CVE-2026-119742 in Nagios XI
Summary
by MITRE • 11/03/2026
Nagios XI before 2026R2 contains an OS command injection vulnerability in the MIB upload feature where the install_mib() function in mibs.php sanitizes uploaded filenames with an incomplete character deny-list that omits semicolons and pipes before passing them to a grep command executed via exec(). An authenticated administrator can upload a MIB file with a crafted filename using tab characters to bypass the deny-list and pipe to sh via a multipart POST request to mibs.php, resulting in arbitrary OS command execution as the web server user.
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Analysis
by VulDB Data Team • 11/03/2026
The vulnerability identified in Nagios XI prior to version 2026R2 represents a critical security flaw within the Management Information Base upload functionality. This issue stems from an operating system command injection weakness located specifically in the install_mib() function found within the mibs.php script. The core of the problem lies in how user-supplied input, namely the filename of an uploaded MIB file, is processed before being passed to underlying system commands. While the application attempts to sanitize this input by employing a character deny-list intended to block dangerous shell metacharacters, the implementation is fundamentally flawed due to its incompleteness. Specifically, the sanitization logic fails to adequately filter out semicolons and pipes, which are critical delimiters in Unix-like operating systems that allow for command chaining and redirection.
The technical mechanism of exploitation relies on the interaction between this incomplete filtering and the use of tab characters within the filename string. When an authenticated administrator uploads a MIB file via a multipart POST request to mibs.php, they can craft the filename to include tab characters interspersed with malicious payloads. The application's validation logic does not properly account for these whitespace variations or fails to strip them before passing the data forward. Consequently, when the sanitized but still dangerous input is passed to the grep command via the exec() function, the shell interprets the tabs and unfiltered special characters as valid syntax. This allows an attacker to break out of the intended context of a simple file search operation and inject arbitrary commands that are executed by the web server process with its associated privileges.
From a classification perspective, this vulnerability aligns closely with CWE-78 Improper Neutralization of Special Elements used in an OS Command commonly known as OS Command Injection. The failure to properly validate or escape special characters before their use in system calls is a classic example of this weakness. Furthermore, the exploitation technique maps directly to MITRE ATT&CK techniques related to command and script interpretation, specifically where attackers leverage legitimate administrative interfaces to execute arbitrary code on the host system. The reliance on an authenticated administrator account for initial access places this within the context of privilege abuse or insider threat scenarios if credentials are compromised through other means such as phishing or credential stuffing attacks.
The operational impact of successfully exploiting this vulnerability is severe, resulting in full remote command execution under the context of the web server user. In many standard deployments, the web server runs with elevated privileges sufficient to read sensitive configuration files, modify system settings, and potentially pivot further into the internal network. An attacker could use this foothold to exfiltrate confidential data stored within Nagios XI or on connected systems, install backdoors for persistent access, or disrupt service availability by terminating critical processes. The ability to execute commands as the web server user significantly lowers the barrier for subsequent privilege escalation attacks against the underlying operating system kernel or other running services with higher permission levels.
Mitigation strategies must prioritize immediate patching of Nagios XI to version 2026R2 or later, where this specific input validation logic has been corrected. Until patches are applied, administrators should restrict access to the MIB upload interface using network-level controls such as firewall rules that limit connectivity to trusted IP addresses only. Additionally, implementing strict web application firewalls can help detect and block requests containing suspicious patterns like tab characters in filenames or attempts to inject shell metacharacters into HTTP parameters. It is also advisable to review audit logs for any unusual activity related to the mibs.php endpoint and ensure that the principle of least privilege is enforced by running the web server with minimal necessary permissions, thereby limiting the blast radius should a compromise occur.