CVE-2005-3774 in PIX
Summary
by MITRE
Cisco PIX 6.3 and 7.0 allows remote attackers to cause a denial of service (blocked new connections) via spoofed TCP packets that cause the PIX to create embryonic connections that that would not produce a valid connection with the end system, including (1) SYN packets with invalid checksums, which do not result in a RST; or, from an external interface, (2) one byte of "meaningless data," or (3) a TTL that is one less than needed to reach the internal destination.
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Analysis
by VulDB Data Team • 06/29/2025
The vulnerability described in CVE-2005-3774 represents a significant denial of service weakness in Cisco PIX firewalls running versions 6.3 and 7.0. This flaw specifically targets the firewall's handling of TCP connection establishment process, creating a scenario where legitimate network traffic becomes blocked due to the accumulation of invalid embryonic connections in the firewall's connection table. The issue stems from the PIX firewall's insufficient validation mechanisms when processing incoming TCP packets that are designed to exploit the connection state machine implementation. From a cybersecurity perspective, this vulnerability aligns with CWE-119, which addresses weaknesses in memory handling, and can be categorized under ATT&CK technique T1498, specifically focusing on network denial of service attacks that target network infrastructure components.
The technical exploitation of this vulnerability occurs through three distinct methods that all leverage the firewall's connection tracking mechanisms. The first method involves sending TCP SYN packets with invalid checksums that fail to generate a proper RST response from the firewall, causing the system to maintain these invalid connections in its embryonic state indefinitely. The second approach utilizes one-byte packets of meaningless data that are sent from external interfaces, creating connection entries that cannot be completed or properly terminated. The third method exploits the Time To Live field in IP packets by sending packets with a TTL value that is exactly one less than what would be required to reach the intended internal destination. This manipulation forces the firewall to establish connection tracking entries that will never successfully complete the TCP handshake process, as the packets cannot reach their intended destination.
The operational impact of this vulnerability is severe for organizations relying on Cisco PIX firewalls for network security. When exploited, the vulnerability results in the exhaustion of connection table resources within the firewall, effectively blocking all new legitimate connections from being established. This creates a cascading effect where network services become unavailable to users, leading to complete service disruption for critical infrastructure components. The attack requires minimal resources from the attacker while generating maximum impact, making it particularly dangerous in enterprise environments where PIX firewalls serve as primary security gateways. The vulnerability essentially transforms the firewall from a security enforcement point into a service disruption vector, undermining its fundamental purpose of protecting network resources.
Organizations can mitigate this vulnerability through several remediation approaches that address both immediate protection and long-term security posture enhancement. The most effective immediate solution involves applying the official Cisco security patches and updates that modify the firewall's TCP connection handling logic to properly validate incoming packets and prevent the creation of invalid embryonic connections. Network administrators should also implement connection rate limiting and connection tracking parameter adjustments to reduce the impact of potential exploitation attempts. Additionally, deploying network monitoring solutions that can detect anomalous TCP packet patterns and connection table exhaustion behaviors provides early warning capabilities. From a defensive perspective, implementing proper network segmentation and access control measures can limit the attack surface, while regular security assessments and vulnerability scanning should be conducted to identify similar weaknesses in network infrastructure components. The mitigation strategies should align with industry best practices for firewall hardening and network security configuration management.