How Can I Securely Verify Uploaded Image File Types in PHP?

Debugging File Type Verification in PHP

Despite using a common approach to validate image file types, some users have encountered errors during the uploading process.

The code snippet provided utilizes the user-provided $_FILES['fupload']['type'] property, which is susceptible to manipulation by malicious users. To address this vulnerability, it has been suggested that using mime_content_type() would improve accuracy.

Resolving the Issue

However, the recommended solution is to avoid relying on user-defined values entirely. Instead, it is advised to conduct your own type verification.

For images, the exif_imagetype() function offers a reliable method for determining the file's true format:

$allowedTypes = array(IMAGETYPE_PNG, IMAGETYPE_JPEG, IMAGETYPE_GIF);
$detectedType = exif_imagetype($_FILES['fupload']['tmp_name']);
$error = !in_array($detectedType, $allowedTypes);

This approach checks the image's binary signature directly, eliminating the possibility of users manipulating the file type.

Alternatively, for more advanced file type verification, consider using the finfo functions if your server supports them.

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