Cisco IOS and IOS XE Smart Install Remote Denial of Service Vulnerability (CVE-2018-0156)
Executive & SecOps Summary
SECOPS ASSESSMENTCisco switches running Cisco IOS or IOS XE Software with the Smart Install client feature enabled
Unauthenticated, remote attackers can trigger an unexpected device reload, leading to a persistent Denial of Service (DoS) condition
Disable the Smart Install feature using the 'no vstack' command, or restrict access to TCP port 4786 using infrastructure ACLs
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Cisco has released software updates that address this vulnerability. Administrators should upgrade to a fixed release of Cisco IOS or IOS XE Software.
# Verify current IOS version and check if TCP port 4786 is active
show version
show tcp brief | include 4786
# Example: Download and apply a secure IOS image via TFTP
copy tftp: flash:
# Address or name of remote host? 192.168.1.100
# Source filename? c2960x-universalk9-mz.152-7.E0a.bin
# Destination filename [c2960x-universalk9-mz.152-7.E0a.bin]?
configure terminal
boot system flash:/c2960x-universalk9-mz.152-7.E0a.bin
end
write memory
reload2. Temporary Workaround & Hardening Configuration
If upgrading immediately is not feasible, disable the Smart Install client feature entirely. If the feature must remain active, implement infrastructure ACLs to restrict access to TCP port 4786.
# Step 1: Disable Smart Install client globally (Highly Recommended)
configure terminal
no vstack
end
write memory
# Step 2: If disabling is not possible, restrict port 4786 access via ACL
configure terminal
ip access-list extended BLOCK_SMART_INSTALL
permit tcp host 10.1.1.50 any eq 4786
deny tcp any any eq 4786
permit ip any any
exit
interface GigabitEthernet0/1
ip access-group BLOCK_SMART_INSTALL in
end
write memory3. Log Audit & IoC Verification Commands
Use the following commands to check if the device has experienced unexpected reloads or if the vulnerable port is exposed.
# Check system reload history and boot logs
show version | include riggered|reboot|uptime
show logging | include Smart Install|vstack|4786
# Verify if the Smart Install socket is actively listening
show sockets | include 4786Stop Exposing VPN Ports to the Internet — Switch to Zero-Trust Access
Eliminate public-facing VPN concentrators and firewall management ports with identity-first Zero-Trust Network Access.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability exists in the Smart Install client feature of Cisco IOS and Cisco IOS XE Software. It is classified under improper input validation (CWE-20) and resource management errors (CWE-399). The root cause is the software's failure to properly validate incoming packet data on the Smart Install listening port. When a malformed packet is processed, it triggers an unhandled exception or memory corruption, forcing the operating system to crash and reload.
2. Exploitation Mechanics & Attack Scenario
An attacker scans the target network to identify devices exposing TCP port 4786, which is the default port for Cisco Smart Install. Because the protocol does not require authentication by design, the attacker can directly transmit a specially crafted TCP packet to the target device. Upon receiving and parsing this malformed packet, the Smart Install daemon crashes, causing an immediate reload of the entire switch and disrupting all active network traffic.
3. Infrastructure & Business Impact
As a CISA KEV-listed vulnerability, this flaw is actively exploited in the wild. A successful exploit on core or distribution switches can cause immediate, widespread network outages, severely impacting business continuity. Because exploitation requires no privileges and can be executed remotely, it poses a severe threat as a tool for infrastructure disruption or lateral movement within compromised networks.
Official Vendor Advisories & References
A vulnerability in the Smart Install feature of Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, remote attacker to trigger a reload of an affected device, resulting in a denial of service (DoS) condition. The vulnerability is due to improper validation of packet data. An attacker could exploit this vulnerability by sending a crafted packet to an affected device on TCP port 4786. Only Smart Install client switches are affected. Cisco devices that are configured as a Smart Install director are not affected by this vulnerability. Cisco Bug IDs: CSCvd40673.
Frequently Asked SecOps Questions (CVE-2018-0156)
FAQQ1. How critical is CVE-2018-0156 and is it actively exploited in the wild?
CVE-2018-0156 is rated CVSS 7.5 (HIGH) with a 30-day EPSS exploitation probability of 9.4%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2018-0156?
Primary impacted vendors include cisco across ios, ios_xe. Scope summary: Cisco switches running Cisco IOS or IOS XE Software configured as Smart Install clients. Smart Install directors are unaffected.
Q3. What temporary mitigations or workarounds exist for CVE-2018-0156 before patching?
[Mitigation] Disable the Smart Install feature using the 'no vstack' command, or restrict access to TCP port 4786 using infrastructure ACLs Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
Generate an Internal Engineering Notice or CISO Report for CVE-2018-0156
Instantly format this advisory into an executive CISO summary or engineering patch ticket.
🔗 Related Advisories in Similar Stack (cisco)
View Category →Cisco Identity Services Engine (ISE) Critical Authentication Bypass Vulnerability (CVE-2026-76460)
[Affected Scope] Cisco Identity Services Engine (ISE) web-based management interface and API endpoints.
Cisco Catalyst SD-WAN Peering Authentication Bypass Vulnerability (CVE-2026-20182)
[Affected Scope] Cisco Catalyst SD-WAN Controller (formerly vSmart), Manager (formerly vManage), and Validator (formerly vBond)
Cisco Secure Firewall Management Center (FMC) Insecure Deserialization Vulnerability (CVE-2026-20131, CVSS 10.0) - Remote Code Execution and Root Privilege Escalation
[Affected Scope] Web-based management interface of Cisco Secure Firewall Management Center (FMC) Software.
Cisco Secure Firewall Management Center (FMC) Authentication Bypass and Root RCE Vulnerability (CVE-2026-20079)
[Affected Scope] Web-based management interface of Cisco Secure Firewall Management Center (FMC) Software
Subscribe to Daily Vulnerability Briefings & KEV Alerts
Concise morning briefings for SecOps, DevSecOps, and SRE teams covering newly exploited CVEs and mitigation priorities.