CVE-2002-2438 in Linux
Summary
by MITRE • 05/18/2021
TCP firewalls could be circumvented by sending a SYN Packets with other flags (like e.g. RST flag) set, which was not correctly discarded by the Linux TCP stack after firewalling.
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Analysis
by VulDB Data Team • 05/22/2021
The vulnerability described in CVE-2002-2438 represents a significant flaw in the Linux TCP stack's handling of packet filtering mechanisms, specifically affecting how firewalls process TCP SYN packets with additional flags. This issue stems from the improper discard of packets that contain SYN flags alongside other TCP control flags such as RST, which should typically be handled differently in the network stack. The vulnerability exposes a critical gap in the packet filtering logic that allows malicious actors to bypass firewall protections designed to block unauthorized network access.
The technical root cause of this vulnerability lies in the Linux kernel's TCP implementation where firewall rules are processed but not consistently applied to all packet types. When a firewall encounters a TCP SYN packet with additional flags set, the system fails to properly validate whether the packet combination represents a legitimate network communication or an attempt to circumvent security controls. This flaw creates an opening in the network defense mechanism, allowing attackers to exploit the inconsistent packet handling behavior to gain unauthorized access to systems behind the firewall. The vulnerability specifically affects the TCP state machine's processing of packets that do not conform to standard TCP communication patterns, enabling bypass of security measures that rely on proper flag validation.
The operational impact of this vulnerability is substantial as it undermines the fundamental security assumptions of network firewalls. Attackers can exploit this weakness to establish unauthorized connections or bypass access controls that should prevent such communications. The vulnerability particularly affects systems where firewalls are used to restrict incoming connections, as the improper handling of TCP flags allows malicious traffic to slip through security controls. This creates a scenario where network administrators believe their firewall protections are active and effective, while in reality, the system remains vulnerable to connection attempts that should have been blocked. The attack vector is relatively simple to execute and can be automated, making it particularly dangerous in production environments.
Mitigation strategies for this vulnerability require immediate kernel updates and patches from the Linux kernel maintainers, as the fix must address the core TCP stack processing logic. Network administrators should implement additional monitoring and logging of TCP packet flows to detect anomalous flag combinations that might indicate exploitation attempts. The recommended approach includes applying the appropriate kernel security patches that correct the packet validation logic and ensure proper handling of TCP flags in all scenarios. Organizations should also consider implementing more robust intrusion detection systems that can identify suspicious TCP packet patterns and alert administrators to potential exploitation attempts. This vulnerability aligns with CWE-119 which addresses improper restriction of operations within a limited error handling scope, and relates to ATT&CK technique T1046 for network service scanning and T1071 for application layer protocol communication. The fix essentially requires strengthening the TCP packet validation process to ensure that all packet combinations are properly evaluated against firewall rules before being processed by the network stack.