CVE-2021-1586
📋 TL;DR
This vulnerability allows unauthenticated remote attackers to cause denial of service by sending crafted TCP traffic to specific ports on Cisco Nexus 9000 Series Fabric Switches in ACI mode configured for Multi-Pod or Multi-Site deployments. The attack forces device restarts, disrupting network operations. Only organizations using these specific Cisco ACI configurations are affected.
💻 Affected Systems
- Cisco Nexus 9000 Series Fabric Switches
📦 What is this software?
Nx Os by Cisco
Nx Os by Cisco
⚠️ Risk & Real-World Impact
Worst Case
Complete network outage affecting multiple pods/sites, extended downtime during device reboots, and potential cascading failures in interconnected systems.
Likely Case
Targeted device restarts causing temporary service disruption, requiring manual intervention to restore operations.
If Mitigated
No impact if proper network segmentation and access controls prevent external access to vulnerable ports.
🎯 Exploit Status
Exploitation requires sending crafted TCP packets to specific listening ports, which is relatively simple for attackers with network access.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Cisco NX-OS Release 14.2(7) or later
Vendor Advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-n9kaci-tcp-dos-YXukt6gM
Restart Required: Yes
Instructions:
1. Download appropriate firmware from Cisco Software Center. 2. Backup current configuration. 3. Upgrade to NX-OS Release 14.2(7) or later. 4. Reboot device to apply update. 5. Verify successful upgrade.
🔧 Temporary Workarounds
Network Access Control
allRestrict access to vulnerable TCP ports using firewall rules or ACLs
access-list ACL-TCP-DENY deny tcp any any eq <vulnerable-port>
access-list ACL-TCP-DENY permit ip any any
interface <interface-name>
ip access-group ACL-TCP-DENY in
Port Isolation
allEnsure vulnerable ports are not exposed to untrusted networks
interface <interface-name>
shutdown
🧯 If You Can't Patch
- Implement strict network segmentation to isolate ACI fabric from untrusted networks
- Deploy intrusion prevention systems (IPS) to detect and block crafted TCP packets targeting vulnerable ports
🔍 How to Verify
Check if Vulnerable:
Check if device is running NX-OS in ACI mode with Multi-Pod/Multi-Site configuration and version earlier than 14.2(7)Check Version:
show version | include "NX-OS: version"Verify Fix Applied:
Verify NX-OS version is 14.2(7) or later using 'show version' command and confirm device operates normally under TCP load📡 Detection & Monitoring
Log Indicators:
- Unexpected device reboots
- TCP connection attempts to specific ports (typically 22, 80, 443, or other listening ports)
- System log messages indicating abnormal TCP traffic
Network Indicators:
- Unusual TCP traffic patterns to ACI fabric ports
- Multiple TCP SYN packets to specific ports from single sources
- Traffic spikes followed by device unavailability
SIEM Query:
source="cisco-nxos" AND (event_type="reboot" OR message="%SYSTEM-5-SYS_RESTART") AND NOT user="admin"