CVE-2016-9493 in PHP FormMail Generator
Summary
by MITRE
The code generated by PHP FormMail Generator prior to 17 December 2016 is vulnerable to stored cross-site scripting. 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 identified as CVE-2016-9493 affects PHP FormMail Generator versions prior to the 17 December 2016 release, presenting a significant security risk through stored cross-site scripting capabilities. This flaw originates from the code generation process where the system creates form.lib.php files containing file upload validation logic. The vulnerability specifically targets the file extension checking mechanism that determines which file types are permitted for upload to the server. The implementation relies on a hard-coded whitelist approach that fails to account for all possible variations of PHP file extensions, creating a critical oversight in the security validation process.
The technical flaw stems from an incomplete validation mechanism that only considers a limited set of file extensions deemed dangerous for upload operations. This hardcoded list does not encompass all potential PHP file variations that could execute server-side code, including but not limited to .php, .php3, .php4, .php5, .phtml, and other potential extensions. The vulnerability is particularly concerning because attackers can exploit this gap by crafting filenames with extensions that bypass the validation checks, especially when combined with the default behavior of appending random strings to filenames. The system's approach of adding only short random strings to filenames provides minimal protection against determined attackers who can guess or enumerate potential filename combinations.
The operational impact of this vulnerability extends beyond simple cross-site scripting attacks to encompass full code execution capabilities on the target server. When an attacker successfully uploads a malicious file with a PHP extension that bypasses the validation checks, the server executes the contained PHP code with the privileges of the web application. This execution can lead to complete server compromise, data exfiltration, and persistent access through backdoor installations. The stored nature of this vulnerability means that once an attacker successfully uploads malicious content, the payload remains active until manually removed, providing sustained access to the compromised system. This vulnerability aligns with CWE-79: Improper Neutralization of Input During Web Page Generation, specifically addressing the failure to properly validate and sanitize user-supplied input that is subsequently stored and executed.
The attack surface for this vulnerability can be exploited through multiple vectors, including social engineering to encourage users to upload malicious files or by compromising legitimate user accounts with upload privileges. The ATT&CK framework categorizes this vulnerability under T1190: Exploit Public-Facing Application, where attackers leverage insecure file upload mechanisms to gain initial access. Additionally, this vulnerability supports T1059.007: Command and Scripting Interpreter: PHP, as it enables execution of PHP code on the target system. The risk is compounded by the fact that many web applications rely on form generators for rapid deployment, making this vulnerability particularly widespread across organizations using outdated versions of the PHP FormMail Generator.
Mitigation strategies should focus on implementing comprehensive file extension validation that accounts for all possible PHP variants and employs more robust file type detection methods beyond simple extension checking. Organizations should immediately update to the patched version released on or after 17 December 2016, which addresses the hardcoded extension list issue. Additional protective measures include implementing proper file type validation using content-based detection rather than relying solely on extensions, employing a whitelist approach for all valid file types, and implementing proper file naming conventions that do not rely on simple random string appending. The system should also implement proper access controls to limit upload capabilities to authorized users only and employ regular security audits to identify similar validation flaws in other components of the web application stack.