CVE-2026-20064: Cisco Secure Firewall FTD DoS Vulnerability

CVE-2026-20064 Overview

A vulnerability in Cisco Secure Firewall Threat Defense (FTD) Software could allow an authenticated, local attacker to cause the device to unexpectedly reload, resulting in a denial of service (DoS) condition. This vulnerability is due to improper validation of user-supplied input at the CLI prompt. An attacker with a low-privileged account could exploit this vulnerability by using crafted commands at the CLI prompt, causing the device to reload.

Critical Impact
Successful exploitation allows low-privileged local attackers to trigger device reloads, disrupting network security operations and potentially leaving networks unprotected during the downtime.

Affected Products

Cisco Secure Firewall Threat Defense (FTD) Software

Discovery Timeline

2026-03-04 - CVE CVE-2026-20064 published to NVD
2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-20064

Vulnerability Analysis

This vulnerability stems from improper input validation within the Cisco Secure Firewall Threat Defense (FTD) Software CLI interface. The root cause is classified as CWE-476 (NULL Pointer Dereference), which occurs when the software dereferences a pointer that it expects to be valid but is actually NULL. When a low-privileged user submits specially crafted input through the CLI, the software fails to properly validate this input before processing, leading to a NULL pointer dereference condition that causes the device to crash and reload.

The local attack vector requires the attacker to have authenticated access to the FTD device's command-line interface, though only low-level privileges are needed. The vulnerability affects the changed scope, meaning the impact extends beyond the vulnerable component itself. While there is no impact to confidentiality or integrity, the availability impact is high due to the complete device reload.

Root Cause

The vulnerability is caused by improper validation of user-supplied input within the CLI processing functions. When certain crafted commands are submitted, the software encounters a NULL pointer dereference condition (CWE-476). This occurs because the input handling routines do not adequately check whether pointer references are valid before attempting to dereference them, leading to an application crash that forces the entire device to reload.

Attack Vector

The attack requires local, authenticated access to the FTD device's CLI. An attacker with even low-privileged credentials can craft malicious CLI commands designed to trigger the NULL pointer dereference condition. When these commands are processed by the vulnerable software, the pointer validation failure causes the device to crash and initiate an unexpected reload.

The exploitation mechanism involves submitting specially formatted input through the CLI that exploits the lack of proper pointer validation. The specific command syntax and parameters that trigger this condition would allow the attacker to reliably cause device reloads on demand, potentially as part of a sustained denial of service attack against the organization's firewall infrastructure.

Detection Methods for CVE-2026-20064

Indicators of Compromise

Unexpected device reloads or crashes in Cisco FTD appliances without scheduled maintenance
System logs showing abnormal CLI command execution followed by device restarts
Crash dump files indicating NULL pointer dereference errors in CLI processing modules
Authentication logs showing repeated login attempts by low-privileged users prior to device crashes

Detection Strategies

Monitor Cisco FTD system logs for unexpected reload events and correlate with CLI activity
Implement alerting on repeated device crashes that occur outside of maintenance windows
Review authentication logs for suspicious patterns of low-privileged user CLI access
Deploy network monitoring to detect when FTD devices become temporarily unavailable

Monitoring Recommendations

Configure SNMP traps or syslog alerts for device reload events on all FTD appliances
Establish baseline metrics for device uptime and alert on deviations from normal patterns
Implement centralized log collection for all firewall CLI sessions for forensic analysis
Monitor for changes in user privilege levels or unusual CLI command patterns

How to Mitigate CVE-2026-20064

Immediate Actions Required

Review the Cisco Security Advisory for detailed mitigation guidance and affected versions
Audit all user accounts with CLI access to FTD devices and remove unnecessary privileges
Restrict CLI access to only essential administrative personnel
Implement additional authentication controls for FTD device access

Patch Information

Cisco has published a security advisory addressing this vulnerability. Organizations should consult the Cisco Security Advisory for specific information about affected software versions and available patches. It is recommended to upgrade to a fixed software release as soon as possible to address this vulnerability.

Workarounds

Limit CLI access to only trusted administrators with a documented business need
Implement network segmentation to restrict which systems can access FTD management interfaces
Enable additional logging and monitoring for all CLI sessions to detect exploitation attempts
Consider implementing out-of-band management access to reduce the attack surface

bash

# Example: Restrict management access to specific IP addresses
# Consult Cisco documentation for specific FTD configuration syntax
# Configure management access controls to limit CLI access
# Enable enhanced logging for CLI sessions