Cisco IOS and IOS XE DHCP Option 82 Encapsulation Input Validation Vulnerability (CVE-2018-0174)
Executive & SecOps Summary
SECOPS ASSESSMENTCisco IOS and IOS XE Software with DHCP Option 82 encapsulation functionality enabled
An unauthenticated, remote attacker can cause the affected device to reload, resulting in a complete Denial of Service (DoS) condition
Apply the official software updates from Cisco, or disable DHCP Option 82 / relay features and restrict DHCP traffic using ACLs
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Upgrading to a non-vulnerable Cisco IOS/IOS XE release is the recommended permanent fix.
# Example: Downloading and applying a secure IOS image via TFTP
Router# copy tftp: flash:
Address or name of remote host? 192.168.1.100
Source filename? c3750e-universalk9-mz.152-4.E8.bin
Destination filename [c3750e-universalk9-mz.152-4.E8.bin]?
# Configure the boot system path and save configuration
Router(config)# boot system flash:/c3750e-universalk9-mz.152-4.E8.bin
Router(config)# end
Router# write memory
Router# reload2. Temporary Workaround & Hardening Configuration
If patching cannot be performed immediately, disable DHCP relay information option processing or block untrusted DHCP traffic.
# Disable DHCP relay information option globally
Router(config)# no ip dhcp relay information option
# Alternatively, apply an ACL to block DHCP traffic on untrusted interfaces
Router(config)# ip access-list extended BLOCK_UNTRUSTED_DHCP
Router(config-ext-nacl)# deny udp any any eq bootps
Router(config-ext-nacl)# deny udp any any eq bootpc
Router(config-ext-nacl)# permit ip any any
Router(config-ext-nacl)# exit
# Apply the ACL to the external-facing interface
Router(config)# interface GigabitEthernet0/1
Router(config-if)# ip access-group BLOCK_UNTRUSTED_DHCP in3. Log Audit & IoC Verification Commands
Verify if DHCP Option 82 is enabled and inspect system logs for unexpected reloads.
# Check if DHCP relay information option is enabled
Router# show running-config | include ip dhcp relay information
# Check system reload history and crash logs
Router# show version | include returned to ROM
Router# show logging | include %SYS-5-RELOAD|%SYSTEM_CONTROLLERStop 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-2018-0174) exists in the DHCP Option 82 (Relay Agent Information Option) encapsulation component of Cisco IOS and IOS XE Software. The root cause is incomplete input validation (CWE-20) of Option 82 data received within DHCPv4 packets from DHCP relay agents. When processing malformed or oversized Option 82 fields, the software fails to properly validate the boundaries, leading to memory corruption or unhandled exceptions that crash the device.
2. Exploitation Mechanics & Attack Scenario
An attacker can exploit this vulnerability by sending crafted DHCPv4 packets containing anomalous Option 82 payloads to an affected device. Because the vulnerability is triggered during the parsing and encapsulation phase, no authentication or user interaction is required. Upon receiving the malicious packet, the parsing engine triggers a system crash, forcing the device to reload and causing an immediate disruption of network traffic.
3. Infrastructure & Business Impact
With a CVSS score of 8.6 (High) and its inclusion in the CISA KEV (Known Exploited Vulnerabilities) catalog, this vulnerability poses a severe threat to enterprise networks. Since Cisco IOS/IOS XE devices often serve as core routing and switching infrastructure, a successful exploit leading to an unexpected reload can disrupt critical network paths, severing business-critical connections and causing significant downtime.
Official Vendor Advisories & References
A vulnerability in the DHCP option 82 encapsulation functionality of 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 exists because the affected software performs incomplete input validation of option 82 information that it receives in DHCP Version 4 (DHCPv4) packets from DHCP relay agents. An attacker could exploit this vulnerability by sending a crafted DHCPv4 packet to an affected device. A successful exploit could allow the attacker to cause the affected device to reload, resulting in a DoS condition. Cisco Bug IDs: CSCuh91645.
Frequently Asked SecOps Questions (CVE-2018-0174)
FAQQ1. How critical is CVE-2018-0174 and is it actively exploited in the wild?
CVE-2018-0174 is rated CVSS 8.6 (HIGH) with a 30-day EPSS exploitation probability of 7.6%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2018-0174?
Primary impacted vendors include cisco across ios, ios xe. Scope summary: All Cisco IOS and IOS XE Software configurations where DHCP Option 82 encapsulation is active (Cisco Bug ID: CSCuh91645)
Q3. What temporary mitigations or workarounds exist for CVE-2018-0174 before patching?
[Mitigation] Apply the official software updates from Cisco, or disable DHCP Option 82 / relay features and restrict DHCP traffic using 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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