Avoiding EVAL()

There are a shed-load of ways to “eval()” code without actually calling the eval() function — usually done simply to avoid the use of the dreaded “evil()” function, but often times because the system has eval() disabled using “disable_functions” in php.ini

Here is another simple way to avoid eval() without writing out files to the filesystem etc:

[php]
<?php
$code = ‘<?php echo “Hello World”; ?>;
include(‘data:text/plaintext;base64,’ . base64_encode($code));
?>
[/php]

This uses the new data: stream wrapper (RFC2397) that was introduced with PHP 5.2.0; and while this seems like a risk, first: The “attacker” already has access to the code on your system, or you’re open to injection anyway, second: PHP 5.2 has also fixed the problem with the introduction of the  ”allow_url_include” php.ini option.

I just thought it was a neat little streams “hack” I would share; I originally thought to do it using the var stream from PHP’s stream_wrapper_register() documentation, but then Evert Pot posted about creating streams from strings using the data: stream, which led to this final “solution”.

- Davey

12 Responses to “Avoiding EVAL()”

1. gasper_k says:

   February 1, 2009 at 5:14 pm

   Nice piece of information, thanks. However, I think either you’re wrong or I understood you wrong, but allow_url_include directive has been available since 5.1.
   • Davey Shafik says:

     February 1, 2009 at 7:56 pm

     As Johannes pointed out, it was also added in 5.2; it was a typo on my part
     • gasper_k says:

       February 2, 2009 at 3:23 am

       Yes, I’ve re-checked it, and it seems it was introduced in 5.2, not in 5.1 as I previously posted.
2. Johannes Schlüter says:

   February 1, 2009 at 5:34 pm

   allow_url_include is no 5.3 but 5.2 introduced feature, data: was no “URL Stream” for one or two versions of that series, but that was fixed soon to avoid troubles like the one above
3. Andrei says:

   February 2, 2009 at 12:40 pm

   Silly and completely useless. I don’t see an serious use case for this “feature”.
   • Davey Shafik says:

     February 2, 2009 at 11:39 pm

     I never claimed it was useful. And there certainly is no *serious* use case; I just enjoy bending the language
     • EllisGL says:

       February 3, 2009 at 8:36 am

       There’s a lot of pay scripts that are eval base64 encoded.. Could use this to no use eval.. Still insecure.
4. Timothy says:

   February 2, 2009 at 1:04 pm

   man… and I thought I wrote sinister code! Thanks for sharing, Davey.
5. links for 2009-02-02 — Mior Muhammad Zaki: PHP & JavaScript Programmer says:

   February 3, 2009 at 12:30 am

   [...] Avoiding EVAL() (tags: PHP) [...]
6. Matt says:

   February 3, 2009 at 7:02 pm

   Lookie:

   http://pastebin.com/f6662eb57

   eval() is significantly faster than include() – on my computer it’s a difference of about 35%.
   • Davey Shafik says:

     February 3, 2009 at 7:58 pm

     This is quite obvious; there is a base64_encode() and a base64_decode() involved in my solution. However, benchmarks in userland are inherently flawed.
7. Žilvinas says:

   February 10, 2009 at 6:37 am

   On the other hand this shows a nice exploit when eval is disabled. You could easily inject your code to execute by exploiting a bug with dynamic variable includes.