CVE-2024-20399

What it actually does

CVE-2024-20399 is a command injection vulnerability in the Cisco NX-OS command-line interface. An attacker who already has Administrator-level access to an affected Cisco Nexus or MDS device can pass crafted arguments to certain configuration CLI commands and cause commands to run on the underlying operating system as root. This moves the attacker from authorized network-device administration into unrestricted root-level control of the appliance OS, enabling file modification, process execution, persistence, credential access, traffic observation, and log tampering.

Why it matters to the business

Although exploitation requires Administrator credentials, the impact is severe because a compromised NX-OS device can affect core data center switching, storage networking, and network availability. Successful exploitation could allow an attacker to disrupt routing or switching, modify device behavior, implant persistent tooling, hide activity, intercept or redirect traffic, and use the device as a privileged foothold deeper in the network. This vulnerability is listed in CISA KEV with active exploitation in the wild and a remediation due date of 2024-07-23, so unpatched exposed administrative planes should be treated as high priority even with the high-privilege prerequisite.

How an attack unfolds

An attacker first obtains Administrator credentials for an affected Cisco NX-OS device, commonly through credential theft, phishing, password reuse, exposed management services, or compromise of a jump host or network management platform. The attacker then establishes an administrative CLI session over an allowed management path such as SSH, console, or another configured administrative access method. From configuration mode, the attacker supplies specially crafted input to an affected CLI command whose arguments are not sufficiently validated. NX-OS passes the crafted argument to the underlying operating system in an unsafe way, causing attacker-controlled commands to execute as root. After gaining root-level execution, the attacker may deploy scripts or binaries, create or alter users, collect configuration and credentials, modify startup or boot-related files, interfere with logging, and pivot through trusted network-management paths. Public reporting and KEV status indicate this issue has been exploited in the wild, so exploitation should be considered realistic where Administrator credentials and vulnerable software are present.

How to fix it

Upgrade affected Cisco NX-OS software to a fixed release identified by Cisco for the specific platform and train in use. No fixed versions were provided in the input, so use the Cisco advisory and Cisco Software Checker to determine the exact corrected release for Nexus 7000, Nexus 7700, MDS 9000, and related NX-OS deployments. Where immediate upgrade is not possible, reduce exposure by tightly restricting management-plane access to trusted administration hosts, enforcing MFA for administrative access where supported through centralized AAA, eliminating shared local Administrator accounts, rotating administrative credentials, reviewing role assignments, and monitoring all configuration commands. Treat devices with suspicious administrative activity as potentially compromised; back up known-good configurations, collect forensic artifacts, rotate secrets that may have transited or been stored on the device, and consider reimaging or replacing the device if root-level compromise is suspected.

How to confirm the fix worked

Confirm exposure by inventorying all Cisco NX-OS devices and comparing their model and software release against Cisco’s official affected/fixed release matrix. Validate that each device has been upgraded to the recommended fixed release using device version output and centralized asset records. Confirm that administrative access is restricted to approved management networks and that AAA command accounting is enabled for privileged sessions. After remediation, verify that unexpected shell metacharacters or OS command invocations are rejected or logged rather than executed, without running destructive tests. Review local users, AAA configuration, startup configuration, boot variables, bootflash/logflash contents, scheduled jobs, and running processes for unauthorized changes before declaring the device clean.

Typical attack path

1. Identify Cisco NX-OS Nexus 7000/7700 or MDS 9000 devices running affected software.
2. Obtain valid Administrator credentials through theft, reuse, compromise of a management host, or abuse of privileged network-management tooling.
3. Connect to the device CLI through an authorized management channel such as SSH or console.
4. Enter configuration context and invoke an affected configuration CLI command with crafted arguments.
5. Trigger command injection and execute commands on the underlying operating system as root.
6. Establish persistence or deploy tooling on writable storage such as bootflash, logflash, or temporary directories.
7. Collect configurations, credentials, logs, and network intelligence, or alter device behavior.
8. Cover tracks by modifying files, clearing command history, changing logs, or removing temporary payloads.

How to detect it

To determine whether exploitation has already occurred, review centralized AAA, TACACS+/RADIUS, ISE, syslog, command accounting, SSH, console-server, and network-management-platform logs for Administrator sessions followed by unusual configuration commands, shell metacharacters, or attempts to invoke operating-system utilities. On affected devices, examine `show accounting log`, `show logging log`, local user configuration, startup and running configurations, boot variables, bootflash/logflash contents, temporary directories, process listings, and any bash or guest-shell history available on the platform. Focus on administrative logins from unusual source IPs, after-hours sessions, command strings containing shell control characters, creation of new users or privilege changes, unexpected files written to bootflash/logflash, outbound connections from the device to non-management hosts, and log clearing or truncation. Because attackers may have root access and can tamper with local evidence, correlate device logs with remote syslog, AAA command accounting, jump-host logs, EDR on management hosts, NetFlow, firewall logs, and configuration backup diffs.

• show accounting log
• show logging log
• bash -c
• /bin/sh
• /bin/bash
• ; sh
• ; bash
• | sh
• | bash
• `sh`
• $(sh
• $(bash
• wget http
• curl http
• scp
• tftp
• python -c
• perl -e
• nc -e
• netcat
• chmod +x
• crontab
• /etc/passwd
• /etc/shadow
• bootflash:
• logflash:
• /tmp/
• guestshell run
• clear logging
• no logging logfile

What your security team should do

• Immediately identify all Cisco NX-OS Nexus 7000/7700 and MDS 9000 devices and prioritize those reachable from broad management networks or administered through shared tooling.
• Upgrade affected devices to Cisco-recommended fixed NX-OS releases; use the vendor advisory and Cisco Software Checker for exact target versions.
• Because this issue is in CISA KEV and has been exploited in the wild, treat overdue systems as urgent and escalate remediation for any devices still vulnerable after the 2024-07-23 deadline.
• Restrict SSH, console server, SNMP, NETCONF/RESTCONF, and other management access to dedicated administration networks and hardened jump hosts.
• Enforce centralized AAA, MFA-backed administrative workflows where feasible, unique named admin accounts, least privilege roles, and command accounting.
• Rotate Administrator credentials and secrets used by network management systems, especially if exploitation or suspicious admin activity is found.
• Hunt for root-level post-exploitation artifacts on device storage, logs, and AAA records, not only failed login attempts.
• Investigate any unexplained configuration changes, new local users, unexpected files on bootflash/logflash, unusual processes, or administrative sessions at abnormal times.
• If compromise is suspected, preserve logs and filesystem artifacts before remediation, then consider reimaging from trusted media and restoring a validated configuration.