CVE-2026-84316 in Nginx UI
Summary
by MITRE • 10/13/2026
Nginx UI is a web user interface for the Nginx web server. Prior to 2.4.1, config.ValidateConfigDirectives splits configuration by newline and checks only the first directive token on each line even though Nginx permits multiple semicolon-separated directives on one line. An authenticated caller can place a benign directive first and a restricted load_module, js_import, perl, or user root directive later on the same line, or can use an unrestricted include directive to load restricted content. These mechanisms bypass validation on REST, nginx_config_modify, site, and stream write paths; when an executable module is available, the injected configuration can execute code in the Nginx worker context. This issue is fixed in version 2.4.1.
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Analysis
by VulDB Data Team • 10/13/2026
The vulnerability identified in Nginx UI prior to version 2.4.1 represents a critical logic flaw within its configuration validation mechanism that allows authenticated users to bypass security restrictions and potentially achieve remote code execution. The core of the issue lies in how the application parses and validates Nginx configuration files, specifically through the config.ValidateConfigDirectives function. This function incorrectly splits configuration lines by newline characters and subsequently inspects only the first directive token on each line for compliance with allowed directives. However, standard Nginx syntax permits multiple semicolon-separated directives to exist on a single line. By exploiting this discrepancy between the parser's simplistic logic and the actual capabilities of the underlying web server software, an attacker can construct malicious configuration lines that appear benign during validation but execute restricted operations when processed by Nginx itself.
An authenticated adversary with write access to the REST API endpoints for nginx_config_modify, site configurations, or stream settings can leverage this flaw to inject arbitrary code into the worker process context. The attack vector involves placing a permitted directive at the beginning of a configuration line followed by a semicolon and then appending restricted directives such as load_module, js_import, perl, or user root. Because the validation routine only checks the first token before the semicolon, it approves the entry. When Nginx subsequently loads this configuration, it interprets all directives on that line, including those hidden after the semicolon. Alternatively, an attacker can use unrestricted include directives to load external files containing restricted content, effectively circumventing the whitelist of allowed operations entirely. This bypass mechanism is particularly dangerous because it allows for the loading of dynamic modules or execution of JavaScript and Perl scripts within the high-privilege Nginx worker process.
The operational impact of this vulnerability is severe, as successful exploitation can lead to arbitrary code execution with the privileges of the user running the Nginx workers. This compromises not only the integrity of the web server but also potentially allows for lateral movement across the network if other services are accessible from the compromised host. The flaw aligns with CWE-20 Improper Input Validation, as the application fails to correctly validate input that contains multiple directives on a single line. Furthermore, it relates to CWE-78 OS Command Injection and CWE-94 Code Injection since the injected configuration can trigger execution of external modules or scripts. In terms of the MITRE ATT&CK framework, this vulnerability facilitates techniques associated with Initial Access via Valid Accounts and Execution through Server Side Scripting, allowing an attacker to establish persistence or escalate privileges within the server environment by manipulating configuration files that are processed at runtime rather than just during startup validation phases.
Mitigation for this issue requires immediate upgrading of Nginx UI to version 2.4.1 or later, where the parsing logic has been corrected to properly handle semicolon-separated directives and validate all tokens on a line against security policies. In environments where an upgrade is not immediately feasible, administrators should enforce strict access controls on the REST API endpoints involved in configuration management, ensuring that only trusted administrative accounts have write permissions. Additionally, implementing network segmentation to restrict access to these management interfaces from untrusted networks can reduce the attack surface. Regular auditing of Nginx configuration files for unexpected directives and monitoring worker process behavior for anomalous activity are also recommended defensive measures to detect potential exploitation attempts before they result in full system compromise.