Cisco IOS and IOS XE DHCP Relay Subsystem Remote Code Execution and DoS Vulnerability (CVE-2017-12240) Security Advisory
Executive & SecOps Summary
SECOPS ASSESSMENTCisco network devices running Cisco IOS Software versions 12.2 through 15.6 or Cisco IOS XE Software with the DHCP relay subsystem enabled.
An unauthenticated, remote attacker can execute arbitrary code with full system privileges (RCE) or trigger a system reload, causing a complete Denial of Service (DoS).
Apply the official Cisco software updates immediately, or implement access control lists (ACLs) to block unauthorized DHCPv4 traffic and disable DHCP relay where not required.
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Upgrade to a patched Cisco IOS/IOS XE release as recommended by Cisco. Download the verified image via TFTP/FTP and update the boot variables.
! 1. Download the patched image from your secure TFTP server
copy tftp: flash:
Address or name of remote host? 192.168.1.100
Source filename? c3900e-universalk9-mz.SPA.156-3.M1b.bin
Destination filename [c3900e-universalk9-mz.SPA.156-3.M1b.bin]?
! 2. Configure the device to boot using the new image
configure terminal
no boot system
boot system flash c3900e-universalk9-mz.SPA.156-3.M1b.bin
end
! 3. Save configuration and reload the device
write memory
reload2. Temporary Workaround & Hardening Configuration
If patching cannot be performed immediately, disable the DHCP relay feature on untrusted interfaces or restrict DHCP traffic using an infrastructure ACL (iACL).
configure terminal
! Disable DHCP relay on untrusted interfaces
interface GigabitEthernet0/1
no ip helper-address
exit
! Restrict DHCPv4 traffic to authorized DHCP servers only
ip access-list extended SECURE_DHCP_RELAY
permit udp host 10.1.1.50 any eq bootps
deny udp any any eq bootps
permit ip any any
exit
interface GigabitEthernet0/1
ip access-group SECURE_DHCP_RELAY in
end
write memory3. Log Audit & IoC Verification Commands
Verify if the DHCP relay feature is active and inspect system logs for signs of memory corruption or unexpected reloads.
! 1. Check running configuration for active DHCP relay configurations
show running-config | include helper-address
show version | include Software
! 2. Audit system logs for memory allocation failures or unexpected reboots
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-2017-12240) is a critical buffer overflow condition within the DHCP relay subsystem of Cisco IOS and Cisco IOS XE Software. The root cause lies in the improper validation of input bounds when processing specific option fields within incoming DHCPv4 packets. Due to this lack of boundary checking, the subsystem fails to restrict the size of the copied data, leading to memory corruption.
2. Exploitation Mechanics & Attack Scenario
An attacker can exploit this vulnerability by sending a specially crafted DHCPv4 packet to an affected system that has the DHCP relay feature active. When the DHCP relay agent processes the malformed packet, the payload overflows the allocated memory buffer, overwriting adjacent memory structures. This allows the attacker to hijack the execution flow to run arbitrary machine code with administrative privileges or corrupt critical system structures, leading to an immediate device crash and reload.
3. Infrastructure & Business Impact
With a CVSS score of 9.8 and its inclusion in the CISA KEV catalog, this vulnerability poses an extreme risk to enterprise infrastructure. Successful exploitation can lead to complete compromise of core routing and switching infrastructure. This enables attackers to intercept network traffic, perform lateral movement within the internal network, or disrupt business continuity entirely by taking down critical network segments.
Official Vendor Advisories & References
The DHCP relay subsystem of Cisco IOS 12.2 through 15.6 and Cisco IOS XE Software contains a vulnerability that could allow an unauthenticated, remote attacker to execute arbitrary code and gain full control of an affected system. The attacker could also cause an affected system to reload, resulting in a denial of service (DoS) condition. The vulnerability is due to a buffer overflow condition in the DHCP relay subsystem of the affected software. An attacker could exploit this vulnerability by sending a crafted DHCP Version 4 (DHCPv4) packet to an affected system. A successful exploit could allow the attacker to execute arbitrary code and gain full control of the affected system or cause the affected system to reload, resulting in a DoS condition. Cisco Bug IDs: CSCsm45390, CSCuw77959.
Frequently Asked SecOps Questions (CVE-2017-12240)
FAQQ1. How critical is CVE-2017-12240 and is it actively exploited in the wild?
CVE-2017-12240 is rated CVSS 9.8 (CRITICAL) with a 30-day EPSS exploitation probability of 13.8%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2017-12240?
Primary impacted vendors include cisco across ios, ios xe. Scope summary: Cisco IOS Software versions 12.2 through 15.6 and Cisco IOS XE Software configured with DHCP relay (ip helper-address)
Q3. What temporary mitigations or workarounds exist for CVE-2017-12240 before patching?
[Mitigation] Apply the official Cisco software updates immediately, or implement access control lists (ACLs) to block unauthorized DHCPv4 traffic and disable DHCP relay where not required. Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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