Cisco IOS and IOS XE Software IKEv1 Denial of Service Vulnerability (CVE-2018-0159)
Executive & SecOps Summary
SECOPS ASSESSMENTCisco IOS and IOS XE Software configured with active Internet Key Exchange Version 1 (IKEv1) functionality.
An unauthenticated, remote attacker can cause the affected device to reload by sending crafted IKEv1 packets, resulting in a complete Denial of Service (DoS) condition.
Apply the official software updates provided by Cisco, or restrict IKEv1 traffic (UDP ports 500 and 4500) to trusted peer IP addresses using Access Control Lists (ACLs).
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Administrators should upgrade to the latest recommended Cisco IOS/IOS XE release that contains the fix for CSCuj73916. Below is an example of upgrading the IOS image via TFTP:
# Verify current version and available flash memory
show version
show flash:
# Download the patched IOS image from your TFTP server
copy tftp: flash:
# Address or name of remote host? 10.1.1.50
# Source filename? c3900e-universalk9-mz.SPA.157-3.M9.bin
# Destination filename [c3900e-universalk9-mz.SPA.157-3.M9.bin]?
# Configure the boot system path and save configuration
configure terminal
no boot system
boot system flash c3900e-universalk9-mz.SPA.157-3.M9.bin
exit
write memory
reload2. Temporary Workaround & Hardening Configuration
If an immediate upgrade is not feasible, implement an infrastructure ACL to restrict IKEv1 traffic (UDP ports 500 and 4500) only to authorized VPN peer IP addresses.
configure terminal
# Define ACL allowing only trusted peer IP (e.g., 198.51.100.20)
ip access-list extended SECURE_IKE_ACL
permit udp host 198.51.100.20 any eq non500-isakmp
permit udp host 198.51.100.20 any eq isakmp
deny udp any any eq non500-isakmp
deny udp any any eq isakmp
permit ip any any
exit
# Apply the ACL to the external interface
interface GigabitEthernet0/0
ip access-group SECURE_IKE_ACL in
end
write memory3. Log Audit & IoC Verification Commands
Run the following commands to check if IKEv1 is enabled and to inspect the reload history of the device.
# Check the reason for the last system reload
show version | include returned to ROM
# Check if ISAKMP (IKEv1) is active and inspect current SAs
show running-config | include crypto isakmp
show crypto isakmp saStop 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-2018-0159) exists in the Internet Key Exchange Version 1 (IKEv1) packet processing engine of Cisco IOS and Cisco IOS XE Software. The root cause is improper validation of specific fields within incoming IKEv1 negotiation packets (CWE-20). When the device processes a malformed or specifically crafted IKEv1 packet, it fails to handle the input safely, leading to an unhandled exception and a subsequent system crash.
2. Exploitation Mechanics & Attack Scenario
An attacker can exploit this vulnerability by sending crafted IKEv1 packets to an affected device during the initial IKE negotiation phase. Because this occurs during the pre-authentication phase, the attacker does not need valid credentials, pre-shared keys, or digital certificates to trigger the vulnerability. The exploit packet is typically targeted at UDP port 500 (ISAKMP) or UDP port 4500 (NAT-T). Upon receiving and parsing the packet, the device immediately reloads.
3. Infrastructure & Business Impact
As a vulnerability listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, CVE-2018-0159 poses a significant threat to enterprise perimeter security. A successful exploit causes an immediate reload of the core router or VPN gateway, terminating all active IPsec VPN tunnels and disrupting network connectivity. Continuous exploitation can trap the device in an infinite reboot loop, leading to prolonged business downtime and loss of critical infrastructure availability.
Official Vendor Advisories & References
A vulnerability in the implementation of Internet Key Exchange Version 1 (IKEv1) functionality in Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause an affected device to reload, resulting in a denial of service (DoS) condition. The vulnerability is due to improper validation of specific IKEv1 packets. An attacker could exploit this vulnerability by sending crafted IKEv1 packets to an affected device during an IKE negotiation. A successful exploit could allow the attacker to cause an affected device to reload, resulting in a DoS condition. Cisco Bug IDs: CSCuj73916.
Frequently Asked SecOps Questions (CVE-2018-0159)
FAQQ1. How critical is CVE-2018-0159 and is it actively exploited in the wild?
CVE-2018-0159 is rated CVSS 7.5 (HIGH) with a 30-day EPSS exploitation probability of 6.9%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2018-0159?
Primary impacted vendors include cisco across ios, ios_xe. Scope summary: Cisco IOS and Cisco IOS XE Software with active IKEv1 processing enabled (Cisco Bug ID: CSCuj73916).
Q3. What temporary mitigations or workarounds exist for CVE-2018-0159 before patching?
[Mitigation] Apply the official software updates provided by Cisco, or restrict IKEv1 traffic (UDP ports 500 and 4500) to trusted peer IP addresses using Access Control Lists (ACLs). Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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