Back to Blog Cisco Secure Firewall FTD Snort 3 Infinite Loop DoS (CVE-2025-20217): Brief Summary and Patch Guidance A brief summary of CVE-2025-20217, a high-severity infinite loop vulnerability in Cisco Secure Firewall Threat Defense (FTD) Snort 3 Detection Engine. This post outlines technical details, affected versions, patch guidance, and detection methods for security teams. CVE Analysis 9 min read ZeroPath CVE Analysis 2025-08-14 Experimental AI-Generated Content This CVE analysis is an experimental publication that is completely AI-generated. The content may contain errors or inaccuracies and is subject to change as more information becomes available. We are continuously refining our process. If you have feedback, questions, or notice any errors, please reach out to us. [email protected] Introduction A single crafted packet stream can temporarily disable a Cisco Secure Firewall Threat Defense (FTD) deployment, leaving critical network segments unprotected until automated recovery kicks in. CVE-2025-20217 demonstrates how a subtle flaw in packet inspection logic can have outsized operational impact for enterprises and service providers relying on Cisco's Snort 3 Detection Engine. About Cisco and Snort: Cisco is a global leader in networking and security, with its Secure Firewall Threat Defense (FTD) platform deployed in thousands of enterprise, government, and service provider environments. The Snort detection engine is a core component of Cisco's network security stack, powering real-time traffic analysis and threat prevention for millions of networks worldwide. Technical Information CVE-2025-20217 is a vulnerability in the Snort 3 Detection Engine of Cisco Secure Firewall Threat Defense (FTD) Software. The flaw is rooted in incorrect processing of network traffic during packet inspection. Specifically, certain crafted packets can trigger a loop with an unreachable exit condition (CWE-835), causing the Snort process to enter an infinite loop and stop processing further traffic. This results in a denial of service (DoS) until the system watchdog detects the hang and restarts the Snort process. Key technical points: The vulnerability can be exploited remotely and does not require authentication. Attackers can send specially crafted traffic through the affected device to trigger the infinite loop. The flaw impacts the core packet inspection logic of Snort 3, which is responsible for analyzing and filtering network traffic for threats. Once the loop is triggered, the device ceases to inspect traffic, creating a temporary security gap until the process is restarted by the system watchdog. No public code snippets or exploit samples are available for this vulnerability. Patch Information To address the vulnerability in the Snort Detection Engine of Cisco Firepower Threat Defense (FTD) Software and Cisco FirePOWER Services, Cisco has released software updates that rectify the improper handling of TCP/IP network traffic. These updates ensure that the Snort engine processes TCP/IP traffic correctly, preventing the denial of service (DoS) condition that previously resulted from the device dropping legitimate network traffic. Administrators are advised to upgrade their devices to the latest software versions to incorporate these fixes. Detailed instructions for upgrading Cisco FTD devices can be found in the Cisco Firepower Management Center Upgrade Guide. ( sec.cloudapps.cisco.com ) Detection Methods Detecting and mitigating denial-of-service (DoS) attacks, particularly those targeting vulnerabilities in Cisco Firepower Threat Defense (FTD) Software, requires a comprehensive approach. Cisco has implemented several features and configurations to enhance the detection and prevention of such attacks. Port Scan Detection Port scanning is a common reconnaissance technique used by attackers to identify open ports and services on a target system. In Cisco FTD release 7.2, the port scan detection capability was moved from the Snort detection engine to the Lina engine. This transition allows for more effective detection, as the Lina engine has visibility into all scan traffic from a given scanner, including distributed port scans involving multiple scanners and targets. When port scan activity is detected, the Firepower Management Center (FMC) registers intrusion events with specific event types, such as TCP portscan (122:1) and TCP distributed portscan (122:4). These events include pseudo packets that provide detailed information about the scan, including source and destination IP addresses, ports, and data. Administrators can configure the system to actively shun identified scanners, blocking their access for a specified duration. ( secure.cisco.com ) Brute-Force Attack Detection and Prevention Brute-force and password spray attacks aim to gain unauthorized access by systematically attempting various password combinations. To combat these threats, Cisco introduced new threat detection capabilities in Cisco ASA and FTD