CVE-2009-3890 in WordPress
Summary
by MITRE
Unrestricted file upload vulnerability in the wp_check_filetype function in wp-includes/functions.php in WordPress before 2.8.6, when a certain configuration of the mod_mime module in the Apache HTTP Server is enabled, allows remote authenticated users to execute arbitrary code by posting an attachment with a multiple-extension filename, and then accessing this attachment via a direct request to a wp-content/uploads/ pathname, as demonstrated by a .php.jpg filename.
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Analysis
by VulDB Data Team • 09/16/2025
The vulnerability described in CVE-2009-3890 represents a critical security flaw in WordPress versions prior to 2.8.6 that stems from improper file type validation within the wp_check_filetype function. This weakness occurs specifically when the Apache HTTP Server is configured with certain mod_mime module settings, creating a dangerous condition where authenticated users can upload malicious files that appear to be benign image files but actually contain executable code. The flaw exploits the way WordPress handles file extensions and MIME type detection, allowing attackers to bypass security measures through carefully crafted multi-extension filenames that can fool both the web server and WordPress's built-in validation mechanisms.
The technical implementation of this vulnerability relies on the specific interaction between WordPress's file validation logic and Apache's mod_mime module configuration. When a user uploads a file with a name like .php.jpg, the WordPress system may incorrectly identify it as a valid image file due to the trailing .jpg extension, while the Apache server's mod_mime module might not properly enforce the file type restrictions. This creates a scenario where the uploaded file is stored with its original .php extension but is served with a .jpg MIME type, allowing the PHP code to execute as a web script rather than being treated as an image file. The vulnerability specifically targets the wp-includes/functions.php file where the wp_check_filetype function performs inadequate validation of file extensions and content types.
The operational impact of this vulnerability is severe and far-reaching, as it allows authenticated attackers to execute arbitrary code on the target WordPress installation with the privileges of the web server user. This means that attackers who have gained access to a valid user account or administrative credentials can upload malicious files that will be executed when accessed through the wp-content/uploads directory. The attack vector is particularly dangerous because it requires minimal privileges and can be performed through normal file upload functionality, making it difficult to detect in standard security monitoring. The vulnerability essentially transforms any authenticated user account into a potential weapon for code execution, potentially leading to complete server compromise, data theft, or further network infiltration.
Mitigation strategies for this vulnerability must address both the immediate security flaw and the broader configuration issues that enable it. The most critical remediation is upgrading to WordPress version 2.8.6 or later, which contains the necessary patches to properly validate file types and prevent the exploitation of multi-extension filenames. Additionally, system administrators should review and configure Apache's mod_mime module to ensure proper MIME type enforcement, disable unnecessary file upload capabilities where possible, and implement strict file type validation at both the web server and application levels. Security best practices should include implementing content validation checks, restricting file upload directories, and monitoring for suspicious file upload activities. This vulnerability aligns with CWE-434 Unrestricted Upload of File with Dangerous Type and follows ATT&CK technique T1190 Exploit Public-Facing Application, emphasizing the need for comprehensive application security measures and proper input validation. Organizations should also consider implementing web application firewalls and regular security audits to detect and prevent similar vulnerabilities in their WordPress installations.