Cisco IOS and IOS XE DHCP Option 82 Encapsulation Heap Overflow Vulnerability (CVE-2018-0172) Analysis and Mitigation Guide
Executive & SecOps Summary
SECOPS ASSESSMENTCisco IOS and IOS XE Software with DHCP Option 82 relay functionality enabled
Unauthenticated, remote attackers can trigger a heap overflow and force a device reload, leading to a complete Denial of Service (DoS)
Apply official Cisco software updates immediately, or disable DHCP Option 82 relay features and restrict untrusted DHCP traffic
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Upgrade to the latest recommended Cisco IOS/IOS XE release that contains the fix for CSCvg62730.
# Example: Downloading and setting boot path for a patched IOS image via TFTP
copy tftp: flash:
Address or name of remote host? 192.168.1.100
Source filename? c3750e-universalk9-mz.152-4.E10.bin
Destination filename [c3750e-universalk9-mz.152-4.E10.bin]?
configure terminal
boot system flash:/c3750e-universalk9-mz.152-4.E10.bin
end
write memory
reload2. Temporary Workaround & Hardening Configuration
If patching is not immediately feasible, disable DHCP Option 82 insertion or restrict untrusted DHCP relay traffic.
configure terminal
# Disable DHCP relay information option globally
no ip dhcp relay information option
# Disable trust on untrusted interfaces to block unauthorized DHCP packets
interface GigabitEthernet0/1
no ip dhcp snooping trust
end
write memory3. Log Audit & IoC Verification Commands
Inspect the running configuration and system logs to identify potential exposure or crash history.
# Check if DHCP relay options are currently enabled
show running-config | include dhcp relay
# Audit system logs for memory allocation failures or unexpected reloads
show logging | include %SYS-2-MALLOCFAIL|%ALIGN-3-SPURIOUS|ReloadStop 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 (CVE-2018-0172) is a critical memory corruption flaw in the DHCP Option 82 (Relay Agent Information Option) encapsulation module of Cisco IOS and IOS XE Software. The root cause lies in the incomplete input validation of Option 82 data received within DHCPv4 packets from DHCP relay agents. This lack of validation allows an out-of-bounds write, leading to a heap overflow condition.
2. Exploitation Mechanics & Attack Scenario
An attacker can exploit this vulnerability by sending specially crafted DHCPv4 packets containing malicious Option 82 payloads to an affected device. When the device processes and attempts to encapsulate this data, it fails to verify the payload length, causing a buffer overflow in the heap memory. This corrupts adjacent memory structures, triggering a kernel panic or process crash, which forces the device to reload and results in a Denial of Service (DoS).
3. Infrastructure & Business Impact
As this vulnerability is listed on the CISA KEV (Known Exploited Vulnerabilities) catalog, it represents a highly active threat vector. Successful exploitation against core routing or switching infrastructure can cause continuous device reloads, leading to severe network downtime, business disruption, and loss of critical connectivity.
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 a heap overflow condition on the affected device, which will cause the device to reload and result in a DoS condition. Cisco Bug IDs: CSCvg62730.
Frequently Asked SecOps Questions (CVE-2018-0172)
FAQQ1. How critical is CVE-2018-0172 and is it actively exploited in the wild?
CVE-2018-0172 is rated CVSS 8.6 (HIGH) with a 30-day EPSS exploitation probability of 7.8%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2018-0172?
Primary impacted vendors include cisco across ios, ios xe. Scope summary: All Cisco IOS and IOS XE Software versions configured with DHCP Option 82 relay functionality (Cisco Bug ID: CSCvg62730)
Q3. What temporary mitigations or workarounds exist for CVE-2018-0172 before patching?
[Mitigation] Apply official Cisco software updates immediately, or disable DHCP Option 82 relay features and restrict untrusted DHCP traffic Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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