CVE-2017-12237

What Is Cisco IOS IKEv2?

Internet Key Exchange Version 2 (IKEv2) is the protocol used to establish IPsec VPN tunnels in modern Cisco IOS deployments. Cisco IOS and IOS XE routers terminate VPN connections from remote employees, branch offices, and partner networks using IKEv2. The IKEv2 implementation is internet-facing on most VPN-enabled routers — making DoS vulnerabilities in IKEv2 particularly dangerous, as the IKEv2 port (UDP 500, 4500) must be accessible from untrusted networks to function.

Overview

CVE-2017-12237 is a denial-of-service vulnerability in the IKEv2 module of Cisco IOS and IOS XE. An unauthenticated remote attacker can cause high CPU utilization, traceback messages, or a device reload by sending specially crafted IKEv2 packets to a VPN-enabled Cisco router. Because IKEv2 endpoints are by design internet-accessible, this vulnerability is exploitable from any internet-connected attacker without network proximity requirements. Patched in the Cisco September 2017 advisory bundle. CISA added CVE-2017-12237 to the KEV catalog in March 2022.

Affected Versions

Cisco IOS and IOS XE versions with IKEv2 enabled — use the Cisco IOS Software Checker with advisory cisco-sa-20170929-ikev2 for specific version identification.

Technical Details

Root Cause: IKEv2 Resource Management Flaw

CVE-2017-12237 is a resource management vulnerability (CWE-399) in the Cisco IOS IKEv2 module. When processing certain crafted IKEv2 packets during the key exchange negotiation phase, the IKEv2 module encounters a resource management error — either exhausting CPU resources (causing traceback messages and high CPU) or triggering a watchdog reset that causes a device reload. An unauthenticated attacker can continuously send crafted IKEv2 packets to sustain the DoS condition.

VPN infrastructure criticality:

• A reloading VPN gateway drops all active IPsec sessions from remote users and branch offices
• Business continuity depends on VPN availability for remote workers and branch connectivity
• IKEv2 endpoints are internet-facing by design, making them accessible to global attackers

Attack Characteristics

Exploitation Context

• VPN infrastructure targeting: Nation-state actors targeting network infrastructure specifically focus on VPN endpoints as high-value targets — disrupting VPN infrastructure disconnects remote workers and branch offices, creating operational disruption during the attack period
• Internet exposure: Unlike internal router vulnerabilities requiring network proximity, IKEv2 DoS vulnerabilities can be exploited from any internet location against any Cisco VPN router with an internet-facing address

Remediation

1. Apply Cisco IOS security update — identify affected IOS/IOS XE versions via cisco-sa-20170929-ikev2 and apply the patched IOS version.

2. Deploy redundant VPN infrastructure — deploy multiple VPN gateway devices in a high-availability cluster so that a single device reload does not cause a complete VPN outage.

3. Rate-limit IKEv2 traffic — implement rate limiting on IKEv2 packets at the perimeter to reduce the impact of DoS attempts:

   crypto isakmp aggressive-mode disable
   

4. Monitor IKEv2 CPU utilization — alert on unusual CPU spikes on VPN gateway devices that may indicate exploitation attempts; correlate with unexpected IKEv2 connection attempts from unknown sources.