CVE-2023-27992: Zyxel NAS326 Firmware RCE Vulnerability

CVE-2023-27992 Overview

CVE-2023-27992 is a critical pre-authentication command injection vulnerability affecting multiple Zyxel Network Attached Storage (NAS) devices. The vulnerability exists in the web management interface of Zyxel NAS326, NAS540, and NAS542 firmware and allows unauthenticated remote attackers to execute arbitrary operating system commands by sending specially crafted HTTP requests to the vulnerable device. This vulnerability requires no authentication, making it particularly dangerous for internet-exposed NAS devices.

Critical Impact

This vulnerability is actively exploited in the wild and has been added to CISA's Known Exploited Vulnerabilities (KEV) catalog. Unauthenticated attackers can achieve complete system compromise through remote command execution without any user interaction.

Affected Products

• Zyxel NAS326 firmware versions prior to V5.21(AAZF.14)C0
• Zyxel NAS540 firmware versions prior to V5.21(AATB.11)C0
• Zyxel NAS542 firmware versions prior to V5.21(ABAG.11)C0

Discovery Timeline

• June 19, 2023 - CVE-2023-27992 published to NVD
• October 27, 2025 - Last updated in NVD database

Technical Details for CVE-2023-27992

Vulnerability Analysis

This vulnerability is classified as CWE-78 (Improper Neutralization of Special Elements used in an OS Command), commonly known as OS Command Injection. The flaw resides in the web management interface of affected Zyxel NAS devices, where user-supplied input in HTTP requests is improperly sanitized before being passed to system-level command execution functions.

The pre-authentication nature of this vulnerability is particularly concerning because attackers do not need valid credentials to exploit it. Any network-accessible Zyxel NAS device running vulnerable firmware versions can be compromised by remote attackers who simply send malicious HTTP requests to the device's web interface. Once exploited, attackers gain the ability to execute operating system commands with the privileges of the web server process, which typically runs with elevated permissions on embedded NAS systems.

Root Cause

The root cause of CVE-2023-27992 is insufficient input validation and sanitization in the HTTP request handling code of the Zyxel NAS web management interface. Specifically, user-controllable parameters in HTTP requests are passed directly to shell command execution functions without proper escaping or filtering of shell metacharacters. This allows attackers to inject arbitrary commands by including shell operators (such as semicolons, backticks, or command substitution sequences) within their HTTP requests.

Attack Vector

The attack is delivered over the network via specially crafted HTTP requests to the vulnerable NAS device's web management interface. An attacker can exploit this vulnerability by:

1. Identifying a vulnerable Zyxel NAS device exposed to the network
2. Crafting an HTTP request containing malicious shell metacharacters and commands in a vulnerable parameter
3. Sending the crafted request to the target device's web interface
4. The injected commands execute on the underlying Linux operating system with the privileges of the web server

The vulnerability manifests when the web application processes certain HTTP request parameters and passes them unsanitized to system command execution. Attackers can leverage shell metacharacters to break out of the intended command context and execute arbitrary commands. For detailed technical information, refer to the Zyxel Security Advisory.

Detection Methods for CVE-2023-27992

Indicators of Compromise

• Unusual HTTP requests to Zyxel NAS web interfaces containing shell metacharacters (;, |, $(), backticks)
• Unexpected outbound network connections from NAS devices to unknown IP addresses
• Creation of unauthorized user accounts or SSH keys on NAS devices
• Suspicious processes running on NAS devices, particularly reverse shells or cryptocurrency miners
• Unexpected modifications to system configuration files or scheduled tasks

Detection Strategies

• Deploy network intrusion detection systems (NIDS) with signatures for command injection patterns in HTTP traffic destined for NAS devices
• Monitor web server access logs on Zyxel NAS devices for requests containing shell metacharacters or encoded command sequences
• Implement behavioral analysis to detect anomalous process execution on NAS devices following HTTP requests
• Configure SentinelOne agents to monitor endpoint behavior for post-exploitation activities including unauthorized shell spawning

Monitoring Recommendations

• Continuously monitor network traffic to and from Zyxel NAS devices, particularly HTTP/HTTPS traffic to management interfaces
• Review NAS device logs regularly for authentication failures, unusual command execution, or configuration changes
• Implement asset inventory to track all Zyxel NAS devices and their firmware versions across the organization
• Configure alerts for any internet-facing exposure of NAS device management interfaces

How to Mitigate CVE-2023-27992

Immediate Actions Required

• Immediately update all affected Zyxel NAS devices to the patched firmware versions: NAS326 V5.21(AAZF.14)C0 or later, NAS540 V5.21(AATB.11)C0 or later, NAS542 V5.21(ABAG.11)C0 or later
• Disconnect vulnerable NAS devices from internet-facing networks until patches can be applied
• Restrict access to NAS management interfaces to trusted internal networks only using firewall rules
• Audit NAS devices for signs of compromise if they were previously exposed to untrusted networks
• Review and remove any unauthorized user accounts, SSH keys, or scheduled tasks on NAS devices

Patch Information

Zyxel has released firmware updates to address CVE-2023-27992. Organizations should apply the following patched firmware versions:

• NAS326: Update to firmware version V5.21(AAZF.14)C0 or later
• NAS540: Update to firmware version V5.21(AATB.11)C0 or later
• NAS542: Update to firmware version V5.21(ABAG.11)C0 or later

Firmware updates are available through the Zyxel Security Advisory. Due to the critical nature and active exploitation of this vulnerability, patching should be treated as the highest priority.

Workarounds

• Block external access to NAS management interfaces at the network perimeter using firewall rules to deny inbound HTTP/HTTPS traffic from untrusted sources
• Implement network segmentation to isolate NAS devices on a dedicated management VLAN accessible only from trusted administrative workstations
• If remote access is required, use a VPN solution to access the internal network rather than exposing NAS interfaces directly to the internet
• Deploy a web application firewall (WAF) in front of NAS devices to filter requests containing command injection patterns

# Example firewall rule to restrict NAS management access (iptables)
# Allow management access only from trusted admin subnet
iptables -A INPUT -p tcp --dport 80 -s 192.168.100.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -s 192.168.100.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -j DROP