Cisco IOS Catalyst Switches SNMP ciscoFlashMIB Denial of Service Vulnerability (CVE-2018-0161) Technical Advisory
Executive & SecOps Summary
SECOPS ASSESSMENTCisco Catalyst 2960-L Series, Catalyst Digital Building Series 8P, and 8U Switches running vulnerable Cisco IOS and configured with SNMPv2 or SNMPv3.
An authenticated remote attacker can trigger a device reload (Denial of Service) via a SYS-3-CPUHOG error by sending a crafted SNMP GET request for the ciscoFlashMIB OID.
Apply the official Cisco IOS software updates, or implement SNMP MIB views to exclude access to the ciscoFlashMIB OID and restrict SNMP access using ACLs.
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Download the recommended stable Cisco IOS release for your specific switch model from the Cisco Software Central. Use the following commands to perform the upgrade via TFTP:
! Enter privileged EXEC mode and verify flash space
enable
show flash:
! Download the patched IOS image from your TFTP server
copy tftp://10.1.1.100/cat2k_caa-universalk9.SPA.152-7.E0a.bin flash:
! Configure the boot system path and save configuration
configure terminal
no boot system
boot system flash:/cat2k_caa-universalk9.SPA.152-7.E0a.bin
end
write memory
reload2. Temporary Workaround & Hardening Configuration
If an immediate upgrade is not feasible, configure an SNMP View to explicitly exclude the ciscoFlashMIB OID (1.3.6.1.4.1.9.9.10) and restrict SNMP access to authorized management hosts only.
configure terminal
! 1. Define an SNMP view that excludes the ciscoFlashMIB OID
snmp-server view BLOCK_FLASH_MIB iso included
snmp-server view BLOCK_FLASH_MIB ciscoFlashMIB excluded
! 2. Create an ACL to restrict SNMP access to trusted hosts
ip access-list standard SNMP_ALLOW_ACL
permit 10.99.1.0 0.0.0.255
deny any log
! 3. Apply the view and ACL to your SNMP community string
snmp-server community READ_ONLY_SECURE view BLOCK_FLASH_MIB RO SNMP_ALLOW_ACL
end
write memory3. Log Audit & IoC Verification Commands
Inspect the device logs to identify unexpected reboots and search for indicators of CPU exhaustion related to SNMP processing.
! Check system uptime and the reason for the last reload
show version | include uptime|returned
! Search for CPU hogging messages in the system logs
show logging | include SYS-3-CPUHOG
! Verify the active SNMP views and community configurations
show snmp view
show snmp communityStop Exposing VPN Ports to the Internet — Switch to Zero-Trust Access
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Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2018-0161) resides in the Simple Network Management Protocol (SNMP) subsystem of Cisco IOS Software. It is classified under resource management errors (CWE-399). The root cause is an input validation and processing flaw when the affected software handles an SNMP read request containing a query for the ciscoFlashMIB Object ID (OID). When such a request is processed, the system fails to release or properly throttle CPU cycles, leading to an infinite loop or resource exhaustion.
2. Exploitation Mechanics & Attack Scenario
To exploit this vulnerability, an attacker must first obtain valid SNMPv2 community strings or SNMPv3 credentials on the target device. Once authenticated, the attacker issues an SNMP GET or GETNEXT request targeting the ciscoFlashMIB OID subtree (1.3.6.1.4.1.9.9.10). The vulnerable IOS daemon attempts to parse this request, causing the control plane CPU utilization to spike to 100%. This triggers a watchdog timeout, generating a SYS-3-CPUHOG system message, and forces the switch to crash and reboot.
3. Infrastructure & Business Impact
As this vulnerability is listed in the CISA KEV catalog, it represents a high-risk vector that has been actively exploited in the wild. Catalyst 2960-L and Digital Building switches are commonly deployed at the access layer to connect critical enterprise endpoints and IoT devices. A successful exploit allows an attacker to repeatedly crash these switches, causing prolonged network downtime, disrupting business operations, and severing connectivity for all downstream devices.
Official Vendor Advisories & References
A vulnerability in the Simple Network Management Protocol (SNMP) subsystem of Cisco IOS Software running on certain models of Cisco Catalyst Switches could allow an authenticated, remote attacker to cause a denial of service (DoS) condition, aka a GET MIB Object ID Denial of Service Vulnerability. The vulnerability is due to a condition that could occur when the affected software processes an SNMP read request that contains a request for the ciscoFlashMIB object ID (OID). An attacker could trigger this vulnerability by issuing an SNMP GET request for the ciscoFlashMIB OID on an affected device. A successful exploit could cause the affected device to restart due to a SYS-3-CPUHOG. This vulnerability affects the following Cisco devices if they are running a vulnerable release of Cisco IOS Software and are configured to use SNMP Version 2 (SNMPv2) or SNMP Version 3 (SNMPv3): Cisco Catalyst 2960-L Series Switches, Cisco Catalyst Digital Building Series Switches 8P, Cisco Catalyst Digital Building Series Switches 8U. Cisco Bug IDs: CSCvd89541.
Frequently Asked SecOps Questions (CVE-2018-0161)
FAQQ1. How critical is CVE-2018-0161 and is it actively exploited in the wild?
CVE-2018-0161 is rated CVSS 6.3 (MEDIUM) with a 30-day EPSS exploitation probability of 4.1%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2018-0161?
Primary impacted vendors include cisco across ios, catalyst_2960-l_series_switches, catalyst_digital_building_series_switches_8p, catalyst_digital_building_series_switches_8u. Scope summary: Cisco Catalyst 2960-L Series Switches, Cisco Catalyst Digital Building Series Switches 8P, and Cisco Catalyst Digital Building Series Switches 8U running vulnerable Cisco IOS Software with SNMPv2/v3 enabled. (Cisco Bug ID: CSCvd89541)
Q3. What temporary mitigations or workarounds exist for CVE-2018-0161 before patching?
[Mitigation] Apply the official Cisco IOS software updates, or implement SNMP MIB views to exclude access to the ciscoFlashMIB OID and restrict SNMP access using ACLs. Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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