decoding eval(base64_decode)) - php

Question

I am trying to decode this code.
I know it can be done by changing eval to echo. But in this case its not working. Is i am making any mistake. This is my encoded_file.php code:
i have tried to change eval to echo but its not working file.
I also tried this decoder:
<?php
// Open and read the content of the encoded file into a variable
$file = file_get_contents('encoded_file.php');
// Strip php tags
$file = str_replace('<?php', "", $file);
$file = str_replace('<?', "", $file);
// Make sure to get rid of short tags....
$file = str_replace('?>', "", $file);
// Strip new lines
$file = str_replace("\n", "", $file);
// Add semi colon to get around a parsing issue.
$file = $file.';';
// Change the Eval function
$file = str_replace('eval', 'echo ', $file);
// Function to eval the new string
function deval()
{
global $file;
ob_start();
eval($file);
$contents = ob_get_contents();
ob_end_clean();
return($contents);
}
// Run the code thru once
$file = deval();
// Counter
$cnt = 1;
// Loop it till it's decoded
while(preg_match('/^\?><\?php eval/', $file))
{
$file = str_replace('?><?php eval', 'echo', $file);
$file = str_replace('?><?', "", $file);
$file = deval();
$cnt;
}
//clean up some tags
$file = str_replace('?><?php', "", $file);
$file = str_replace('?><?', "", $file);
echo $cnt,' iterations<br/><br/>';
echo $file;
?>
but it also not working well. Any solution how to decode it or what's wrong in my decoder code.

Answer 1

Here are the steps which are needed to decode this (note - I've renamed variables/functions for clarity):
1. We see that this script reads content of itself, so we can assume - we cannot change this file
so lets create new file with this content and change this file:
$encoded=file('another_file.txt');
2. Then we can change first eval to echo and all other evals should be commented:
here is first line:
echo base64_decode("aWYoIWZ1bmN0aW9uX2V4aXN0cygiWWl1bklVWTc2YkJodWhOWUlPOCIpKXtmdW5jdGlvbiBZaXVuSVVZNzZiQmh1aE5ZSU84KCRnLCRiPTApeyRhPWltcGxvZGUoIlxuIiwkZyk7JGQ9YXJyYXkoNjU1LDIzNiw0MCk7aWYoJGI9PTApICRmPXN1YnN0cigkYSwkZFswXSwkZFsxXSk7ZWxzZWlmKCRiPT0xKSAkZj1zdWJzdHIoJGEsJGRbMF0rJGRbMV0sJGRbMl0pO2Vsc2UgJGY9dHJpbShzdWJzdHIoJGEsJGRbMF0rJGRbMV0rJGRbMl0pKTtyZXR1cm4oJGYpO319");
this will give us:
if(!function_exists("getSubString"))
{
function getSubString($g,$b=0)
{
$a=implode("\n",$g);
$d=array(655,236,40);
if($b==0) $f=substr($a,$d[0],$d[1]);
elseif($b==1) $f=substr($a,$d[0]+$d[1],$d[2]);
else $f=trim(substr($a,$d[0]+$d[1]+$d[2]));
return $f;
}
}
3. Now we can remove first echo/eval and go to 2nd one:
here is 2nd line:
echo base64_decode(getSubString($encoded));
give us:
if(!function_exists("decodeCode"))
{
function decodeCode($a,$h)
{
if($h==sha1($a))
{
return(gzinflate(base64_decode($a)));
}
else
{
echo("Error: File Modified");
}
}
}
4. we can remove it and go to last eval:
here is it:
echo decodeCode(getSubString($encoded,2),getSubString($encoded,1));
and we see final code:
/**
* #site #####
* #copyright 2010
*/
include 'config.php';
$id=$_GET['id'];
if(isset($id))
{
header("Content-type: image/jpeg");
$url='http://#####/siteuploads/thumb/'.$id;
$path=pathinfo($url);
header('Content-Disposition: attachment; filename="'.$path['basename'].'"');
$img=imagecreatefromjpeg($url);
$red=imagecolorallocate($img,255,155,255);
imagestring($img,2,1,2,$site,$red);
echo imagejpeg($img);
}