CVE-2017-16798 in CMS Made Simple
Summary
by MITRE
In CMS Made Simple 2.2.3.1, the is_file_acceptable function in modules/FileManager/action.upload.php only blocks file extensions that begin or end with a "php" substring, which allows remote attackers to bypass intended access restrictions or trigger XSS via other extensions, as demonstrated by .phtml, .pht, .html, or .svg.
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Analysis
by VulDB Data Team • 12/05/2019
The vulnerability identified as CVE-2017-16798 represents a critical security flaw in CMS Made Simple version 2.2.3.1 that stems from inadequate file extension validation within the file upload functionality. This weakness exists in the is_file_acceptable function located in the modules/FileManager/action.upload.php file, where the security mechanism fails to properly sanitize file extensions that could potentially execute malicious code or bypass access controls. The flaw demonstrates a classic case of insufficient input validation and sanitization, which is categorized under CWE-20 - Improper Input Validation, making it particularly dangerous for web applications that rely on user-uploaded content.
The technical implementation of this vulnerability exploits a specific pattern matching approach that only examines file extensions beginning or ending with "php" as potentially malicious. This simplistic validation mechanism creates a significant security gap because it fails to account for other file extensions that can execute code or be interpreted as executable content by web servers. Attackers can leverage this oversight by uploading files with extensions such as .phtml, .pht, .html, or .svg which while not directly matching the blocked pattern, can still execute code or enable cross-site scripting attacks. The vulnerability specifically targets the file upload process where the system should validate all file types against a comprehensive whitelist rather than relying on a limited blacklist approach.
The operational impact of this vulnerability extends beyond simple bypass of access restrictions to encompass potential full system compromise through code execution and cross-site scripting attacks. Remote attackers can leverage this flaw to upload malicious files that execute arbitrary code on the server, potentially leading to complete system takeover. The attack vector allows for persistent malicious code execution through extensions like .phtml and .pht which are often interpreted by web servers as PHP files, while .svg files can be used to deliver XSS payloads that target users of the CMS. This vulnerability aligns with ATT&CK technique T1190 - Exploit Public-Facing Application, where adversaries exploit weaknesses in web applications to gain unauthorized access and execute malicious code.
The security implications of this vulnerability are particularly severe given that it affects a core functionality of the CMS platform - the file management system that users rely on for content management. The flaw demonstrates poor security engineering practices where the developers implemented a validation mechanism that is easily circumvented through simple extension manipulation, creating a false sense of security. Organizations using CMS Made Simple 2.2.3.1 are at risk of unauthorized code execution, data breaches, and potential compromise of the entire web application infrastructure. The vulnerability also reflects a broader pattern of security issues in web applications where developers fail to implement proper content validation and sanitization mechanisms.
Mitigation strategies for this vulnerability must include immediate implementation of a comprehensive file extension whitelist approach that validates all uploaded files against a predetermined list of safe extensions rather than relying on blacklisting patterns. Organizations should also implement additional security measures such as file content inspection, MIME type validation, and proper file handling procedures that prevent execution of uploaded files in web-accessible directories. The fix should involve updating the is_file_acceptable function to perform thorough extension validation and ensure that all file types are properly sanitized before being stored or made accessible to users. Additionally, implementing proper access controls and file permissions, along with regular security audits, can help prevent exploitation of similar vulnerabilities in other parts of the application stack. This vulnerability underscores the importance of following secure coding practices and implementing defense-in-depth strategies to protect against common web application security flaws.