CVE-2018-0159

What is IKEv1 in Cisco IOS/IOS XE?

Internet Key Exchange version 1 (IKEv1) negotiates the security associations underlying IPsec VPN tunnels. Every Cisco router or switch acting as a VPN endpoint runs IKEv1 negotiation logic reachable over UDP/500, so a crash bug in that code path directly threatens the availability of the VPN service the device provides.

Overview

CVE-2018-0159 is a high-severity denial-of-service vulnerability in the IKEv1 implementation of Cisco IOS and IOS XE Software. An unauthenticated, remote attacker can send a single crafted IKEv1 packet to a VPN-terminating interface to trigger a crash and forced reload of the device. It was published in the same March 2018 advisory batch as a related IKEv1 memory-leak DoS vulnerability, CVE-2018-0158.

Technical Details

The issue is classified under CWE-20 (improper input validation): the IKEv1 negotiation parser does not correctly validate certain packet fields, and processing a malformed packet causes the negotiation process to crash outright rather than simply rejecting the invalid input. Because IKEv1 negotiation begins before peer authentication completes, an attacker requires no valid pre-shared key or certificate — only network reachability to UDP/500 on the target interface and a single crafted packet.

Discovery

Cisco identified and fixed this issue internally as part of the March 2018 semiannual IOS/IOS XE security advisory bundle.

Exploitation Context

CISA's KEV catalog listing confirms real-world exploitation of this vulnerability. A single-packet, unauthenticated crash of a VPN-terminating router is an efficient and repeatable denial-of-service primitive, making it attractive both to criminal DDoS actors and to more sophisticated groups seeking to disrupt an organization's site-to-site or remote-access VPN connectivity as part of a broader operation.

Remediation

1. Identify Cisco IOS/IOS XE devices configured as IKEv1 VPN endpoints and confirm their current software version.
2. Apply the Cisco-published fixed software release from the corresponding security advisory.
3. Restrict which external hosts can reach UDP/500 on VPN-terminating interfaces using infrastructure ACLs.
4. Monitor for unexpected device reloads correlated with VPN-facing interfaces.
5. Where practical, migrate legacy IKEv1 configurations to IKEv2.