CVE-2004-1799 in OpenBSD
Summary
by MITRE
PF in certain OpenBSD versions, when stateful filtering is enabled, does not limit packets for a session to the original interface, which allows remote attackers to bypass intended packet filters via spoofed packets to other interfaces.
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Analysis
by VulDB Data Team • 06/22/2018
The vulnerability described in CVE-2004-1799 represents a critical flaw in the packet filter implementation of OpenBSD operating systems. This issue specifically affects the stateful filtering mechanism within the pf firewall component, which is designed to track and control network sessions based on established connections. When stateful filtering is enabled, pf maintains state information about active connections and should enforce strict boundaries between network interfaces to prevent unauthorized traffic flow. However, this particular vulnerability creates a bypass mechanism that allows malicious actors to circumvent these security controls through the careful crafting of spoofed network packets.
The technical root cause of this vulnerability lies in the improper handling of session boundaries within pf's state tracking system. Normally, when a session is established through one interface, pf should restrict subsequent packets belonging to that session to return through the same interface where the initial connection was accepted. This interface binding ensures that network traffic follows the intended security policies and prevents attackers from using spoofed packets to traverse between interfaces in ways that would otherwise be blocked by the firewall rules. The flaw occurs when pf fails to properly validate that incoming packets for an existing session originate from the correct interface, allowing packets to be accepted even when they appear to come from a different network interface than where the session was originally established.
From an operational perspective, this vulnerability creates significant security implications for OpenBSD systems running pf with stateful filtering enabled. Attackers can exploit this weakness to bypass firewall rules that are designed to restrict traffic flow between different network segments or interfaces. This allows for unauthorized access to systems behind the firewall, potentially enabling lateral movement within a network, data exfiltration, or the establishment of unauthorized communication channels. The vulnerability is particularly dangerous because it operates at the network layer, where it can affect all applications and services running on the compromised system without requiring special privileges or specific application-level exploits. The impact extends beyond simple packet filtering bypasses to potentially enable more sophisticated attacks such as network reconnaissance, man-in-the-middle attacks, or the circumvention of network segmentation policies that are fundamental to modern security architectures.
The security implications of this vulnerability align with several elements of the Common Weakness Enumeration framework, particularly CWE-284 which addresses improper access control, and CWE-308 which covers use of a predictable algorithm for a security token. The vulnerability also maps to techniques described in the MITRE ATT&CK framework under the T1046 category for network service scanning and potentially T1562 for evasion techniques. Organizations affected by this vulnerability should implement immediate mitigations including disabling stateful filtering when spoofing is a concern, implementing additional network segmentation controls, or upgrading to patched versions of OpenBSD. The recommended approach involves either configuring pf to enforce stricter interface binding for sessions or applying the official OpenBSD security patches that address the core flaw in the state tracking mechanism. Additionally, network administrators should conduct thorough audits of their firewall configurations to identify any potential exploitation vectors and consider implementing additional monitoring for unusual traffic patterns that might indicate attempted exploitation of this vulnerability.