CVE-2025-20217: Cisco Secure Firewall FTD DoS Vulnerability

CVE-2025-20217 Overview

A vulnerability exists in the packet inspection functionality of the Snort 3 Detection Engine within Cisco Secure Firewall Threat Defense (FTD) Software that could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device. This vulnerability stems from incorrect processing of traffic that is inspected by an affected device. An attacker could exploit this vulnerability by sending crafted traffic through the affected device, causing it to enter an infinite loop while inspecting traffic, resulting in a DoS condition. The system watchdog will automatically restart the Snort process.

Critical Impact

Unauthenticated remote attackers can disrupt network security inspection capabilities by triggering an infinite loop in the Snort 3 Detection Engine, causing service interruption for traffic inspection on Cisco Secure Firewall devices.

Affected Products

• Cisco Secure Firewall Threat Defense (FTD) Software with Snort 3 Detection Engine
• Devices running vulnerable versions of Cisco FTD Software with packet inspection enabled

Discovery Timeline

• 2025-08-14 - CVE-2025-20217 published to NVD
• 2025-08-15 - Last updated in NVD database

Technical Details for CVE-2025-20217

Vulnerability Analysis

This vulnerability is classified under CWE-835 (Loop with Unreachable Exit Condition), commonly known as an infinite loop vulnerability. The flaw exists within the packet inspection functionality of the Snort 3 Detection Engine, which is a core component responsible for deep packet inspection and threat detection in Cisco Secure Firewall Threat Defense deployments.

When the affected device processes specially crafted network traffic, the Snort 3 Detection Engine enters an infinite loop during the inspection phase. This prevents the engine from completing its traffic analysis and progressing to process subsequent packets. The denial of service condition persists until the system watchdog detects the unresponsive process and automatically restarts the Snort service.

The vulnerability can be exploited remotely without requiring authentication or user interaction, making it particularly dangerous for internet-facing firewall deployments. While the Snort process automatically recovers after the watchdog restart, persistent exploitation could result in repeated service interruptions, effectively degrading the security posture of the affected network.

Root Cause

The root cause of CVE-2025-20217 is improper handling of specific traffic patterns during packet inspection in the Snort 3 Detection Engine. The vulnerable code path contains a loop with an unreachable exit condition (CWE-835) that can be triggered when processing malformed or specially crafted packets. This logic flaw causes the inspection routine to iterate indefinitely without progressing, consuming CPU resources and preventing normal traffic processing.

Attack Vector

The attack is network-based and can be executed by an unauthenticated remote attacker. The exploitation requires sending crafted traffic through a device protected by Cisco Secure Firewall Threat Defense with the Snort 3 Detection Engine enabled for packet inspection.

The attack flow involves:

1. Attacker identifies a target network protected by Cisco FTD with Snort 3
2. Attacker crafts malicious network packets designed to trigger the infinite loop condition
3. Crafted packets are sent through the firewall where they are subjected to deep packet inspection
4. The Snort 3 Detection Engine enters an infinite loop while processing the malicious traffic
5. Traffic inspection halts until the system watchdog restarts the Snort process
6. During the restart period, network security inspection may be degraded

For detailed technical information regarding this vulnerability, refer to the Cisco Security Advisory.

Detection Methods for CVE-2025-20217

Indicators of Compromise

• Unusual Snort process restarts or watchdog events in system logs
• Elevated CPU utilization on the Snort process prior to restart events
• Traffic inspection latency spikes or dropped connections during inspection
• Repeated SNORT3-5-RELOAD or similar process restart messages in syslog

Detection Strategies

• Monitor system logs for Snort 3 Detection Engine crash or restart events
• Implement alerting on abnormal Snort process behavior including high CPU usage patterns
• Configure SNMP traps or syslog alerts for watchdog-triggered process restarts
• Review network traffic patterns for anomalous packets targeting the firewall

Monitoring Recommendations

• Enable detailed logging for the Snort 3 Detection Engine and monitor for loop-related events
• Implement real-time monitoring of firewall health status including process states
• Configure baseline alerts for normal Snort restart frequency to detect exploitation attempts
• Deploy network traffic analysis to identify potential malicious traffic patterns

How to Mitigate CVE-2025-20217

Immediate Actions Required

• Review the Cisco Security Advisory for specific affected versions and patches
• Identify all Cisco Secure Firewall Threat Defense devices in your environment using Snort 3
• Plan and schedule patch deployment following Cisco's remediation guidance
• Increase monitoring on affected devices while awaiting patch deployment

Patch Information

Cisco has released security updates to address this vulnerability. Administrators should consult the Cisco Security Advisory for detailed information on affected software versions and the availability of fixed releases. It is strongly recommended to upgrade to a patched version of Cisco Secure Firewall Threat Defense Software as soon as possible.

Workarounds

• Review Cisco's security advisory for any available temporary mitigations or configuration changes
• Consider implementing rate limiting on traffic sources if exploitation is detected
• Ensure watchdog monitoring is enabled and properly configured for automatic recovery
• Evaluate network segmentation to limit exposure of affected devices to untrusted traffic

# Check current Snort 3 status and version
show snort3 status
show version

# Review Snort process restart history
show process cpu-usage
show log | include SNORT