CVE-2024-20342: Cisco Snort Auth Bypass Vulnerability

CVE-2024-20342 Overview

CVE-2024-20342 is a high-severity vulnerability in the rate filtering feature of the Cisco Snort detection engine. The flaw allows an unauthenticated, remote attacker to bypass configured rate limiting filters by sending traffic that exceeds the configured threshold. The vulnerability stems from an incorrect connection count comparison within the Snort rate filter logic. Affected deployments include Cisco Firepower Threat Defense (FTD) Software running vulnerable Snort versions. A successful bypass permits unintended traffic to traverse the affected device and reach the protected network, undermining inspection and policy enforcement controls.

Critical Impact
Remote attackers can bypass Snort rate filters without authentication, allowing unintended traffic into networks protected by Cisco FTD devices.

Affected Products

Cisco Snort detection engine
Cisco Firepower Threat Defense Software 7.1.0
Cisco Firepower Threat Defense Software 7.3.0

Discovery Timeline

2024-10-23 - CVE-2024-20342 published to NVD
2025-08-11 - Last updated in NVD database

Technical Details for CVE-2024-20342

Vulnerability Analysis

The vulnerability resides in the rate filtering subsystem of the Snort detection engine integrated with Cisco Firepower Threat Defense Software. Rate filters in Snort apply thresholds to connection counts and trigger actions such as dropping or alerting when traffic exceeds defined limits. The flaw is categorized under [CWE-1025] Comparison Using Wrong Factors, indicating that the engine evaluates connection counts against the wrong reference value during enforcement. Because Snort sits in the data plane of FTD, a bypass of the rate filter directly affects the security posture of the network behind the device.

Root Cause

The root cause is an incorrect connection count comparison in the rate filter evaluation logic. When the engine compares the observed connection rate to the configured filter threshold, the comparison operates on an inaccurate value. This logic flaw permits traffic that should match the filter to evade the configured action, allowing the session to pass through inspection unimpeded.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker sends traffic through an affected device at a rate exceeding the configured rate filter. Due to the faulty comparison, the filter fails to engage. The attacker gains the ability to push higher-volume traffic into the protected network than policy permits. The vulnerability does not directly compromise confidentiality or availability, but it weakens integrity by allowing policy-violating traffic to reach internal assets. See the Cisco Security Advisory for technical details.

Detection Methods for CVE-2024-20342

Indicators of Compromise

Unexpected traffic volume reaching internal hosts despite active rate filter policies on Cisco FTD
Snort connection event logs showing connection counts exceeding configured thresholds without corresponding filter actions
Anomalous spikes in session counts that do not produce expected rate-filter alerts

Detection Strategies

Correlate firewall flow telemetry with configured rate filter thresholds to identify sessions that should have been throttled
Compare Snort intrusion event logs against policy expectations to flag missing rate-filter triggers
Apply behavioral analytics to ingress traffic patterns to identify volumetric anomalies bypassing inspection

Monitoring Recommendations

Forward Cisco FTD and Snort event logs to a centralized analytics platform for continuous review
Build alerts for traffic patterns that match the conditions configured rate filters were intended to suppress
Monitor for unauthenticated, high-rate connection attempts traversing affected FTD appliances

How to Mitigate CVE-2024-20342

Immediate Actions Required

Identify Cisco FTD deployments running affected Snort versions, including FTD 7.1.0 and 7.3.0
Apply the fixed software release referenced in the Cisco Security Advisory cisco-sa-snort-rf-bypass-OY8f3pnM
Audit existing rate filter configurations and validate their enforcement after patching

Patch Information

Cisco has published guidance and fixed releases through the Cisco Security Advisory cisco-sa-snort-rf-bypass-OY8f3pnM. Administrators should upgrade Cisco Firepower Threat Defense Software to a release that incorporates the corrected Snort rate filter logic. Verify version information using show version on the FTD command line before and after patching.

Workarounds

No workarounds are listed in the Cisco Security Advisory; upgrading is required to remediate the vulnerability
As a compensating control, supplement rate filtering with upstream traffic-shaping or access control policies on perimeter routers
Restrict exposure of inspection paths by tightening interface access policies until patching completes

bash

# Verify Cisco FTD software version and Snort engine version
show version
show snort3 status
# After upgrade, validate rate filter configuration
show running-config | include rate-filter