CVE-2018-0158

What is IKEv1 in Cisco IOS/IOS XE?

Internet Key Exchange version 1 (IKEv1) is the protocol used to negotiate and establish the security associations that underpin IPsec VPN tunnels. Any Cisco router or switch configured as a VPN endpoint runs an IKEv1 (or IKEv2) negotiation process reachable on UDP/500, making the correctness of that parsing code directly relevant to the availability of every VPN tunnel the device serves.

Overview

CVE-2018-0158 is a high-severity denial-of-service vulnerability caused by a memory leak in the IKEv1 implementation of Cisco IOS and IOS XE Software. An unauthenticated, remote attacker can send a stream of crafted IKEv1 packets to a VPN-terminating interface, causing the device to progressively leak memory until resources are exhausted and the device reloads, disrupting all VPN and non-VPN traffic it handles.

Technical Details

The vulnerability is classified under CWE-20 (improper input validation): malformed or unexpected content in received IKEv1 negotiation packets is not properly rejected or freed, causing allocated memory associated with each negotiation attempt to accumulate rather than being released. Because IKEv1 negotiation begins before any peer authentication is completed, an attacker needs no valid credentials or pre-shared key — only the ability to send UDP/500 traffic to the target interface, repeated over time to exhaust available memory. The Scope: Changed rating on the CVSS vector reflects that the resulting memory exhaustion can affect the device beyond the VPN subsystem itself.

Discovery

Cisco identified and fixed this issue internally as part of its March 2018 semiannual IOS/IOS XE security advisory bundle, alongside a companion IKEv1 crash vulnerability (CVE-2018-0159).

Exploitation Context

CISA's KEV catalog listing confirms this vulnerability has been used in real-world attacks. IKEv1/IKEv2 parsing flaws in VPN-terminating network gear have historically drawn sustained interest from both criminal DDoS-for-hire operators and more sophisticated actors, since disrupting a site's VPN connectivity can serve as either a standalone denial-of-service goal or a distraction while other intrusion activity occurs elsewhere on the network.

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 where feasible.
4. Monitor device memory utilization and reload history for signs of an ongoing memory-leak attack.
5. Where practical, migrate legacy IKEv1 configurations to IKEv2, which has undergone more recent security scrutiny.