CVE-2016-9492 in PHP FormMail Generator
Summary
by MITRE
The code generated by PHP FormMail Generator prior to 17 December 2016 is vulnerable to unrestricted upload of dangerous file types. In the generated form.lib.php file, upload file types are checked against a hard-coded list of dangerous extensions. This list does not include all variations of PHP files, which may lead to execution of the contained PHP code if the attacker can guess the uploaded filename. The form by default appends a short random string to the end of the filename.
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Analysis
by VulDB Data Team • 12/27/2024
The vulnerability described in CVE-2016-9492 represents a critical security flaw in PHP FormMail Generator software versions released before December 17, 2016, where the application suffers from insecure file upload functionality that enables remote code execution. This issue stems from inadequate validation of uploaded file types within the generated form.lib.php script, creating a pathway for malicious actors to bypass security controls and execute arbitrary code on the target server. The vulnerability specifically targets the file extension validation mechanism that relies on a hardcoded whitelist of dangerous file extensions, which fails to account for all possible variations of PHP file extensions that could be exploited for code execution.
The technical implementation of this flaw occurs within the file upload validation logic where the software maintains a static list of file extensions deemed dangerous for upload, particularly focusing on PHP-related extensions. However, this validation mechanism contains gaps in its coverage as it does not include all possible variations of PHP file extensions such as .php3, .php4, .php5, .phtml, or other less common variants that could be interpreted by the web server as executable scripts. The security weakness becomes apparent when considering that the application's default behavior appends a short random string to uploaded filenames, which creates a scenario where attackers can guess or predict the complete filename including the appended random component, thereby enabling successful execution of malicious code.
The operational impact of this vulnerability extends beyond simple file upload restrictions and creates a significant threat vector for attackers seeking to compromise affected systems. When an attacker successfully uploads a malicious PHP file through this vulnerability, they can execute arbitrary code on the web server with the privileges of the web application, potentially leading to complete system compromise, data exfiltration, or the establishment of persistent backdoors. The vulnerability aligns with CWE-434 which describes "Unrestricted Upload of File with Dangerous Type" and represents a classic example of insecure file handling that allows attackers to circumvent security controls through predictable filename generation and incomplete validation logic. This weakness can be exploited through various attack vectors including social engineering to trick users into uploading malicious files, or direct exploitation if the application is accessible to unauthenticated users.
The implications of this vulnerability are particularly severe in environments where web applications are not properly hardened against file upload attacks, as it directly enables the execution of arbitrary code without requiring authentication or advanced exploitation techniques. Organizations running affected versions of PHP FormMail Generator face significant risk of unauthorized access and potential data breaches, as the vulnerability provides attackers with a straightforward path to achieve remote code execution. Mitigation strategies should include implementing comprehensive file extension validation that covers all known PHP variants, enforcing strict file type checking beyond simple extension matching, and employing additional security controls such as file content validation and proper file storage practices. The vulnerability demonstrates the critical importance of maintaining up-to-date security practices and the necessity of thorough testing of file upload mechanisms against comprehensive attack patterns, aligning with ATT&CK technique T1190 which describes "Exploit Public-Facing Application" and emphasizes the need for proper input validation and file handling controls in web applications.