CVE-2024-20407 in Firepower Threat Defense
Summary
by MITRE • 10/23/2024
A vulnerability in the interaction between the TCP Intercept feature and the Snort 3 detection engine on Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to bypass configured policies on an affected system. Devices that are configured with Snort 2 are not affected by this vulnerability.
This vulnerability is due to a logic error when handling embryonic (half-open) TCP connections. An attacker could exploit this vulnerability by sending a crafted traffic pattern through an affected device. A successful exploit could allow unintended traffic to enter the network protected by the affected device.
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Analysis
by VulDB Data Team • 08/05/2025
The vulnerability identified as CVE-2024-20407 represents a critical security flaw within Cisco Firepower Threat Defense software that specifically impacts systems utilizing Snort 3 detection engine configurations. This weakness stems from a fundamental logic error in how the TCP Intercept feature interacts with the Snort 3 engine when processing embryonic or half-open TCP connections. The flaw exists exclusively in FTD devices configured with Snort 2 detection engine, making it a targeted issue for organizations running the newer Snort 3 framework. The vulnerability creates a bypass mechanism that allows unauthorized network access despite active policy configurations, fundamentally undermining the security posture of affected systems. This type of vulnerability falls under CWE-284, which addresses improper access control mechanisms, and specifically relates to improper handling of network connection states within security appliances.
The technical implementation of this vulnerability occurs when the TCP Intercept feature fails to properly validate connection states during the embryonic phase of TCP handshakes. During normal network operations, TCP connections progress through established states before becoming fully operational, but embryonic connections exist in a transitional state where they are not yet fully established. The logic error in the Snort 3 engine causes it to incorrectly process these half-open connections, allowing malicious traffic patterns to bypass the intended policy enforcement mechanisms. Attackers can exploit this by crafting specific traffic sequences that trigger the flawed connection handling logic, effectively creating a tunnel through the security controls that should prevent such traffic from entering the protected network. This exploitation technique aligns with ATT&CK tactic TA0011 (Command and Control) and technique T1071.004 (Application Layer Protocol: DNS) as attackers could leverage this bypass to establish covert communication channels or exfiltrate data through the compromised connection handling logic.
The operational impact of CVE-2024-20407 extends beyond simple policy bypass, as it creates persistent security gaps that could enable various attack vectors including lateral movement, data exfiltration, and command and control communications. Network administrators face the challenge of maintaining security controls while potentially allowing unauthorized traffic to traverse their protected environments. The vulnerability particularly affects organizations that rely heavily on Firepower appliances for network segmentation and access control, as it undermines the fundamental premise that these devices provide robust policy enforcement. The flaw's impact is amplified because it operates at the network layer where traffic is processed before reaching higher-level security controls, making it a critical point of failure in network defense architectures. Organizations using Snort 3 on their FTD devices must consider this vulnerability as a potential entry point for sophisticated attacks that could compromise their entire network infrastructure.
Mitigation strategies for CVE-2024-20407 require immediate attention from security teams, with the most effective approach being the deployment of official Cisco patches and updates that address the specific logic error in the TCP Intercept and Snort 3 interaction. Network administrators should implement additional monitoring procedures to detect anomalous connection patterns that might indicate exploitation attempts, particularly focusing on unusual embryonic connection handling behaviors. The remediation process should include thorough testing of updated configurations to ensure that the patch does not introduce compatibility issues with existing network policies or performance characteristics. Organizations should also consider implementing temporary network segmentation measures or alternative access controls while applying the official patches. The vulnerability's nature makes it particularly challenging to detect through traditional signature-based methods, requiring behavioral monitoring and anomaly detection systems to identify potential exploitation attempts. Security teams must also review their incident response procedures to account for this specific bypass mechanism, as it could enable attackers to establish persistent access that might not be immediately apparent through standard security monitoring activities.