CVE-2026-20154: Cisco Secure Firewall DoS Vulnerability

CVE-2026-20154 Overview

CVE-2026-20154 is a denial-of-service vulnerability in the system rate-limiting process for syslog message 419002 within Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software. An unauthenticated, remote attacker can trigger high CPU utilization on an affected device by sending a flood of TCP synchronization (SYN) packets. Successful exploitation leads to performance degradation and a potential denial-of-service condition. The weakness is categorized under [CWE-835] (Loop with Unreachable Exit Condition).

Critical Impact
A remote, unauthenticated attacker can exhaust CPU resources on a perimeter firewall, degrading traffic inspection and forwarding for the entire protected network.

Affected Products

Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
Cisco Secure Firewall Threat Defense (FTD) Software
Cisco firewall appliances processing syslog message 419002

Discovery Timeline

2026-09-16 - CVE-2026-20154 published to the National Vulnerability Database
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-20154

Vulnerability Analysis

The vulnerability resides in how Cisco ASA and FTD Software rate-limit syslog message 419002. This syslog message is generated when the device detects duplicate TCP SYN packets carrying different initial sequence numbers for the same connection. The rate-limiting logic intended to suppress repeat log generation does not function correctly under high input rates.

When an attacker sends a sustained flood of TCP SYN packets that trigger syslog 419002 generation, the logging subsystem consumes disproportionate CPU cycles. The device continues processing and formatting log events beyond intended thresholds, starving other packet-processing tasks. The result is performance degradation across all traffic transiting the firewall.

Because the attack targets a control-plane logging function reached through data-plane traffic, exploitation does not require authentication, valid session state, or knowledge of internal topology. The vulnerability affects availability only; confidentiality and integrity of traffic are not directly impacted.

Root Cause

The root cause is improper rate limiting for syslog message 419002. The logging pipeline fails to enforce a bounded generation rate when repeated triggering conditions arrive at high volume, resulting in unbounded log processing consistent with [CWE-835].

Attack Vector

Exploitation requires only network reachability to an interface of the affected device. An attacker generates a high-rate stream of TCP SYN packets crafted to trigger the 419002 syslog condition. No credentials, prior access, or user interaction are needed. See the Cisco Security Advisory for authoritative technical details.

// No verified public proof-of-concept is available.
// Refer to the Cisco Security Advisory for exploitation prerequisites and affected releases.

Detection Methods for CVE-2026-20154

Indicators of Compromise

Sustained high CPU utilization on ASA or FTD devices without a corresponding increase in legitimate connection throughput
Abnormally high volume of syslog message 419002 events emitted from the affected firewall
Unusual bursts of inbound TCP SYN traffic destined to firewall-hosted or transiting services

Detection Strategies

Baseline normal rates of syslog message 419002 and alert on statistical deviations sustained over short windows
Correlate firewall CPU utilization spikes with inbound SYN packet rate and syslog throughput
Deploy network flow analysis at the perimeter to identify SYN flood patterns targeting firewall interfaces

Monitoring Recommendations

Ingest ASA and FTD syslog and SNMP CPU metrics into a centralized analytics platform for correlation
Configure alerting on show cpu usage thresholds exceeding operational baselines
Monitor interface counters for asymmetric SYN-to-SYN-ACK ratios indicative of flooding

How to Mitigate CVE-2026-20154

Immediate Actions Required

Review the Cisco Security Advisory to identify affected releases in your environment
Apply the fixed ASA or FTD software release provided by Cisco as soon as change windows permit
Restrict management and exposed service interfaces to trusted source networks where feasible

Patch Information

Cisco has published guidance in the referenced security advisory cisco-sa-asa-ftd-logging-dos-ZXXNesfN. Consult the advisory for fixed release tables corresponding to your deployed ASA or FTD train and upgrade accordingly.

Workarounds

Suppress or disable syslog message 419002 using no logging message 419002 if operational logging requirements permit
Deploy upstream SYN flood protection such as TCP intercept, connection limits, or scrubbing services
Apply access control lists to drop unsolicited SYN traffic from untrusted sources at the network edge

bash

# Disable generation of syslog message 419002 as a temporary workaround
configure terminal
no logging message 419002
end
write memory