CVE-2017-12232

What Is Cisco IOS?

Cisco Integrated Services Routers Generation 2 (ISR G2) are mid-range branch office and WAN access routers running Cisco IOS, widely deployed for connecting remote offices and branch networks to enterprise WAN infrastructure. ISR G2 routers handle business-critical connectivity for enterprise and government networks. As with all Cisco IOS devices, ISR G2 routers are subject to network-accessible DoS vulnerabilities that can disrupt connectivity when exploited.

Overview

CVE-2017-12232 is a denial-of-service vulnerability in Cisco IOS running on Cisco Integrated Services Routers Generation 2 (ISR G2). An unauthenticated attacker on the same network segment (adjacent network, AV:A) can trigger a device reload by sending specially crafted protocol packets to an affected device. Patched in the Cisco September 2017 advisory bundle. CISA added CVE-2017-12232 to the KEV catalog in March 2022 as part of a batch of Cisco IOS vulnerabilities confirmed in exploitation by nation-state actors targeting US critical infrastructure networks.

Affected Versions

Cisco ISR G2 devices (ISR 1900, 2900, 3900 series) running affected Cisco IOS versions. Consult cisco-sa-20170929-isr and the Cisco IOS Software Checker for specific affected version details.

Technical Details

Root Cause: Resource Management Flaw in ISR G2 Protocol Handler

CVE-2017-12232 is a resource management vulnerability (CWE-399) in the Cisco IOS implementation on ISR G2 hardware. A crafted protocol message sent from the adjacent network segment triggers an error condition in the IOS protocol handler that is not properly managed, causing a process crash and device reload. The AV:A (Adjacent Vector) attack vector means the attacker must be able to send layer-2 or layer-3 traffic directly reachable to the target device's interfaces — the attack does not traverse arbitrary network hops.

Why adjacent-vector DoS matters: Nation-state actors who have already gained access to an internal network segment (via a compromised internal host or physical access) can use AV:A vulnerabilities to disrupt network infrastructure from within the perimeter, complicating incident response and potentially disrupting monitoring infrastructure.

Attack Characteristics

Discovery

Reported to Cisco through coordinated disclosure; patched in September 2017 advisory bundle.

Exploitation Context

• Part of March 2022 KEV batch: CVE-2017-12232 was added to KEV alongside 8 other Cisco IOS CVEs from the same September 2017 advisory bundle, reflecting CISA's assessment that nation-state actors exploiting Cisco infrastructure vulnerabilities represents an ongoing threat to US critical infrastructure
• Branch network targeting: ISR G2 routers are common at branch offices and remote sites; disrupting a branch router cuts off connectivity for that site's users and systems

Remediation

1. Apply Cisco IOS security update — identify affected IOS versions via the Cisco IOS Software Checker using advisory cisco-sa-20170929-isr, then upgrade to the fixed IOS release.

2. Restrict adjacent-network access — implement port security and 802.1X authentication on switch ports connected to ISR G2 devices; restrict which hosts can send traffic to the router's interfaces.

3. Maintain IOS patch currency — establish a regular patching cadence for all Cisco IOS devices; apply security updates during planned maintenance windows.