Cisco IOS and IOS XE DHCPv4 Option 82 Input Validation Remote Denial of Service Vulnerability (CVE-2018-0173)
Executive & SecOps Summary
SECOPS ASSESSMENTCisco devices running Cisco IOS and IOS XE Software with encapsulated DHCPv4 Option 82 restoration enabled
An unauthenticated, remote attacker can cause an affected device to reload, resulting in a complete Denial of Service (DoS) condition
Apply official software updates from Cisco or implement workarounds by hardening DHCP snooping and relay configurations
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Download the recommended fixed release image from the Cisco Software Center. Use the following commands to download the image via TFTP and update the boot variable.
# Download the secure IOS XE image from your TFTP server
copy tftp: flash:
# Address or name of remote host? 10.1.1.50
# Source filename? cat3k_caa-universalk9.16.06.08.SPA.bin
# Destination filename [cat3k_caa-universalk9.16.06.08.SPA.bin]?
# Configure the system to boot from the new image
configure terminal
no boot system
boot system flash:cat3k_caa-universalk9.16.06.08.SPA.bin
end
# Save configuration and reload the device
write memory
reload2. Temporary Workaround & Hardening Configuration
If patching cannot be performed immediately, mitigate the risk by disabling untrusted Option 82 processing or disabling Option 82 insertion globally if acceptable.
configure terminal
# Disable trust for DHCP relay information on untrusted interfaces
interface GigabitEthernet1/0/1
no ip dhcp relay information trusted
exit
# Disable global DHCP snooping information option insertion
no ip dhcp snooping information option
end
write memory3. Log Audit & IoC Verification Commands
Verify the running software version, check if DHCP Option 82 features are active, and inspect system logs for unexpected reloads.
# Check current running software version and uptime
show version | include Software|uptime
# Verify if DHCP snooping and Option 82 are enabled
show ip dhcp snooping | include Information option
# Search system logs for crash or reload signatures
show logging | include %SYSTEM-3-RST|RELOAD|ExceptionStop 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 DHCP Version 4 (DHCPv4) packet processing engine of Cisco IOS and IOS XE Software. The root cause is incomplete input validation of encapsulated Option 82 (DHCP Relay Agent Information Option) information received within DHCPOFFER messages from DHCPv4 servers. When the software attempts to restore this malformed encapsulated data, it triggers an unhandled exception or memory corruption, leading to a device reload.
2. Exploitation Mechanics & Attack Scenario
An attacker can exploit this vulnerability by sending a crafted DHCPv4 request packet to an affected device. The device, acting as a relay agent, forwards this request to the DHCPv4 server. When the DHCPv4 server responds with a DHCPOFFER containing the encapsulated Option 82 data, the vulnerable Cisco device processes the response. Due to the lack of proper validation on the encapsulated payload, the parsing routine fails, causing the device to crash and reload. This attack requires no authentication and can be initiated remotely.
3. Infrastructure & Business Impact
Since Cisco IOS and IOS XE devices form the backbone of enterprise and service provider networks, an unexpected reload of these core switches or routers disrupts all transit network traffic. This leads to a severe Denial of Service (DoS) condition. Given that this vulnerability is actively tracked in the CISA KEV catalog, it represents a high-priority threat that could be leveraged by malicious actors to cause widespread operational downtime.
Official Vendor Advisories & References
A vulnerability in the Cisco IOS Software and Cisco IOS XE Software function that restores encapsulated option 82 information in DHCP Version 4 (DHCPv4) packets could allow an unauthenticated, remote attacker to cause an affected device to reload, resulting in a Relay Reply denial of service (DoS) condition. The vulnerability exists because the affected software performs incomplete input validation of encapsulated option 82 information that it receives in DHCPOFFER messages from DHCPv4 servers. An attacker could exploit this vulnerability by sending a crafted DHCPv4 packet to an affected device, which the device would then forward to a DHCPv4 server. When the affected software processes the option 82 information that is encapsulated in the response from the server, an error could occur. A successful exploit could allow the attacker to cause the affected device to reload, resulting in a DoS condition. Cisco Bug IDs: CSCvg62754.
Frequently Asked SecOps Questions (CVE-2018-0173)
FAQQ1. How critical is CVE-2018-0173 and is it actively exploited in the wild?
CVE-2018-0173 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-2018-0173?
Primary impacted vendors include cisco across ios, ios_xe. Scope summary: All Cisco IOS and IOS XE Software versions configured with DHCPv4 Option 82 relay/snooping features (Cisco Bug ID: CSCvg62754)
Q3. What temporary mitigations or workarounds exist for CVE-2018-0173 before patching?
[Mitigation] Apply official software updates from Cisco or implement workarounds by hardening DHCP snooping and relay configurations Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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