CVE-2001-0183 in FreeBSD
Summary
by MITRE
ipfw and ip6fw in FreeBSD 4.2 and earlier allows remote attackers to bypass access restrictions by setting the ECE flag in a TCP packet, which makes the packet appear to be part of an established connection.
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Analysis
by VulDB Data Team • 08/18/2024
The vulnerability described in CVE-2001-0183 represents a significant flaw in the FreeBSD firewall implementations ipfw and ip6fw versions 4.2 and earlier. This security issue stems from improper handling of the Explicit Congestion Notification (ECN) ECE (ECN-Echo) flag within TCP packets, creating a bypass mechanism that allows unauthorized access to protected network resources. The flaw specifically affects the packet filtering logic that determines whether incoming TCP packets should be accepted or rejected based on connection state. When an attacker crafts a TCP packet with the ECE flag set, the firewall incorrectly interprets this packet as belonging to an existing connection rather than as a new connection attempt, thereby circumventing the intended access controls.
The technical implementation of this vulnerability exploits a fundamental weakness in the stateful packet filtering mechanism of FreeBSD's networking stack. The ECE flag is part of the Explicit Congestion Notification RFC 3168 standard designed to signal network congestion without dropping packets, but FreeBSD's ipfw and ip6fw implementations failed to properly validate the context in which this flag appears. When a TCP packet carries the ECE flag, the firewall's connection tracking logic incorrectly assumes the packet is part of an already established session, even though the packet may be attempting to initiate a new connection or access restricted resources. This misinterpretation occurs because the firewall's state machine does not adequately differentiate between legitimate ECN signaling and malicious attempts to exploit the flag for access bypass.
The operational impact of this vulnerability extends beyond simple access control bypass, potentially allowing remote attackers to gain unauthorized network access to systems running affected FreeBSD versions. Attackers can leverage this flaw to establish connections to services that should be restricted or protected by firewall rules, effectively neutralizing the security posture provided by the ipfw and ip6fw implementations. The vulnerability is particularly concerning because it operates at the network layer, making it difficult to detect through traditional application-level monitoring systems. The attack requires minimal privileges and can be executed remotely, making it a low-effort, high-impact vector for unauthorized access. Systems that rely on FreeBSD's built-in firewall capabilities for network segmentation, access control, or security enforcement become vulnerable to this type of attack, potentially exposing sensitive data and services to unauthorized access.
Organizations affected by this vulnerability should immediately implement mitigations including upgrading to FreeBSD versions that contain patches addressing this issue, typically FreeBSD 4.3 or later. The patch mechanism involves modifying the firewall's TCP packet processing logic to properly validate the ECE flag context and prevent its misuse for connection state manipulation. Network administrators should also consider implementing additional monitoring and intrusion detection measures to detect potential exploitation attempts. From a cybersecurity framework perspective, this vulnerability aligns with CWE-284 Access Control Bypass and can be mapped to ATT&CK technique T1071.004 Application Layer Protocol: DNS, as attackers may use this bypass to establish covert communication channels. The vulnerability demonstrates the importance of proper state machine validation in network security implementations and serves as a reminder of the critical need for thorough testing of security features against edge cases and protocol nuances. Organizations should also review their overall firewall configurations and implement defense-in-depth strategies that do not rely solely on a single layer of protection.