Understanding File Upload Vulnerabilities
File upload vulnerabilities occur when web applications fail to properly validate uploaded files. This can allow attackers to upload malicious files, potentially leading to remote code execution on the server.
Let's examine a typical file upload implementation in PHP using the move_uploaded_file() function:
<?php
// The move_uploaded_file() function moves an uploaded file to a new location
// Returns true on success, false on failure
move_uploaded_file($file, $newloc);
Parameters:
$file: The uploaded file to move$newloc: The destination path for the file
A secure implementation should include proper validation, as shown in this example:
<?php
// Define allowed image extensions
$allowedExts = array("gif", "jpeg", "jpg", "png");
$temp = explode(".", $_FILES["file"]["name"]);
$extension = end($temp); // Get the file extension
if ((($_FILES["file"]["type"] == "image/gif")
|| ($_FILES["file"]["type"] == "image/jpeg")
|| ($_FILES["file"]["type"] == "image/jpg")
|| ($_FILES["file"]["type"] == "image/pjpeg")
|| ($_FILES["file"]["type"] == "image/x-png")
|| ($_FILES["file"]["type"] == "image/png"))
&& ($_FILES["file"]["size"] < 204800) // Less than 200 KB
&& in_array($extension, $allowedExts))
{
if ($_FILES["file"]["error"] > 0)
{
echo "Error: " . $_FILES["file"]["error"] . "<br>";
}
else
{
echo "Uploaded file name: " . $_FILES["file"]["name"] . "<br>";
echo "File type: " . $_FILES["file"]["type"] . "<br>";
echo "File size: " . ($_FILES["file"]["size"] / 1024) . " KB<br>";
echo "Temporary file location: " . $_FILES["file"]["tmp_name"];
}
}
else
{
echo "Invalid file format";
}
?>
This code demonstrates several important security checks:
- File extension validation
- MIME type verification
- File size limitation
- Error handling
Without these validations, attackers could potentially upload malicious files that execute on the server.
