Cisco IOS and IOS XE IKEv2 Protocol Processing Remote Denial of Service Vulnerability (CVE-2017-12237)
Executive & SecOps Summary
SECOPS ASSESSMENTCisco IOS 15.0 through 15.6 and Cisco IOS XE 3.5 through 16.5 with ISAKMP enabled
Unauthenticated, remote attackers can trigger high CPU utilization, traceback messages, or device reloads (DoS) by sending crafted IKEv2 packets
Apply official Cisco software patches immediately, or restrict UDP 500/4500 traffic at the network boundary using Infrastructure ACLs (iACLs).
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Upgrade to the latest recommended Cisco IOS / IOS XE release that contains the fix for CSCvc41277. Below is an example of upgrading the image via TFTP:
! Verify current boot image and free space
show flash:
show version
! Download the patched IOS XE image from your TFTP server
copy tftp://10.1.1.100/cat3k_caa-universalk9.SPA.03.06.06.E.152-2.E6.bin flash:
! Configure the system to boot from the new image
configure terminal
no boot system
boot system flash flash:cat3k_caa-universalk9.SPA.03.06.06.E.152-2.E6.bin
end
write memory
! Reload the device to apply the patch
reload2. Temporary Workaround & Hardening Configuration
If patching cannot be performed immediately, implement Infrastructure ACLs (iACLs) to restrict IKEv2 traffic (UDP ports 500 and 4500) only to trusted VPN peers.
configure terminal
! Define ACL to permit only trusted peer IPs and deny all other IKEv2 traffic
ip access-list extended ACL_RESTRICT_IKE
permit udp host 203.0.113.10 host 198.51.100.1 eq 500
permit udp host 203.0.113.10 host 198.51.100.1 eq 4500
deny udp any any eq 500
deny udp any any eq 4500
permit ip any any
exit
! Apply the ACL to the external interface
interface GigabitEthernet0/0
ip access-group ACL_RESTRICT_IKE in
end
write memory3. Log Audit & IoC Verification Commands
Verify if ISAKMP is enabled and monitor the device for high CPU utilization or unexpected reloads.
! Check if ISAKMP or IKEv2 is enabled on the device
show run | include crypto isakmp|crypto ikev2
! Monitor CPU utilization and identify high-consuming processes
show processes cpu sorted | exclude 0.00%
! Search system logs for tracebacks or unexpected reloads
show logging | include Traceback|RELOAD|IKEv2Stop Exposing VPN Ports to the Internet — Switch to Zero-Trust Access
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Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2017-12237) exists in the Internet Key Exchange Version 2 (IKEv2) module of Cisco IOS and Cisco IOS XE Software. The root cause is improper resource management (CWE-399) when processing specific incoming IKEv2 packets. Devices running vulnerable software versions are susceptible if Internet Security Association and Key Management Protocol (ISAKMP) is enabled, even if no explicit IKEv2 features are configured.
2. Exploitation Mechanics & Attack Scenario
An unauthenticated remote attacker can exploit this vulnerability by sending crafted IKEv2 packets to the UDP ports 500 or 4500 of an affected device. When the device attempts to parse and process these malicious packets, it triggers an unhandled exception or resource exhaustion state within the IKEv2 state machine. This leads to 100% CPU utilization, generation of traceback messages in the system logs, and an eventual device reload due to watchdog timeouts, resulting in a complete Denial of Service (DoS).
3. Infrastructure & Business Impact
As this vulnerability is listed in the CISA KEV (Known Exploited Vulnerabilities) catalog, it represents a high-risk threat with confirmed active exploitation history. Successful exploitation disrupts critical VPN services, including LAN-to-LAN VPNs, Remote-access VPNs (excluding SSL VPN), DMVPN, and FlexVPN. A reload of core or edge routers can cause severe enterprise network downtime and business interruption.
Official Vendor Advisories & References
A vulnerability in the Internet Key Exchange Version 2 (IKEv2) module of Cisco IOS 15.0 through 15.6 and Cisco IOS XE 3.5 through 16.5 could allow an unauthenticated, remote attacker to cause high CPU utilization, traceback messages, or a reload of an affected device that leads to a denial of service (DoS) condition. The vulnerability is due to how an affected device processes certain IKEv2 packets. An attacker could exploit this vulnerability by sending specific IKEv2 packets to an affected device to be processed. A successful exploit could allow the attacker to cause high CPU utilization, traceback messages, or a reload of the affected device that leads to a DoS condition. This vulnerability affects Cisco devices that have the Internet Security Association and Key Management Protocol (ISAKMP) enabled. Although only IKEv2 packets can be used to trigger this vulnerability, devices that are running Cisco IOS Software or Cisco IOS XE Software are vulnerable when ISAKMP is enabled. A device does not need to be configured with any IKEv2-specific features to be vulnerable. Many features use IKEv2, including different types of VPNs such as the following: LAN-to-LAN VPN; Remote-access VPN, excluding SSL VPN; Dynamic Multipoint VPN (DMVPN); and FlexVPN. Cisco Bug IDs: CSCvc41277.
Frequently Asked SecOps Questions (CVE-2017-12237)
FAQQ1. How critical is CVE-2017-12237 and is it actively exploited in the wild?
CVE-2017-12237 is rated CVSS 7.5 (HIGH) with a 30-day EPSS exploitation probability of 7.1%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2017-12237?
Primary impacted vendors include cisco across ios, ios_xe. Scope summary: Cisco IOS 15.0 through 15.6 and Cisco IOS XE 3.5 through 16.5 with ISAKMP enabled (Cisco Bug ID: CSCvc41277)
Q3. What temporary mitigations or workarounds exist for CVE-2017-12237 before patching?
[Mitigation] Apply official Cisco software patches immediately, or restrict UDP 500/4500 traffic at the network boundary using Infrastructure ACLs (iACLs). Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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