CVE-2017-12240

What Is Cisco IOS?

Cisco IOS and IOS XE are the operating systems powering the majority of Cisco routers and switches deployed in enterprise, service provider, government, and critical infrastructure networks worldwide. The DHCP relay feature in Cisco IOS allows routers to forward DHCP (Dynamic Host Configuration Protocol) requests from clients on one subnet to a DHCP server on another — a ubiquitous feature in enterprise networks where routers separate subnets. CVE-2017-12240 stands out as the only RCE vulnerability in the September 2017 Cisco advisory bundle — all companion CVEs are DoS-only, but CVE-2017-12240 allows full code execution on the IOS device itself.

Overview

CVE-2017-12240 is a critical remote code execution vulnerability in the DHCP relay subsystem of Cisco IOS and IOS XE. An unauthenticated remote attacker can execute arbitrary code and gain full control of an affected Cisco router or switch by sending crafted DHCP packets to a device with DHCP relay configured. A compromised IOS device gives the attacker complete control over all traffic traversing that network device — enabling traffic interception, routing manipulation, credential harvesting, and lateral movement to connected network segments. Patched in the Cisco September 2017 advisory bundle. CISA added CVE-2017-12240 to the KEV catalog in March 2022.

Affected Versions

Cisco IOS and IOS XE versions with DHCP relay functionality (ip helper-address configured). Use cisco-sa-20170929-dhcp and the Cisco IOS Software Checker for specific affected version identification.

Technical Details

Root Cause: DHCP Relay Heap Overflow Leading to Code Execution

CVE-2017-12240 is an improper input validation vulnerability (CWE-20) in the Cisco IOS DHCP relay subsystem that leads to code execution. When the DHCP relay processes a specially crafted DHCP packet, insufficient validation allows the packet to overflow a heap buffer in the IOS DHCP relay processing code. Unlike a simple DoS, the overflow can be controlled to overwrite function pointers or IOS heap metadata to redirect execution to attacker-controlled code.

Full device compromise implications: Code execution on Cisco IOS gives an attacker:

• Complete visibility into all forwarded traffic (including in-band management traffic)
• Ability to modify routing tables and redirect traffic
• Access to IOS device configuration (credentials, SNMP community strings, etc.)
• Persistent access by modifying IOS startup configuration
• Pivot point to attack devices on all connected network segments

DHCP relay ubiquity: The ip helper-address command (DHCP relay) is configured on nearly every enterprise Cisco router interface — it's required for clients on subnets that don't have a local DHCP server. This makes CVE-2017-12240 relevant on a very high percentage of enterprise IOS deployments.

Attack Characteristics

Discovery

Reported to Cisco through coordinated disclosure; patched in the September 2017 security advisory bundle as the highest-severity vulnerability in the batch.

Exploitation Context

• Complete network device control: Unlike the companion DoS-only CVEs in the September 2017 bundle, CVE-2017-12240 provides full code execution — a compromised IOS device is the ultimate network pivot point, enabling traffic interception on all connected segments
• Nation-state network infrastructure operations: The March 2022 KEV addition reflects CISA's assessment that nation-state actors (Russian state-sponsored groups in particular) were exploiting Cisco IOS vulnerabilities in campaigns targeting US critical infrastructure; gaining persistent access to core routers enables long-term traffic collection and network mapping
• IOS patching urgency: The CRITICAL severity (9.8) and RCE impact make CVE-2017-12240 the highest priority in the September 2017 bundle; organizations that patched the DoS-only vulnerabilities but deferred the DHCP RCE patch remain at significantly elevated risk

Remediation

1. Apply Cisco IOS security update immediately — CVE-2017-12240 is CRITICAL (9.8 RCE) and should be treated as the highest-priority patch in the September 2017 bundle. Upgrade to fixed IOS/IOS XE versions per cisco-sa-20170929-dhcp.

2. Restrict DHCP relay traffic — if DHCP relay is required, restrict inbound DHCP traffic (UDP 67) to only expected subnets and DHCP servers; use ACLs to block crafted DHCP packets from untrusted sources.

3. Implement infrastructure ACLs (iACL) — deploy control plane policing (CoPP) on affected IOS devices to rate-limit DHCP-related traffic and prevent packet floods from triggering the vulnerability.

4. Audit all IOS devices for DHCP relay — identify all Cisco IOS devices with ip helper-address configured using your network management infrastructure; these are all potentially affected by CVE-2017-12240 and require patching.

5. Implement out-of-band management — manage Cisco IOS devices via dedicated out-of-band management networks that are not accessible from internet-facing or untrusted subnets; this limits attacker ability to interact with management interfaces after exploitation.