CVE-2016-9836 in Joomla
Summary
by MITRE
The file scanning mechanism of JFilterInput::isFileSafe() in Joomla! CMS before 3.6.5 does not consider alternative PHP file extensions when checking uploaded files for PHP content, which enables a user to upload and execute files with the `.php6`, `.php7`, `.phtml`, and `.phpt` extensions. Additionally, JHelperMedia::canUpload() did not blacklist these file extensions as uploadable file types.
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Analysis
by VulDB Data Team • 10/05/2022
The vulnerability CVE-2016-9836 represents a critical security flaw in Joomla! CMS versions prior to 3.6.5 that stems from inadequate file extension validation during the upload process. This weakness resides within the JFilterInput::isFileSafe() function which is responsible for determining whether uploaded files contain potentially dangerous content. The flaw demonstrates a classic bypass mechanism where the security checks fail to account for alternative PHP file extensions that can execute server-side code, effectively creating a pathway for arbitrary code execution. The vulnerability specifically targets the file scanning mechanism's inability to recognize that extensions such as .php6, .php7, .phtml, and .phpt are functionally equivalent to standard .php files in terms of execution capabilities, allowing attackers to circumvent the intended security controls.
The technical implementation of this vulnerability exposes a fundamental flaw in the input validation logic where the file extension checking mechanism operates with a limited whitelist approach that fails to consider the full spectrum of PHP file extensions that could be interpreted by the web server. This oversight creates a direct path for privilege escalation attacks where authenticated users can upload malicious files with these alternative extensions, bypassing the security controls that were designed to prevent PHP code execution. The vulnerability operates at the application layer and specifically affects the file upload functionality within Joomla! CMS, making it particularly dangerous as it leverages legitimate user permissions to execute malicious code on the server. The flaw aligns with CWE-732: Incorrect Permissions for Critical Resource, as it allows unauthorized code execution through legitimate upload mechanisms.
The operational impact of this vulnerability extends beyond simple code execution to encompass full system compromise potential, as attackers can leverage these alternative extensions to upload web shells, backdoors, or other malicious payloads that can persist on the server. The attack vector requires authenticated access to the CMS, but once achieved, the attacker can establish persistent access and potentially escalate privileges to gain full control over the web server. This vulnerability particularly affects organizations that rely on Joomla! for content management and may not have implemented additional security controls to prevent such bypasses. The security implications are significant as it undermines the integrity of the file upload validation process and can lead to complete system compromise, data exfiltration, and service disruption.
Mitigation strategies for CVE-2016-9836 should focus on immediate patching of affected Joomla! installations to version 3.6.5 or later, where the file extension validation has been properly enhanced to include the additional PHP extensions. Organizations should also implement additional security layers such as web application firewalls that can detect and block attempts to upload files with these alternative extensions. The remediation process involves updating the JHelperMedia::canUpload() function to properly blacklist the identified file extensions and ensuring that the JFilterInput::isFileSafe() function performs comprehensive validation across all known PHP file extensions. Security administrators should also conduct thorough audits of uploaded file directories and implement monitoring for suspicious file uploads. This vulnerability demonstrates the importance of comprehensive input validation and the need for security controls to consider all potential attack vectors, including alternative file extensions that may serve the same functional purpose as primary threat vectors. The mitigation approach should align with ATT&CK technique T1190: Exploit Public-Facing Application to ensure that all potential entry points are secured against similar bypass mechanisms.