CVE-2021-25297: Nagios XI RCE Vulnerability Explained

CVE-2021-25297 Overview

CVE-2021-25297 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/switch/switch.inc.php due to improper sanitization of authenticated user-controlled input. An attacker with valid credentials can exploit this flaw via a single HTTP request to achieve command injection on the Nagios XI server, potentially leading to full system compromise.

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 the underlying Nagios XI server, potentially leading to complete system takeover, data exfiltration, and lateral movement within the network.

Affected Products

• Nagios XI version 5.7.5 and potentially earlier versions
• Systems running the vulnerable switch.inc.php configuration wizard component
• Enterprise monitoring infrastructure utilizing Nagios XI

Discovery Timeline

• 2021-02-15 - CVE-2021-25297 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2021-25297

Vulnerability Analysis

This command injection vulnerability (CWE-78) stems from insufficient input validation in the Nagios XI switch configuration wizard. The vulnerable component processes user-supplied input without proper sanitization before passing it to system commands. Since Nagios XI typically runs with elevated privileges to perform monitoring operations, successful exploitation grants attackers the ability to execute commands with the same privilege level as the web server process.

The attack requires authentication, meaning an attacker must first obtain valid credentials to the Nagios XI web interface. However, once authenticated (even with low-privilege user credentials), the exploitation is straightforward and can be accomplished through a single HTTP request. This makes it particularly dangerous in environments where multiple users have access to the Nagios XI interface or where credentials may have been compromised.

Root Cause

The root cause lies in the improper handling of user-controlled input within the switch.inc.php configuration wizard file. The application fails to adequately sanitize or escape special characters and shell metacharacters before incorporating user input into OS commands. This allows attackers to inject arbitrary commands that are then executed by the underlying operating system.

Attack Vector

The attack is network-based and requires low privileges (authenticated access) with no user interaction. An authenticated attacker sends a specially crafted HTTP request to the vulnerable endpoint containing malicious OS commands embedded within expected parameters. The Nagios XI application processes this input and inadvertently executes the injected commands on the server.

The exploitation mechanism involves manipulating parameters that are eventually passed to shell execution functions. By including shell metacharacters such as semicolons, pipes, or command substitution syntax, attackers can break out of the intended command context and execute arbitrary commands.

For detailed technical analysis and exploitation techniques, refer to the Fastly Blog Command Injection Analysis and the GitHub Nagios XI Bugs Report.

Detection Methods for CVE-2021-25297

Indicators of Compromise

• Unusual HTTP requests to /nagiosxi/includes/configwizards/switch/ endpoints containing shell metacharacters
• Unexpected child processes spawned by the Nagios XI web server process (typically httpd or apache2)
• Suspicious command execution patterns in system logs originating from the web server user context
• Network connections initiated from the Nagios XI server to unexpected external destinations

Detection Strategies

• Monitor web server access logs for requests to the vulnerable switch.inc.php endpoint with suspicious parameter values
• Implement web application firewall (WAF) rules to detect command injection patterns in HTTP requests
• Deploy endpoint detection and response (EDR) solutions to identify anomalous process creation from web server contexts
• Analyze system command history and audit logs for commands executed by the web server user account

Monitoring Recommendations

• Enable detailed logging for the Nagios XI application and underlying web server
• Configure alerts for process execution anomalies on Nagios XI servers
• Monitor outbound network traffic from monitoring infrastructure for signs of reverse shells or data exfiltration
• Implement file integrity monitoring on critical Nagios XI configuration and script files

How to Mitigate CVE-2021-25297

Immediate Actions Required

• Upgrade Nagios XI to the latest available version that addresses this vulnerability
• Review and restrict user access to the Nagios XI web interface, limiting accounts to only those with legitimate need
• Implement network segmentation to isolate Nagios XI servers from sensitive network segments
• Audit existing Nagios XI user accounts and remove or disable any unnecessary accounts

Patch Information

Nagios has released security updates to address this vulnerability. Organizations should upgrade to a patched version as soon as possible. Check the Nagios XI Version Information page for the latest secure versions and upgrade instructions. Given that this vulnerability is listed in the CISA Known Exploited Vulnerabilities Catalog, immediate patching is strongly recommended.

Workarounds

• Restrict network access to the Nagios XI web interface using firewall rules or VPN requirements
• Implement a web application firewall (WAF) with rules to block command injection attempts
• Disable or restrict access to the switch configuration wizard if not required for operations
• Apply principle of least privilege to all Nagios XI user accounts

# Example: Restrict access to Nagios XI web interface via iptables
# Allow only specific management network to access Nagios XI
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