CVE-2025-9129 in Flexi Plugin
Summary
by MITRE • 10/03/2025
The Flexi plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the plugin for WordPress's flexi-form-tag shortcode in all versions up to, and including, 4.28 due to insufficient input sanitization and output escaping on user supplied attributes. This makes it possible for authenticated attackers, with contributor-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page.
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Analysis
by VulDB Data Team • 10/03/2025
The vulnerability in the Flexi plugin for WordPress represents a critical stored cross-site scripting flaw that undermines the security integrity of affected websites. This issue affects all versions up to and including 4.28, creating a persistent threat vector that can be exploited by malicious actors with relatively low privileges. The vulnerability specifically resides within the flexi-form-tag shortcode implementation, where user-supplied attributes are not properly sanitized or escaped before being rendered in web pages. This architectural weakness allows attackers to inject malicious scripts that persist in the database and execute whenever affected pages are accessed, making it particularly dangerous for content management systems where multiple users with varying permission levels can interact with plugin functionality.
The technical flaw stems from inadequate input validation mechanisms within the plugin's shortcode processing logic. When administrators or contributors use the flexi-form-tag shortcode with user-supplied attributes, the system fails to properly escape special characters and validate the integrity of submitted data. This insufficient sanitization creates a pathway for attackers to inject malicious JavaScript code through seemingly benign form parameters. The vulnerability operates at the application layer and specifically targets the output rendering process where attributes are directly embedded into HTML responses without proper context-aware escaping mechanisms. According to CWE-79, this represents a classic stored cross-site scripting vulnerability that allows attackers to execute arbitrary scripts in the victim's browser context.
The operational impact of this vulnerability extends beyond simple script execution, as it enables authenticated attackers with contributor-level access or higher to compromise user sessions and potentially escalate privileges within the WordPress environment. Once deployed, malicious scripts can capture user credentials, hijack sessions, redirect users to malicious sites, or perform actions on behalf of authenticated users. The persistence aspect of stored XSS means that even after the initial injection, the malicious code continues to execute whenever any user accesses pages containing the compromised shortcode, creating a continuous threat vector that can affect multiple users over extended periods. This vulnerability particularly impacts websites where contributors have access to form creation functionality or where plugins are used to generate dynamic content through shortcode parameters.
Mitigation strategies should focus on immediate patching of the affected plugin to version 4.29 or later, which contains the necessary sanitization and escaping fixes. Organizations should implement restrictive user permission policies that limit contributor-level access to form-related functionalities and establish regular security audits of installed plugins. Network-based solutions such as web application firewalls can provide additional protection layers by detecting and blocking suspicious script injection attempts. The vulnerability aligns with ATT&CK technique T1566.001, which covers the exploitation of web applications through input validation weaknesses, emphasizing the importance of proper parameter validation and output encoding practices. Security teams should also consider implementing content security policies that restrict script execution and monitor for unusual shortcode usage patterns that might indicate exploitation attempts.