CVE-2021-36828 in WP Maintenance Plugin
Summary
by MITRE • 04/15/2022
Authenticated (admin+) Stored Cross-Site Scripting (XSS) in WP Maintenance (WordPress plugin)
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Analysis
by VulDB Data Team • 05/01/2025
The vulnerability CVE-2021-36828 represents a critical authenticated stored cross-site scripting flaw discovered in the WP Maintenance WordPress plugin, affecting versions prior to 2.2.0. This vulnerability specifically targets administrators and users with elevated privileges, making it particularly dangerous within WordPress environments where administrative access is often compromised or mismanaged. The flaw exists within the plugin's handling of user input in maintenance mode configuration settings, where unfiltered data is directly stored in the database and subsequently reflected in subsequent page renders without proper sanitization or output encoding. The vulnerability falls under CWE-79 which specifically addresses cross-site scripting weaknesses in web applications, and aligns with ATT&CK technique T1190 which covers exploitation of vulnerabilities in web applications.
The technical implementation of this vulnerability allows authenticated users with administrator or higher privileges to inject malicious JavaScript code through the plugin's maintenance mode settings interface. When administrators configure maintenance mode parameters, the plugin fails to properly sanitize or escape user-supplied input before storing it in the WordPress database. This stored data is then retrieved and displayed on subsequent administrative pages without appropriate output encoding, creating a persistent XSS vector that can execute malicious scripts in the context of any user who views the affected pages. The vulnerability is particularly concerning because it requires only administrative privileges to exploit, which are often less strictly controlled than other system access levels.
The operational impact of this vulnerability extends beyond simple script execution, as it can be leveraged to steal administrative sessions, modify website content, redirect users to malicious sites, or even escalate privileges within the WordPress environment. An attacker who gains access to an administrator account can craft malicious payloads that will execute whenever any administrator visits the maintenance mode configuration pages, potentially leading to complete compromise of the WordPress installation. The stored nature of the vulnerability means that the malicious scripts will persist until manually removed from the database, allowing for prolonged exploitation without requiring repeated authentication. This makes the vulnerability particularly dangerous in shared hosting environments or multi-user WordPress installations where administrators may not regularly monitor all configuration pages.
Mitigation strategies for CVE-2021-36828 should prioritize immediate patching to version 2.2.0 or later of the WP Maintenance plugin, which includes proper input sanitization and output encoding measures. Organizations should also implement network monitoring to detect suspicious activities related to maintenance mode configuration changes and consider implementing additional access controls for administrative functions. Security teams should conduct thorough audits of all installed WordPress plugins to identify similar vulnerabilities and establish regular update procedures. The vulnerability demonstrates the critical importance of proper input validation and output encoding in web applications, as recommended by OWASP Top Ten and the CWE guidelines for preventing cross-site scripting attacks. Additionally, implementing content security policies and regular security scanning of WordPress installations can help detect and prevent exploitation of similar vulnerabilities in the broader WordPress ecosystem.