CVE-2026-20249 Overview
CVE-2026-20249 is a denial-of-service vulnerability in the Internet Key Exchange version 2 (IKEv2) certificate authentication feature of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software. An unauthenticated, remote attacker can trigger an unexpected device reload by initiating an IKEv2 VPN connection using a crafted certificate. The flaw is classified under CWE-704: Incorrect Type Conversion or Cast and originates from a logic error during the certificate authentication phase of IKEv2 connection setup.
Critical Impact
Successful exploitation crashes the IKEv2 process and forces the affected firewall to reload, disrupting VPN termination and traffic passing through the device.
Affected Products
- Cisco Secure Firewall Adaptive Security Appliance (ASA) Software with IKEv2 certificate authentication enabled
- Cisco Secure Firewall Threat Defense (FTD) Software with IKEv2 certificate authentication enabled
- Refer to the Cisco Security Advisory for the authoritative list of fixed releases
Discovery Timeline
- 2026-09-16 - CVE-2026-20249 published to NVD
- 2026-09-16 - Last updated in NVD database
Technical Details for CVE-2026-20249
Vulnerability Analysis
The vulnerability resides in the IKEv2 certificate authentication code path used by Cisco ASA and FTD Software when negotiating VPN tunnels. During the IKE_AUTH exchange, the responder processes the initiator's certificate payload to validate identity before completing the tunnel. A logic error in this validation path causes the IKEv2 process to enter an unrecoverable state when a specifically crafted certificate is presented. The process then crashes, and the affected appliance reloads.
Because the flaw is reachable prior to authentication, no valid credentials, pre-shared key, or trust relationship is required. An attacker only needs network reachability to the IKEv2 listener (UDP 500 or 4500) on an interface configured for IKEv2 with certificate authentication. Repeated exploitation produces a sustained denial-of-service condition affecting all traffic traversing the firewall.
Root Cause
The issue is an incorrect type conversion or cast [CWE-704] during parsing or validation of fields inside the attacker-supplied certificate. When the malformed value flows into subsequent authentication logic, the process reaches an invalid state and terminates. Full technical detail is available in the Cisco Security Advisory.
Attack Vector
The attack is network-based and unauthenticated. An attacker initiates an IKEv2 SA_INIT exchange with the target device and proceeds to IKE_AUTH, presenting a crafted X.509 certificate designed to trigger the logic error. No user interaction is required, and the exploit does not depend on prior tunnel establishment or valid certificate trust chains. Devices exposing IKEv2 to untrusted networks for remote-access or site-to-site VPN are directly reachable targets.
Detection Methods for CVE-2026-20249
Indicators of Compromise
- Unexpected traceback or crash entries in ASA/FTD syslog referencing the IKEv2 process
- Unscheduled device reloads correlated with inbound IKEv2 traffic on UDP 500 or 4500
- Repeated IKE_AUTH failures from a single or small set of source addresses immediately preceding a reload
- VPN tunnel flaps affecting multiple peers at the same timestamp
Detection Strategies
- Alert on ASA/FTD syslog message IDs associated with process crashes and unexpected reloads
- Correlate IKEv2 negotiation failures with subsequent loss of firewall telemetry to identify DoS attempts
- Baseline normal IKEv2 peer sources and flag certificate-based IKE_AUTH attempts from unknown initiators
Monitoring Recommendations
- Forward ASA/FTD syslog and NetFlow to a centralized analytics platform and monitor for reload events
- Track NetFlow spikes on UDP 500 and UDP 4500 from external sources, especially short-lived flows that never establish tunnels
- Monitor device uptime counters via SNMP or REST API to detect unplanned restarts
How to Mitigate CVE-2026-20249
Immediate Actions Required
- Inventory all ASA and FTD appliances with IKEv2 certificate authentication enabled and identify internet-exposed interfaces
- Apply the fixed software releases documented in the Cisco Security Advisory
- Restrict IKEv2 access using interface access control lists to known peer IP ranges where feasible
- Enable centralized logging and alerting for process crashes and unexpected device reloads
Patch Information
Cisco has released fixed software for ASA and FTD. Consult the Cisco Security Advisory for the fixed release matrix mapped to affected platforms and train branches. Cisco has not published a permanent workaround; upgrading to a fixed release is the remediation.
Workarounds
- No official software workaround eliminates the vulnerability; upgrade remains the required fix
- Where certificate authentication is not required, switch remote-access and site-to-site tunnels to pre-shared key or EAP-based authentication until patching is complete
- Filter inbound UDP 500 and UDP 4500 at upstream network devices to permit only trusted VPN peer addresses
- Terminate remote-access VPN on a segmented appliance so a crash of the IKEv2 process does not disrupt other firewall functions
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
