CVE-2026-20100: Cisco Secure Firewall DoS Vulnerability

CVE-2026-20100 Overview

A vulnerability exists in the LUA interpreter of the Remote Access SSL VPN feature within Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Secure Firewall Threat Defense (FTD) Software. This input validation flaw could allow an authenticated, remote attacker with a valid VPN connection to cause the device to reload unexpectedly, resulting in a denial of service (DoS) condition. The vulnerability is classified as CWE-120 (Buffer Copy without Checking Size of Input), indicating improper handling of user-supplied input in the LUA interpreter component.

Critical Impact

Authenticated attackers can exploit this vulnerability to crash Cisco ASA/FTD devices, disrupting VPN services and network connectivity for all users relying on the affected SSL VPN infrastructure.

Affected Products

• Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
• Cisco Secure Firewall Threat Defense (FTD) Software
• Remote Access SSL VPN feature deployments

Discovery Timeline

• 2026-03-04 - CVE-2026-20100 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-20100

Vulnerability Analysis

This vulnerability stems from improper input validation within the LUA interpreter used by the Remote Access SSL VPN feature. When processing HTTP packets from authenticated VPN users, the interpreter fails to properly validate user-supplied input before processing it. This lack of validation allows specially crafted input to trigger a buffer overflow condition (CWE-120), ultimately causing the device to reload unexpectedly.

The vulnerability specifically affects the Remote Access SSL VPN server component and does not impact the management or MUS (Management Unified Services) interfaces. This scope limitation means that while VPN services are at risk, administrative access to the device through other interfaces remains unaffected during exploitation attempts.

Root Cause

The root cause of this vulnerability is the LUA interpreter's trust of user input without proper validation. The interpreter processes HTTP packet data from authenticated VPN sessions without adequate boundary checking, allowing malformed or oversized input to corrupt memory structures. This is a classic buffer copy vulnerability where input size is not verified before being written to a fixed-size buffer, leading to memory corruption and subsequent device instability.

Attack Vector

The attack vector for CVE-2026-20100 requires network access and a valid VPN connection to the target device. An attacker must first authenticate to the Remote Access SSL VPN server, establishing a legitimate VPN session. Once authenticated, the attacker can send specially crafted HTTP packets to the SSL VPN server that exploit the input validation weakness in the LUA interpreter.

The exploitation flow involves:

1. Establishing an authenticated VPN session to the target ASA/FTD device
2. Crafting malicious HTTP packets designed to trigger the buffer overflow
3. Sending the crafted packets to the Remote Access SSL VPN server
4. The LUA interpreter processes the malicious input without validation
5. Buffer overflow occurs, causing memory corruption
6. Device reloads unexpectedly, resulting in denial of service

The network-based attack vector with low complexity makes this vulnerability relatively straightforward to exploit once authentication is achieved. The impact extends beyond the attacker's session, affecting all users connected to the VPN service when the device reloads.

Detection Methods for CVE-2026-20100

Indicators of Compromise

• Unexpected device reloads or crashes on ASA/FTD appliances running Remote Access SSL VPN
• Crash logs referencing the LUA interpreter or SSL VPN subsystem components
• Unusual HTTP traffic patterns from authenticated VPN sessions preceding device failures
• Multiple device reload events occurring in short time intervals

Detection Strategies

• Monitor ASA/FTD syslog messages for unexpected reload events with crashinfo data
• Implement network intrusion detection rules to identify anomalous HTTP traffic to SSL VPN endpoints
• Configure SNMP traps for device reload notifications to enable rapid incident response
• Review VPN session logs for unusual activity patterns from authenticated users

Monitoring Recommendations

• Enable detailed logging for the Remote Access SSL VPN feature to capture HTTP request anomalies
• Configure real-time alerting for device reload events through Cisco Security Manager or similar tools
• Establish baseline metrics for normal VPN traffic patterns to identify deviation
• Implement centralized log collection for correlation analysis across multiple ASA/FTD devices

How to Mitigate CVE-2026-20100

Immediate Actions Required

• Review the Cisco Security Advisory for specific patch information and affected versions
• Identify all ASA and FTD devices in your environment running Remote Access SSL VPN features
• Prioritize patching based on exposure and criticality of affected VPN services
• Implement enhanced monitoring on vulnerable devices until patches can be applied

Patch Information

Cisco has released security updates to address this vulnerability. Organizations should consult the Cisco Security Advisory for specific version information and upgrade paths. The advisory contains detailed guidance on determining whether your software version is affected and identifies the appropriate fixed software releases.

As this vulnerability affects both ASA Software and FTD Software, ensure that patching efforts cover all applicable platforms in your environment. Coordinate maintenance windows to minimize service disruption during the upgrade process.

Workarounds

• Restrict VPN access to trusted IP ranges using access control lists where operationally feasible
• Implement additional authentication factors to limit the pool of potentially authenticated attackers
• Consider deploying redundant ASA/FTD devices in high-availability configurations to minimize DoS impact
• Monitor for suspicious VPN user activity and implement session limits where appropriate

# Example: Configure access-list to restrict VPN access to trusted networks
access-list VPN_RESTRICT extended permit ip 10.0.0.0 255.0.0.0 any
access-list VPN_RESTRICT extended deny ip any any log

# Apply to VPN group policy
group-policy DfltGrpPolicy attributes
 vpn-filter value VPN_RESTRICT