Natas - Level 14

2 November 2017
OverTheWire Natas

Connection information

Information given

The white box is composed of four parts:

  1. The first part is a text displaying : " For security reasons, we now only accept image files!. Choose a JPEG to upload (max 1KB):"
  2. The second part is an buttonh named Browse...
  3. The third is a button named Upload File
  4. The last is a link View sourcecode pointing to http://natas12.natas.labs.overthewire.org/index-source.html

Source code

<?
function genRandomString() {
$length = 10;
$characters = "0123456789abcdefghijklmnopqrstuvwxyz";
$string = "";
for ($p = 0; $p < $length; $p++) {
$string .= $characters[mt rand(0, strlen($characters)−1)];
}
return $string;
}
function makeRandomPath($dir, $ext) {
35do {
$path = $dir."/".genRandomString().".".$ext;
} while(file exists($path));
return $path;
}
function makeRandomPathFromFilename($dir, $fn) {
$ext = pathinfo($fn, PATHINFO EXTENSION);
return makeRandomPath($dir, $ext);
}
if(array key exists("filename", $ POST)) {
$target path = makeRandomPathFromFilename("upload", $ POST["filename"]);
if(filesize($ FILES[’uploadedfile’][’tmp name’]) > 1000) {
echo "File is too big";
} else if (! exif imagetype($ FILES[’uploadedfile’][’tmp name’])) {
echo "File is not an image";
} else {
if(move uploaded file($ FILES[’uploadedfile’][’tmp name’], $target path)) {
echo "The file <a href=\"$target path\">$target path</a> has been uploaded";
} else{
echo "There was an error uploading the file, please try again!";
}
}
} else {
?>
<form enctype="multipart/form−data" action="index.php" method="POST">
<input type="hidden" name="MAX FILE SIZE" value="1000" />
<input type="hidden" name="filename" value="<? print genRandomString(); ?>.jpg" />
Choose a JPEG to upload (max 1KB):<br/>
<input name="uploadedfile" type="file" /><br />
<input type="submit" value="Upload File" />
</form>
<? } ?>

Getting the password

https://phocean.net/2013/09/29/file-upload-vulnerabilities-appending-php-code-to-an-image.html

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