsystem that specifically affects version 2.6 and earlier releases. This issue manifests within the tcp_find_option function which is responsible for processing TCP options in network packets when iptables rules are applied. The vulnerability arises from improper handling of option length parameters during packet inspection, creating a scenario where maliciously crafted TCP packets can trigger unexpected behavior in the kernel's network processing logic. The flaw is particularly dangerous because it can be exploited remotely without requiring any authentication or privileged access, making it an attractive target for denial of service attacks that can consume significant system resources.
The technical root cause of this vulnerability stems from a type casting operation that converts a large unsigned integer value to a signed char type. When the TCP option length exceeds certain thresholds, this casting operation results in a negative integer value that is then used in a loop condition within the tcp_find_option function. The negative value causes the loop to become infinite, as the loop termination condition can never be met due to the sign reversal. This infinite loop consumes excessive CPU cycles and can effectively bring the targeted system to a halt, preventing legitimate network traffic from being processed. The vulnerability specifically impacts systems running Linux kernel versions 2.6 and earlier that utilize iptables for network packet filtering and the netfilter framework for packet handling. The flaw is classified under CWE-191 Integer Underflow/Overflow, which is a well-documented category of vulnerabilities involving improper handling of integer arithmetic operations. This type of vulnerability falls under the ATT&CK technique T1499.004 Network Denial of Service, where adversaries exploit weaknesses in network protocols to exhaust system resources.
The operational impact of this vulnerability extends beyond simple system unavailability as it can be leveraged to create sustained denial of service conditions that are difficult to detect and mitigate. Attackers can craft TCP packets with specifically designed option lengths that trigger the infinite loop, causing the affected system to become unresponsive to legitimate network traffic. The CPU consumption pattern is particularly concerning because it can be sustained for extended periods without requiring additional network activity from the attacker, making detection challenging. Systems that rely heavily on network processing, such as firewalls, routers, and network appliances running Linux, are particularly vulnerable to this attack vector. The vulnerability can also be amplified through distributed denial of service attacks where multiple compromised systems simultaneously send malicious packets to target victims. Organizations using older Linux kernel versions or those unable to patch immediately face significant risk exposure, as the vulnerability can be exploited by anyone with network access to the affected system. The impact is particularly severe in environments where network availability is critical, such as enterprise data centers, cloud infrastructure, and network security appliances that depend on stable kernel operations.
Mitigation strategies for CVE-2004-0626 primarily focus on immediate kernel upgrades to versions that contain the appropriate patches addressing the integer underflow condition in the tcp_find_option function. System administrators should prioritize applying the official kernel security patches released by the Linux kernel development team, which typically involve correcting the type casting operations and adding proper bounds checking to prevent negative values from entering the loop conditions. Network administrators can also implement temporary workarounds such as disabling specific iptables rules that involve TCP option matching or limiting the number of TCP options that can be processed by the netfilter subsystem. Additionally, monitoring network traffic for unusual patterns that might indicate exploitation attempts can help detect attacks before they cause significant system impact. Organizations should also consider implementing rate limiting and connection tracking rules to prevent a single source from overwhelming the system with malicious packets. The vulnerability highlights the importance of maintaining up-to-date system software and following security best practices for kernel management. Regular security assessments and vulnerability scanning should be conducted to identify and remediate similar issues before they can be exploited in production environments. Network segmentation and intrusion detection systems can provide additional layers of protection by monitoring for suspicious traffic patterns that might indicate exploitation attempts.