Cisco Catalyst 4500 and 4900 Series Switches BFD Offload Denial of Service Vulnerability Advisory (CVE-2018-0155)
Executive & SecOps Summary
SECOPS ASSESSMENTCisco Catalyst 4500, 4500-X, 4900M, and 4948E Series Switches with BFD hardware offload enabled
Unauthenticated, remote attackers can transmit crafted BFD packets to trigger an iosd process crash, resulting in a complete system reload (DoS)
Apply official Cisco IOS/IOS-XE software updates or disable BFD hardware offloading as a temporary workaround
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Upgrade to the latest Cisco IOS/IOS-XE release that contains the fix for this vulnerability. Below is an example of upgrading the switch image via TFTP.
# Enter privileged EXEC mode
enable
# Verify flash space and backup current image
show flash:
copy bootflash:cat4500e-universalk9.SPA.bin tftp://10.1.1.100/cat4500e-backup.bin
# Download the patched image
copy tftp://10.1.1.100/cat4500e-universalk9.SPA.patched.bin bootflash:
# Configure boot system to use the new image
configure terminal
no boot system flash bootflash:cat4500e-universalk9.SPA.bin
boot system flash bootflash:cat4500e-universalk9.SPA.patched.bin
end
write memory2. Temporary Workaround & Hardening Configuration
If patching cannot be scheduled immediately, disable BFD hardware offloading to force software-based processing, or implement ACLs to drop unauthorized BFD traffic.
# Enter global configuration mode
configure terminal
# Disable BFD hardware offloading
no ip bfd offload
# Create ACL to block untrusted BFD traffic (UDP 3784/4784)
ip access-list extended BLOCK_UNTRUSTED_BFD
deny udp any any eq 3784
deny udp any any eq 4784
permit ip any any
# Apply ACL to the external interface
interface GigabitEthernet0/1
ip access-group BLOCK_UNTRUSTED_BFD in
end
write memory3. Log Audit & IoC Verification Commands
Verify if BFD offloading is active and inspect system logs for unexpected reloads or iosd crashes.
# Check if BFD offload is enabled in the configuration
show running-config | include bfd
# Check for system crash logs and reload history
show logging | include %SYSTEM-3-RST
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Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2018-0155) exists in the Bidirectional Forwarding Detection (BFD) hardware offload implementation of Cisco Catalyst 4500 and 4900 Series Switches. BFD is designed to rapidly detect forwarding path failures between adjacent devices, offloading packet processing to hardware ASICs for performance optimization. However, due to insufficient error handling when processing incomplete or malformed BFD headers (CWE-755, CWE-388), the system fails to gracefully handle exception states, leading to a crash of the core 'iosd' process.
2. Exploitation Mechanics & Attack Scenario
An attacker can exploit this vulnerability by sending a specially crafted BFD message with an incomplete header to or across an affected switch. Because BFD typically operates over UDP ports 3784 (single-hop) or 4784 (multi-hop), an attacker does not require authentication to inject these malicious packets. When the hardware offload engine attempts to parse the malformed packet, the unhandled exception triggers a kernel-level crash of the iosd process, forcing the entire switch to reload.
3. Infrastructure & Business Impact
With a CVSS score of 8.6 (High) and its inclusion in the CISA KEV catalog, this vulnerability represents a severe threat to enterprise network availability. A successful exploit causes immediate network disruption, tearing down routing adjacencies and causing a denial of service (DoS) condition across the core or distribution layers of the enterprise infrastructure.
Official Vendor Advisories & References
A vulnerability in the Bidirectional Forwarding Detection (BFD) offload implementation of Cisco Catalyst 4500 Series Switches and Cisco Catalyst 4500-X Series Switches could allow an unauthenticated, remote attacker to cause a crash of the iosd process, causing a denial of service (DoS) condition. The vulnerability is due to insufficient error handling when the BFD header in a BFD packet is incomplete. An attacker could exploit this vulnerability by sending a crafted BFD message to or across an affected switch. A successful exploit could allow the attacker to trigger a reload of the system. This vulnerability affects Catalyst 4500 Supervisor Engine 6-E (K5), Catalyst 4500 Supervisor Engine 6L-E (K10), Catalyst 4500 Supervisor Engine 7-E (K10), Catalyst 4500 Supervisor Engine 7L-E (K10), Catalyst 4500E Supervisor Engine 8-E (K10), Catalyst 4500E Supervisor Engine 8L-E (K10), Catalyst 4500E Supervisor Engine 9-E (K10), Catalyst 4500-X Series Switches (K10), Catalyst 4900M Switch (K5), Catalyst 4948E Ethernet Switch (K5). Cisco Bug IDs: CSCvc40729.
Frequently Asked SecOps Questions (CVE-2018-0155)
FAQQ1. How critical is CVE-2018-0155 and is it actively exploited in the wild?
CVE-2018-0155 is rated CVSS 8.6 (HIGH) with a 30-day EPSS exploitation probability of 7.7%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2018-0155?
Primary impacted vendors include cisco across catalyst 4500, catalyst 4500-x, catalyst 4900m, catalyst 4948e. Scope summary: Catalyst 4500 Supervisor Engine 6-E/6L-E/7-E/7L-E/8-E/8L-E/9-E, Catalyst 4500-X Series, Catalyst 4900M, and Catalyst 4948E Switches
Q3. What temporary mitigations or workarounds exist for CVE-2018-0155 before patching?
[Mitigation] Apply official Cisco IOS/IOS-XE software updates or disable BFD hardware offloading as a temporary workaround Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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