CVE-2016-9187 in Moodle
Summary
by MITRE
Unrestricted file upload vulnerability in the double extension support in the "image" module in Moodle 3.1.2 allows remote authenticated users to execute arbitrary code by uploading a file with an executable extension, and then accessing it via unspecified vectors.
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Analysis
by VulDB Data Team • 09/29/2022
The CVE-2016-9187 vulnerability represents a critical security flaw in Moodle version 3.1.2 that stems from inadequate file validation mechanisms within the image module's double extension support functionality. This vulnerability specifically targets the file upload process where the system fails to properly sanitize file extensions, creating an avenue for malicious actors to bypass security controls. The flaw exists in the way Moodle handles file uploads with double extensions, where a file might appear to have a legitimate extension like .jpg while actually containing executable code with an extension such as .php. This vulnerability is particularly dangerous because it allows authenticated users to upload malicious files that can subsequently be executed on the server, potentially leading to complete system compromise.
The technical implementation of this vulnerability resides in the insufficient input validation and sanitization processes within Moodle's image handling module. When users upload files, the system should verify that the file type matches its extension and that no malicious code is embedded within the file. However, in this case, the double extension support feature fails to properly validate file content, allowing attackers to exploit a known weakness in file extension checking mechanisms. The vulnerability operates through a specific attack pattern where an attacker uploads a file named something like shell.php.jpg, which appears to be a legitimate image file but contains executable PHP code. This flaw aligns with CWE-434, which addresses insecure file upload vulnerabilities where applications fail to validate file types and content properly.
The operational impact of this vulnerability extends beyond simple code execution, as it provides attackers with a pathway to establish persistent access to the Moodle platform. Once an attacker successfully uploads a malicious file, they can execute arbitrary commands on the server, potentially leading to data exfiltration, privilege escalation, or the installation of backdoors. The unspecified vectors mentioned in the vulnerability description suggest that there are multiple ways attackers can access the uploaded files, including direct web access to the upload directory or through application logic that processes the uploaded content. This vulnerability affects the integrity and confidentiality of the entire Moodle platform, as it allows unauthorized access to the underlying system resources and can be leveraged to compromise the entire educational institution's online learning environment.
Organizations should implement multiple layers of defense to mitigate this vulnerability, including immediate patching of the affected Moodle version to 3.1.3 or later, where the issue has been resolved. Network segmentation and web application firewalls should be configured to monitor and block suspicious file upload patterns, particularly those involving double extensions. Additionally, implementing strict file type validation at both the application and server levels can prevent malicious files from being processed. The mitigation strategy should also include regular security audits of file upload mechanisms, proper access controls to restrict upload permissions, and monitoring for unauthorized file access attempts. This vulnerability demonstrates the importance of following secure coding practices as outlined in the OWASP Top Ten and aligns with ATT&CK technique T1190, which covers exploit public-facing application vulnerabilities, emphasizing the need for comprehensive application security measures that address both the technical implementation flaws and the operational security controls required to prevent exploitation.