CVE-2021-25298: Nagios XI OS Command Injection Vulnerability

CVE-2021-25298 Overview

CVE-2021-25298 is an OS command injection vulnerability affecting Nagios XI version 5.7.5. The vulnerability exists in the file /usr/local/nagiosxi/html/includes/configwizards/cloud-vm/cloud-vm.inc.php due to improper sanitization of authenticated user-controlled input. An attacker can exploit this flaw through a single HTTP request to achieve remote code execution on the Nagios XI server.

Critical Impact

This vulnerability is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, indicating active exploitation in the wild. Authenticated attackers can execute arbitrary OS commands on affected Nagios XI servers, potentially leading to complete system compromise.

Affected Products

• Nagios XI version 5.7.5
• Nagios XI versions prior to patched releases

Discovery Timeline

• 2021-02-15 - CVE-2021-25298 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2021-25298

Vulnerability Analysis

This command injection vulnerability stems from insufficient input validation in the Cloud VM configuration wizard component of Nagios XI. The affected PHP file cloud-vm.inc.php processes user-supplied input without proper sanitization before passing it to system shell commands. Since this is an authenticated vulnerability, an attacker must first obtain valid credentials to the Nagios XI web interface, though the required privilege level is low.

The vulnerability allows attackers to inject arbitrary shell commands that execute with the privileges of the Nagios XI web server process. Successful exploitation can result in unauthorized access to sensitive monitoring data, lateral movement within the network, installation of backdoors, or complete takeover of the monitoring infrastructure.

Root Cause

The root cause of CVE-2021-25298 is improper input sanitization (CWE-78: Improper Neutralization of Special Elements used in an OS Command). The cloud-vm.inc.php file fails to adequately sanitize user-controlled parameters before incorporating them into shell commands. This allows metacharacters and command separators to break out of the intended command context and execute attacker-supplied commands.

Attack Vector

The attack is network-based and requires low-privilege authenticated access to the Nagios XI web interface. An attacker can craft a malicious HTTP request targeting the Cloud VM configuration wizard endpoint. The request contains specially crafted input that, when processed by the vulnerable PHP script, results in OS command execution.

The vulnerability can be exploited remotely without user interaction once authenticated. Exploitation payloads typically use shell metacharacters such as backticks, semicolons, or command substitution syntax to inject malicious commands alongside legitimate input parameters.

Public exploit code is available through Packet Storm Security, and additional technical analysis can be found in the Fastly blog post covering related Nagios XI command injection vulnerabilities (CVE-2021-25296/25297/25298).

Detection Methods for CVE-2021-25298

Indicators of Compromise

• Suspicious HTTP requests to /nagiosxi/includes/configwizards/cloud-vm/ endpoints containing shell metacharacters
• Unexpected child processes spawned by the Apache/PHP web server process
• Unusual outbound network connections from the Nagios XI server
• Web server logs showing requests with encoded shell commands or unusual parameter values
• Creation of unauthorized files in web-accessible directories

Detection Strategies

• Monitor web application firewall (WAF) logs for command injection patterns targeting Nagios XI endpoints
• Implement file integrity monitoring on Nagios XI configuration and wizard directories
• Configure SIEM rules to alert on shell command execution by web server processes
• Review Nagios XI access logs for authentication from unexpected IP addresses followed by wizard endpoint access

Monitoring Recommendations

• Enable verbose logging for Nagios XI web application components
• Deploy endpoint detection and response (EDR) solutions on Nagios XI servers to detect post-exploitation activity
• Implement network segmentation to limit lateral movement from compromised monitoring infrastructure
• Configure alerts for any new process execution by the Nagios XI web server user account

How to Mitigate CVE-2021-25298

Immediate Actions Required

• Upgrade Nagios XI to a patched version immediately; consult the Nagios XI Versions page for available updates
• Restrict network access to Nagios XI administrative interfaces to trusted IP addresses only
• Review Nagios XI user accounts and remove unnecessary privileged access
• Audit recent authentication logs for suspicious login activity
• Consider taking vulnerable Nagios XI instances offline until patching is complete

Patch Information

Nagios has released updated versions of Nagios XI that address this vulnerability. Organizations should upgrade to the latest available version of Nagios XI as documented on the official Nagios XI downloads page. Given that this vulnerability is listed in CISA's Known Exploited Vulnerabilities catalog, federal agencies are required to apply patches according to CISA deadlines, and all organizations should prioritize remediation.

Workarounds

• Implement strict IP-based access controls to limit Nagios XI web interface access to authorized administrators only
• Deploy a web application firewall (WAF) with rules to block command injection attempts targeting known vulnerable endpoints
• Disable or remove the Cloud VM configuration wizard if not actively in use
• Implement network segmentation to isolate Nagios XI servers from critical infrastructure

# Example: Restrict access to Nagios XI via iptables
# Allow only trusted management network
iptables -A INPUT -p tcp --dport 80 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -j DROP