解码eval（base64_decode））

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("
", "", $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)
    {
        $a=implode("
",$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);
}