Cisco ISR G2 Routers IOS Ethernet Frame Misclassification Denial of Service Vulnerability (CVE-2017-12232)
Executive & SecOps Summary
SECOPS ASSESSMENTCisco Integrated Services Routers Generation 2 (ISR G2) running Cisco IOS Software versions 15.0 through 15.6.
An unauthenticated, adjacent attacker can cause the affected device to reload, leading to a complete Denial of Service (DoS) condition.
Apply the official Cisco IOS software updates or implement Layer 2 security controls to restrict unauthorized adjacent traffic.
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Download the fixed Cisco IOS image from the Cisco Software Central and upgrade the device using the following commands:
! 1. Copy the secure IOS image from your TFTP server to flash
copy tftp: flash:
Address or name of remote host? 192.168.1.100
Source filename? c1900-universalk9-mz.SPA.156-3.M11.bin
Destination filename [c1900-universalk9-mz.SPA.156-3.M11.bin]?
! 2. Configure the router to boot from the new image
configure terminal
no boot system
boot system flash c1900-universalk9-mz.SPA.156-3.M11.bin
end
! 3. Save configuration and reload the router
write memory
reload2. Temporary Workaround & Hardening Configuration
If immediate patching is not feasible, harden the adjacent Layer 2 interfaces to restrict unauthorized frame injection and limit MAC spoofing:
! Enable Port Security on adjacent switch ports to restrict unauthorized devices
interface GigabitEthernet0/1
switchport mode access
switchport port-security
switchport port-security maximum 2
switchport port-security violation shutdown
ip dhcp snooping limit rate 100
end3. Log Audit & IoC Verification Commands
Verify the current running IOS version and inspect system logs for unexpected reloads or crash signatures:
! 1. Check the current IOS version and system uptime
show version | include Software|uptime|System image
! 2. Identify the reason for the last reload and check for crash logs
show version | include returned to ROM
show logging | include %SYSTEM-3-RELOAD|%SYS-2-MALLOCFAILStop 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-2017-12232) exists in the protocol implementation of Cisco IOS Software (versions 15.0 through 15.6) running on Cisco ISR G2 routers. The root cause is a flaw in how the device classifies incoming Ethernet frames. Due to this misclassification, processing a specifically crafted Ethernet frame triggers an unhandled exception or memory corruption within the IOS kernel, causing the device to crash and reload.
2. Exploitation Mechanics & Attack Scenario
To exploit this vulnerability, an attacker must be located on the same local Layer 2 network segment or broadcast domain as the target device (Adjacent vector). The attacker transmits a maliciously crafted Ethernet frame directly to the interface of the vulnerable Cisco ISR G2 router. Upon receiving the frame, the router's parser misclassifies the packet, triggering a kernel panic or system reload without requiring any authentication or user interaction.
3. Infrastructure & Business Impact
As this vulnerability is listed in the CISA KEV (Known Exploited Vulnerabilities) catalog, it represents a highly practical threat that has been actively exploited in the wild. A successful exploit causes an immediate device reload, disrupting all transit network traffic. If the attacker continuously floods the device with crafted frames, it can result in a persistent Denial of Service (DoS) loop, paralyzing branch-to-HQ WAN connectivity and critical business operations.
Official Vendor Advisories & References
A vulnerability in the implementation of a protocol in Cisco Integrated Services Routers Generation 2 (ISR G2) Routers running Cisco IOS 15.0 through 15.6 could allow an unauthenticated, adjacent attacker to cause an affected device to reload, resulting in a denial of service (DoS) condition. The vulnerability is due to a misclassification of Ethernet frames. An attacker could exploit this vulnerability by sending a crafted Ethernet frame to an affected device. A successful exploit could allow the attacker to cause the affected device to reload, resulting in a DoS condition. Cisco Bug IDs: CSCvc03809.
Frequently Asked SecOps Questions (CVE-2017-12232)
FAQQ1. How critical is CVE-2017-12232 and is it actively exploited in the wild?
CVE-2017-12232 is rated CVSS 6.5 (MEDIUM) with a 30-day EPSS exploitation probability of 2.1%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2017-12232?
Primary impacted vendors include cisco across ios, integrated_services_routers_generation_2. Scope summary: Cisco Integrated Services Routers Generation 2 (ISR G2) running Cisco IOS Software versions 15.0 through 15.6. (Cisco Bug ID: CSCvc03809)
Q3. What temporary mitigations or workarounds exist for CVE-2017-12232 before patching?
[Mitigation] Apply the official Cisco IOS software updates or implement Layer 2 security controls to restrict unauthorized adjacent 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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