CVE-2026-20105: Cisco Secure Firewall DoS Vulnerability

CVE-2026-20105 Overview

A vulnerability in the Remote Access SSL VPN functionality of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Secure Firewall Threat Defense (FTD) Software could allow an authenticated, remote attacker with a valid VPN connection to exhaust device memory resulting in a denial of service (DoS) condition. This vulnerability is classified as a Memory Leak (CWE-401) issue, where the affected system fails to properly release allocated memory resources when processing specially crafted VPN packets.

The vulnerability is due to trusting user input without validation. An attacker could exploit this vulnerability by sending crafted packets to the Remote Access SSL VPN server. A successful exploit could allow the attacker to cause the device to reload, resulting in a DoS condition. Notably, this vulnerability does not affect the management or MUS interfaces.

Critical Impact

Authenticated attackers with valid VPN credentials can trigger device reload, disrupting network connectivity for all users relying on the affected firewall for secure remote access.

Affected Products

• Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
• Cisco Secure Firewall Threat Defense (FTD) Software
• Remote Access SSL VPN functionality on affected platforms

Discovery Timeline

• 2026-03-04 - CVE-2026-20105 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-20105

Vulnerability Analysis

This vulnerability stems from improper input validation within the Remote Access SSL VPN processing functionality. The affected software fails to properly validate user-supplied input, leading to a memory leak condition (CWE-401: Missing Release of Memory after Effective Lifetime). When the SSL VPN server processes maliciously crafted packets, memory resources are consumed but not properly deallocated, eventually leading to memory exhaustion.

The attack requires authentication, meaning the attacker must possess valid VPN credentials to establish a connection before attempting exploitation. Once authenticated, the attacker can send specially crafted packets that trigger the memory leak, progressively consuming available system memory until the device is forced to reload. This results in a complete denial of service for all VPN users and potentially disrupts other firewall functions during the reload cycle.

Root Cause

The root cause is a missing release of memory after effective lifetime (CWE-401). The SSL VPN processing code allocates memory to handle incoming packets but fails to properly free this memory under certain conditions when processing malformed or crafted input. This improper resource management allows an attacker to systematically exhaust available memory by repeatedly sending exploit packets.

Attack Vector

The attack is executed over the network against the Remote Access SSL VPN service. The attacker must first authenticate using valid VPN credentials, then send a series of crafted packets to the SSL VPN server. The crafted packets trigger the memory leak condition, and repeated exploitation leads to progressive memory exhaustion until the device is forced to reload.

The vulnerability mechanism involves sending malformed SSL VPN packets that cause the device to allocate memory without proper deallocation. As memory resources become exhausted, the device's stability degrades until it reaches a critical threshold forcing an automatic reload. For technical details regarding specific packet structures and exploitation methodology, refer to the Cisco Security Advisory.

Detection Methods for CVE-2026-20105

Indicators of Compromise

• Unusual memory consumption patterns on ASA/FTD devices, particularly steady increases without corresponding traffic spikes
• Unexpected device reloads or crashinfo files indicating memory exhaustion
• Anomalous SSL VPN session behavior from authenticated users, such as repeated connections with unusual packet patterns
• Log entries indicating memory allocation failures or resource exhaustion events

Detection Strategies

• Monitor system memory utilization on ASA/FTD devices using show memory and show blocks commands for abnormal growth patterns
• Enable syslog monitoring for memory-related warnings and device reload events
• Implement network traffic analysis to detect unusual packet patterns targeting the SSL VPN service
• Configure SNMP traps for memory threshold alerts to provide early warning of potential exploitation

Monitoring Recommendations

• Establish baseline memory utilization metrics and configure alerts for deviations exceeding normal operational thresholds
• Monitor VPN session logs for authenticated users sending unusual volumes of traffic or malformed packets
• Implement regular automated health checks on ASA/FTD devices to detect memory leaks before they cause service disruptions
• Review crashinfo files following any unexpected device reloads to identify memory exhaustion as a potential cause

How to Mitigate CVE-2026-20105

Immediate Actions Required

• Consult the Cisco Security Advisory for the latest patch information and fixed software versions
• Evaluate VPN user accounts and revoke or disable credentials for any suspicious or unnecessary accounts
• Implement rate limiting on SSL VPN connections where possible to slow potential exploitation attempts
• Increase monitoring of ASA/FTD device memory utilization to detect early signs of exploitation

Patch Information

Cisco has released a security advisory addressing this vulnerability. Organizations should review the Cisco Security Advisory to determine the appropriate fixed software version for their deployment. Apply the vendor-recommended patches as soon as possible after appropriate testing in a non-production environment.

Workarounds

• Restrict SSL VPN access to only trusted IP ranges using access control lists (ACLs) to limit potential attacker sources
• Implement additional authentication requirements such as multi-factor authentication to reduce the risk of credential compromise
• Consider temporarily disabling Remote Access SSL VPN functionality if not operationally critical while awaiting patch deployment
• Configure scheduled device memory monitoring and automatic alerts to enable rapid response to potential exploitation attempts

# Example: Configure memory threshold monitoring
# Access ASA/FTD CLI and enable memory monitoring
show memory detail
show blocks
# Set up syslog for memory events
logging enable
logging trap warnings
logging host inside 10.1.1.100