CVE-1999-1018 in Linux
Summary
by MITRE
IPChains in Linux kernels 2.2.10 and earlier does not reassemble IP fragments before checking the header information, which allows a remote attacker to bypass the filtering rules using several fragments with 0 offsets.
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Analysis
by VulDB Data Team • 09/11/2024
The vulnerability described in CVE-1999-1018 represents a critical flaw in the IPChains packet filtering implementation within Linux kernels version 2.2.10 and earlier. This issue stems from the improper handling of IP fragment reassembly processes, specifically when the system fails to reconstruct fragmented packets before performing header validation checks. The fundamental problem lies in the timing of security rule enforcement, where filtering mechanisms operate on individual fragments rather than complete reconstructed packets, creating a window of opportunity for malicious actors to exploit the system's defenses. This weakness directly impacts the integrity of network access control policies and undermines the core security assumptions that administrators rely upon when configuring firewall rules.
The technical implementation flaw occurs within the kernel's network stack processing where IP fragments are individually examined for filtering criteria before the complete packet is reassembled. When attackers send multiple IP fragments with zero offset values, they can manipulate the filtering system by ensuring that each fragment individually passes through the rules without triggering any suspicious behavior. This approach exploits the fact that the filtering rules are applied to fragments in isolation rather than considering the complete packet structure that would normally be subject to comprehensive inspection. The vulnerability specifically affects the IPChains module which was the primary packet filtering framework for Linux systems before the adoption of iptables, making it a significant concern for legacy systems and early adopters of Linux networking security.
The operational impact of this vulnerability extends beyond simple bypass of firewall rules, as it fundamentally compromises the security posture of systems relying on IPChains for network protection. Attackers can craft fragmented packets that appear benign when examined individually but collectively form malicious traffic patterns that would otherwise be blocked by proper filtering rules. This technique allows for the circumvention of port filtering, protocol restrictions, and other security measures that depend on accurate packet header inspection. The vulnerability essentially enables a form of packet crafting attack where the attacker can subvert the intended security policy by exploiting the timing gap between fragment reception and complete packet reconstruction, potentially allowing unauthorized access to protected network resources or enabling other malicious activities that would normally be prevented by the firewall configuration.
Mitigation strategies for this vulnerability require immediate system updates to newer kernel versions where the IPChains implementation properly handles fragment reassembly before rule evaluation. Organizations should prioritize upgrading their Linux systems to versions that address this specific flaw, as the vulnerability cannot be effectively patched through configuration changes alone. The recommended approach involves implementing proper kernel version management and security patching procedures to ensure that systems remain protected against known vulnerabilities. Additionally, network administrators should consider implementing redundant security measures such as intrusion detection systems and monitoring solutions that can detect anomalous fragmentation patterns, as well as adopting more modern packet filtering frameworks like iptables that provide better handling of fragmented traffic. This vulnerability aligns with CWE-119 and CWE-121 categories related to improper handling of memory and buffer operations, while also mapping to ATT&CK techniques involving evasion and privilege escalation through network traffic manipulation.