CVE-2026-20212: Cisco Nexus 9000 Series RCE Vulnerability

CVE-2026-20212 Overview

Cisco disclosed CVE-2026-20212, an unauthenticated remote code execution vulnerability in the Silicon One integration for Cisco Nexus 9000 Series Switches. The flaw stems from TCP ports 43210 and 43211 being reachable in the default Layer 3 (L3) virtual routing and forwarding (VRF) instance. A remote attacker can connect to an affected device and send crafted input that executes as code with root privileges. Successful exploitation can also crash the S1HAL process, causing the switch to reload. The vulnerability is tracked under CWE-1327: Binding to an Unrestricted IP Address.

Critical Impact
Unauthenticated network attackers can gain root code execution on affected Nexus 9000 switches and force a device reload, disrupting data center fabrics.

Affected Products

Cisco Nexus 9000 Series Switches with Silicon One integration
Devices exposing TCP port 43210 in the default L3 VRF
Devices exposing TCP port 43211 in the default L3 VRF

Discovery Timeline

2026-09-02 - CVE-2026-20212 published to the National Vulnerability Database
2026-09-02 - Last updated in NVD database

Technical Details for CVE-2026-20212

Vulnerability Analysis

The vulnerability resides in the Silicon One Hardware Abstraction Layer (S1HAL) service on Cisco Nexus 9000 switches. The service binds to TCP ports 43210 and 43211 and processes input received on those sockets without adequate authentication or input validation. Because the listeners are exposed in the default Layer 3 VRF, any host with routable access to the switch management or data plane can reach them.

An attacker who submits crafted payloads to these ports can trigger execution of attacker-controlled code in the context of the S1HAL process. That process runs with root privileges, giving the attacker full control over the switch operating system. Malformed input can alternatively cause S1HAL to crash, which forces a device reload and degrades network availability.

Root Cause

The root cause is an insecure service binding classified under [CWE-1327]. The Silicon One integration exposes management endpoints on the default L3 VRF rather than isolating them to a management-only or loopback interface. Combined with missing authentication and unsafe handling of received data, this exposure converts a diagnostic interface into a network-reachable code execution primitive.

Attack Vector

Exploitation requires only network reachability to TCP 43210 or 43211 on an affected switch. No credentials, user interaction, or prior foothold are needed. An attacker on the same routed segment, or one that can traverse ACLs to reach the default VRF, can send crafted TCP payloads that are parsed by S1HAL and result in root-level code execution or a process crash and reload. Refer to the Cisco Security Advisory for vendor-specific technical detail.

Detection Methods for CVE-2026-20212

Indicators of Compromise

Unexpected inbound TCP connections to ports 43210 or 43211 on Nexus 9000 switches from non-administrative hosts.
Unscheduled reloads or crashes of the S1HAL process recorded in switch logs.
New or unusual child processes spawned under the S1HAL process tree.
Configuration changes or new user accounts on the switch that do not correlate with authorized change tickets.

Detection Strategies

Inspect NetFlow, IPFIX, or firewall logs for traffic destined to TCP 43210 and 43211 on switch management or SVI addresses.
Alert on S1HAL process restarts, core dumps, or repeated device reloads within short windows.
Correlate switch syslog with authentication events to identify unauthenticated access that precedes crashes or configuration changes.

Monitoring Recommendations

Forward Nexus 9000 syslog and process telemetry to a centralized SIEM for retention and correlation.
Baseline expected administrative source ranges and alert on any deviation reaching the Silicon One ports.
Monitor control-plane policing (CoPP) counters for spikes on TCP ports 43210 and 43211.

How to Mitigate CVE-2026-20212

Immediate Actions Required

Apply the fixed NX-OS release published in the Cisco Security Advisory as soon as maintenance windows allow.
Restrict network reachability to TCP 43210 and 43211 using infrastructure ACLs or CoPP policies until patches are deployed.
Audit all Nexus 9000 switches for exposure of these ports in the default L3 VRF.
Review switch logs for prior evidence of exploitation, including S1HAL crashes and unexplained reloads.

Patch Information

Cisco has published fixed software addressing CVE-2026-20212. Consult the Cisco Security Advisory cisco-sa-n9k-s1-rce-EH8dEtr for the specific fixed NX-OS versions applicable to each affected Nexus 9000 platform.

Workarounds

Apply infrastructure ACLs (iACLs) on upstream devices to block inbound TCP 43210 and 43211 toward Nexus 9000 switch addresses.
Configure a CoPP policy on the switch to drop traffic destined to these ports from untrusted sources.
Segment management traffic into a dedicated management VRF and remove exposure from the default L3 VRF where feasible.

bash

# Example: block Silicon One ports at the network edge using an infrastructure ACL
ip access-list extended BLOCK_S1HAL
  deny tcp any any eq 43210
  deny tcp any any eq 43211
  permit ip any any
!
interface Ethernet1/1
  ip access-group BLOCK_S1HAL in