Cisco NX-OS Heap Buffer Overflow in NX-API Allows Unauthenticated Root Code Execution

Networking infrastructure leader Cisco Systems has issued an emergency security advisory documenting a critical heap-based buffer overflow vulnerability, tracked as CVE-2026-76471 (Advisory ID: cisco-sa-napi-rce-r2shwu2j, CVSS 9.8), in the NX-API management interface of Cisco NX-OS Software. The flaw allows remote, unauthenticated threat actors to execute arbitrary code with root operating system privileges on mission-critical Nexus datacenter switches and enterprise fabric interconnects.

Root Cause Analysis: Heap Corruption in NX-API HTTP Parser

The NX-API subsystem is an embedded programmatic management engine that translates structured HTTP/HTTPS requests (JSON or XML payloads) into internal command-line interface (CLI) instructions. The vulnerability resides within the C-based daemon handling incoming HTTP request headers and payload chunks.

When parsing specially formatted multi-part MIME headers or oversized JSON request buffers, the parser allocates a fixed-size heap chunk without verifying the decoded payload length against the allocated buffer boundary (CWE-122). By transmitting an HTTP request containing mismatched content-length descriptors and crafted multi-byte boundary headers, an unauthenticated attacker triggers memory corruption in adjacent heap structures:

/* Conceptual layout of HTTP request triggering NX-API heap boundary corruption */
POST /ins HTTP/1.1
Host: 10.100.20.1:443
User-Agent: Mozilla/5.0
Content-Type: application/json
Content-Length: 4096
X-NXAPI-Chunk-Control: OVERFLOW_DESCRIPTOR_CHUNK_STREAM

{
  "ins_api": {
    "version": "1.0",
    "type": "cli_conf",
    "chunk": "AAAA...[Controlled Heap Padding]...[Shellcode Trampoline Address]..."
  }
}

If the heap metadata is overwritten with valid pointers to injected instructions, the control flow of the underlying nginx/nxapi supervisor daemon is diverted, executing arbitrary binaries under root privileges within the Linux kernel subsystem of the switch. Alternatively, incomplete overwrites cause the supervisor process to terminate unexpectedly, triggering a watchdog reset and an immediate reload of the network switch.

Impacted Cisco Nexus Switch Families

The flaw specifically affects devices running vulnerable builds of Cisco NX-OS that have the NX-API feature enabled (configured via the feature nxapi command):

Defensive Checklist & Remediation Protocols

Cisco has confirmed that no software workarounds exist to neutralize the heap overflow vulnerability while keeping NX-API operational. Network operations and security engineering teams must execute the following protocol:

1. Verify NX-API Activation Status: Connect to the switch console and execute the following verification command:

   # Check whether the vulnerable NX-API subsystem is running
   switch# show feature | include nxapi
   nxapi                 1          enabled

   If NX-API is enabled but not actively required for automation pipelines, immediately disable it by issuing no feature nxapi in configuration mode.
2. Apply Official Maintenance Releases: Upgrade affected switches to Cisco NX-OS maintenance releases that incorporate Bug IDs CSCwu46199 and CSCwu58579.
3. Enforce Management Plane ACLs: Ensure that switch management interfaces (mgmt0) and any in-band interfaces listening on ports 80/443 are strictly isolated within a dedicated Out-of-Band (OOB) management VRF, protected by strict access control lists (ACLs) permitting traffic only from authenticated configuration management bastions.