CVE-2026-103339 in Metform Plugin
Summary
by MITRE • 10/01/2026
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') vulnerability in Wpmet Metform metform allows Stored XSS.This issue affects Metform: from n/a through 4.3.0.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 10/01/2026
The identified security flaw represents a classic instance of Improper Neutralization of Input During Web Page Generation, commonly known as Cross-Site Scripting or XSS. Specifically, this vulnerability manifests as a Stored XSS within the Wpmet Metform plugin for WordPress. Unlike reflected XSS attacks where malicious scripts are executed immediately via a crafted link, stored vulnerabilities persist on the target server. In the context of Metform, which is designed to create and manage forms on websites, an attacker can inject malicious script payloads into form fields that are subsequently saved in the database without proper sanitization or validation. This means that every time a user views a page containing data from these compromised forms, the embedded scripts execute within their browser session under the context of the vulnerable website.
The technical root cause lies in the insufficient input handling mechanisms during the rendering process. When form submissions are processed and later displayed to other users or administrators, the application fails to encode special characters such as angle brackets, quotes, and ampersands that have syntactic meaning in HTML and JavaScript. This oversight allows an attacker to break out of the expected data context and inject executable code. The vulnerability affects versions of Metform ranging from no specific lower bound through version 4.3.0, indicating a long-standing issue where input validation has not been adequately enforced across multiple releases.
The operational impact of this stored XSS is severe because it facilitates persistent attacks against all visitors who access the affected pages. Attackers can leverage this capability to steal session cookies, hijack user sessions, and perform actions on behalf of authenticated users without their knowledge. If an administrator views a page containing the malicious payload, the attacker could potentially gain administrative privileges by executing arbitrary JavaScript commands that interact with the WordPress admin interface. This leads to unauthorized access, data exfiltration, defacement of the website content, or even using the compromised server as a pivot point for further network intrusions. The trust users place in the web application is fundamentally undermined when their browsers are tricked into running untrusted code from what appears to be a legitimate source.
From an industry standards perspective, this vulnerability aligns with CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting'). It also maps closely to MITRE ATT&CK techniques such as T1059.007 (JavaScript) and potentially T1213 (Data from Information Repositories) if the attacker uses stored data to facilitate further attacks. The persistence mechanism characteristic of stored XSS places it in a higher risk category compared to reflected variants due to its ability to affect multiple victims automatically without requiring social engineering for each individual attack instance.
To mitigate this vulnerability, immediate action is required by updating the Metform plugin to a version later than 4.3.0 where the input validation and output encoding issues have been resolved. Developers should implement strict server-side input sanitization using whitelisting approaches rather than blacklisting specific characters, ensuring that only expected data formats are accepted. Furthermore, all dynamic content rendered in web pages must be properly encoded based on its context, such as HTML entity encoding for text nodes or attribute values. Implementing Content Security Policy headers can also provide an additional layer of defense by restricting the sources from which scripts can be loaded and executed, thereby reducing the impact even if a bypass occurs. Regular security audits and code reviews focusing on form handling logic are essential to prevent similar flaws in future development cycles.