CVE-2026-20049: Cisco Secure Firewall DoS Vulnerability

CVE-2026-20049 Overview

A vulnerability exists in the processing of Galois/Counter Mode (GCM)-encrypted Internet Key Exchange version 2 (IKEv2) IPsec traffic in Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software. This vulnerability could allow an authenticated, remote attacker to cause a denial of service (DoS) condition on an affected device by exploiting an insufficient memory allocation issue.

The vulnerability stems from improper memory allocation when handling GCM-encrypted IPsec traffic. An attacker with valid VPN credentials can send specially crafted packets to trigger an unexpected device reload, effectively disrupting network security operations and VPN connectivity for legitimate users.

Critical Impact

An authenticated attacker can cause affected Cisco ASA and FTD devices to unexpectedly reload, resulting in denial of service and disruption of VPN services and network security functions.

Affected Products

• Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
• Cisco Secure Firewall Threat Defense (FTD) Software
• IKEv2 IPsec VPN implementations using GCM encryption

Discovery Timeline

• 2026-03-04 - CVE-2026-20049 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-20049

Vulnerability Analysis

This vulnerability is classified as CWE-131 (Incorrect Calculation of Buffer Size), indicating that the affected software fails to properly calculate the required memory buffer size when processing GCM-encrypted IKEv2 IPsec traffic. When the device allocates an insufficiently sized memory block for processing encrypted VPN traffic, subsequent operations can overflow the allocated buffer or cause memory corruption, leading to a device crash and reload.

The attack requires network access and valid VPN credentials, meaning the attacker must first authenticate to establish an IKEv2 VPN connection with the target device. Once authenticated, the attacker can send specially crafted GCM-encrypted IPsec packets that trigger the memory allocation flaw, causing the firewall to reload unexpectedly.

The scope of impact extends beyond the vulnerable component itself—when a firewall device reloads, all traffic inspection, VPN tunnels, and security policies are temporarily unavailable, potentially affecting an entire network segment.

Root Cause

The root cause is an incorrect calculation of buffer size (CWE-131) in the IPsec processing code. When handling GCM-encrypted IKEv2 traffic, the software allocates a memory block that is smaller than required for the incoming data. This miscalculation can occur when processing packets with specific characteristics that cause the size calculation to underestimate the necessary buffer, resulting in memory corruption and device instability.

Attack Vector

The attack vector is network-based, requiring the attacker to have valid credentials to establish a VPN connection with the target device. The attack flow is as follows:

1. The attacker authenticates to the Cisco ASA or FTD device using valid VPN credentials to establish an IKEv2 IPsec tunnel
2. Once the VPN session is established with GCM encryption, the attacker crafts malicious IPsec packets designed to exploit the buffer size miscalculation
3. The affected device processes the crafted GCM-encrypted traffic and allocates an insufficient memory buffer
4. The memory allocation error causes device instability, resulting in an unexpected reload
5. All active VPN sessions and firewall functions are disrupted until the device completes its reload cycle

While authentication is required, any user with VPN access credentials—including compromised accounts or insider threats—could potentially exploit this vulnerability.

Detection Methods for CVE-2026-20049

Indicators of Compromise

• Unexpected device reloads or crashes on Cisco ASA or FTD appliances
• Crash logs indicating memory allocation failures or corruption in IPsec processing modules
• Repeated device reloads following patterns of GCM-encrypted IKEv2 VPN traffic
• Anomalous VPN connection patterns from authenticated users followed by device instability

Detection Strategies

• Monitor Cisco ASA/FTD crash logs and reload events for patterns indicating exploitation attempts
• Implement logging and alerting for unusual IPsec traffic patterns from authenticated VPN users
• Configure SNMP traps or syslog alerts for device reload events
• Review VPN session logs for anomalous connection behavior preceding device crashes

Monitoring Recommendations

• Enable detailed logging for IKEv2 IPsec VPN sessions on affected devices
• Implement real-time monitoring for device availability and automatic restart events
• Configure network monitoring to detect service disruptions affecting VPN connectivity
• Establish baseline traffic patterns for VPN users to identify anomalous behavior

How to Mitigate CVE-2026-20049

Immediate Actions Required

• Review the Cisco Security Advisory for specific affected versions and available patches
• Identify all Cisco ASA and FTD devices in your environment that may be vulnerable
• Prioritize patching for devices handling critical VPN infrastructure
• Audit VPN user credentials and access permissions to limit potential attack surface
• Implement network segmentation to minimize the impact of potential device reloads

Patch Information

Cisco has released security updates to address this vulnerability. Administrators should consult the Cisco Security Advisory for detailed information on affected versions and the specific software releases that contain the fix. It is recommended to upgrade to the latest available patched version appropriate for your deployment.

Workarounds

• Evaluate whether GCM encryption can be temporarily replaced with alternative IPsec encryption algorithms if operationally feasible
• Implement strict access controls for VPN credentials to limit the pool of potential attackers
• Deploy additional monitoring and alerting for device stability issues
• Consider implementing redundant firewall configurations to maintain availability during potential exploitation attempts
• Review and restrict VPN access to only essential personnel until patches can be applied

# Example: Enable enhanced logging for IPsec on Cisco ASA
# Consult Cisco documentation for your specific software version
logging enable
logging timestamp
logging trap informational
logging facility 20
logging host inside 192.168.1.100
# Monitor crash logs
show crashinfo