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 software. These features can detect and mitigate:
- Repeated failed authentication attempts to remote access VPN services.
- Client initiation attacks, where an attacker initiates but does not complete connection attempts to a remote access VPN headend multiple times from a single host.
- Connection attempts to invalid remote access VPN services, targeting built-in tunnel groups intended solely for internal device functions.
Administrators can enable these features using specific commands, such as:
threat-detection service invalid-vpn-accessthreat-detection service remote-access-client-initiations hold-down <minutes> threshold <count>threat-detection service remote-access-authentication hold-down <minutes> threshold <count>
These configurations help in identifying and blocking IP addresses exhibiting suspicious behavior, thereby reducing the risk of unauthorized access. (bleepingcomputer.com)
Denial-of-Service (DoS) Attack Detection
While Cisco Firepower devices are not specifically designed to detect Distributed Denial-of-Service (DDoS) attacks, they offer mechanisms to detect and mitigate certain types of DoS attacks. For instance, administrators can configure rate-based attack prevention settings to detect and respond to SYN floods and TCP/IP connection floods. By setting rate-based filters for individual intrusion or preprocessor rules, the system can trigger alerts or take action when defined rate conditions are exceeded. This proactive approach helps in identifying and mitigating potential DoS attacks before they can cause significant disruption. (community.cisco.com)
In summary, Cisco FTD provides robust detection methods for various attack vectors, including port scans, brute-force attempts, and certain DoS attacks. By leveraging these built-in features and configuring them appropriately, administrators can enhance their network's resilience against these threats.
Affected Systems and Versions
- Cisco Secure Firewall Threat Defense (FTD) Software with Snort 3 Detection Engine
- Cisco FirePOWER Services
- Specific affected versions are not provided in the available advisories. Administrators should refer to Cisco's official advisories for detailed version information and ensure all FTD and FirePOWER devices are updated to the latest software releases.
Vendor Security History
Cisco has a history of vulnerabilities in its Snort detection engine and FTD product line, including:
- Multiple denial of service vulnerabilities related to packet inspection and traffic handling
- Bypass vulnerabilities in Snort rule processing
- Regular release of advisories and patches, with generally prompt response times
These recurring issues highlight the complexity of developing secure, high-performance packet inspection engines and the importance of timely patch management for organizations relying on Cisco security products.
References
- NVD Entry for CVE-2025-20217
- Cisco Security Advisory: cisco-sa-ftd-dos-SvKhtjgt
- Cisco Patch Advisory: cisco-sa-sa-ftd-snort-fw-BCJTZPMu
- Cisco Port Scan Detection Documentation
- BleepingComputer: New Cisco ASA and FTD Features Block VPN Brute-Force Password Attacks
- Cisco Community: FTD FMC Snort Rule Detect DDoS Attack