CVE-2024-3920 in Flattr Plugin
Summary
by MITRE • 05/23/2024
The Flattr WordPress plugin through 1.2.2 does not sanitise and escape some of its settings, which could allow high privilege users such as admin to perform Stored Cross-Site Scripting attacks even when the unfiltered_html capability is disallowed (for example in multisite setup)
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Analysis
by VulDB Data Team • 03/28/2025
The Flattr WordPress plugin version 1.2.2 contains a critical stored cross-site scripting vulnerability that affects high-privilege users including administrators. This vulnerability stems from inadequate input sanitization and output escaping mechanisms within the plugin's settings handling functionality. The flaw specifically targets the plugin's ability to process and store user-supplied data without proper validation, creating an avenue for malicious code injection that can persist across user sessions. The vulnerability is particularly concerning because it operates even when WordPress security measures such as the unfiltered_html capability are restricted, which typically prevents non-admin users from executing potentially harmful scripts. This scenario commonly occurs in multisite WordPress installations where security policies are more stringent and user permissions are carefully managed.
The technical implementation of this vulnerability involves the plugin's failure to properly sanitize user inputs before storing them in the WordPress database and subsequently outputting them to web pages without appropriate HTML escaping. When administrators access plugin settings or view pages where these stored values are rendered, the malicious scripts execute within the context of other users' browsers. The vulnerability is classified under CWE-79 as a failure to sanitize user inputs, which directly enables XSS attacks through stored data. This type of vulnerability allows attackers to inject malicious scripts that can steal session cookies, redirect users to malicious sites, or perform actions on behalf of authenticated users. The attack vector specifically targets the plugin's administrative interface where users with sufficient privileges can manipulate settings that are later reflected in web pages without proper security measures.
The operational impact of this vulnerability extends beyond simple script execution as it provides attackers with persistent access to administrative functions and user data within the WordPress environment. In a multisite setup where the unfiltered_html capability is disabled, the vulnerability becomes particularly dangerous because it circumvents the expected security boundaries that typically protect against such attacks. An attacker who gains access to an administrator account can inject malicious scripts that will execute whenever other users view pages containing the compromised plugin data. The stored nature of this XSS vulnerability means that the malicious code remains active until manually removed from the database, potentially affecting all users who interact with the compromised WordPress site. This vulnerability can be exploited to escalate privileges, steal sensitive information, or redirect users to phishing sites that can harvest credentials and other confidential data.
Mitigation strategies for this vulnerability should focus on immediate patching of the Flattr plugin to version 1.2.3 or later, which addresses the sanitization and escaping issues. Administrators should also implement additional security measures including regular monitoring of plugin updates, implementing content security policies to limit script execution, and conducting thorough security audits of all installed plugins. The vulnerability demonstrates the importance of proper input validation and output escaping as fundamental security practices that should be enforced throughout WordPress plugin development. Organizations should also consider implementing web application firewalls and regular security scanning to detect similar issues in other components of their WordPress installations. The ATT&CK framework categorizes this vulnerability under T1566 as a malicious input delivery technique, highlighting the need for comprehensive security controls that address both the technical flaw and potential attack scenarios. Security teams must ensure that all administrative interfaces properly validate and sanitize inputs, particularly in environments where user permissions are tightly controlled and security policies are enforced through capabilities like unfiltered_html restrictions.