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:

    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.

  2. 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:

    Silly and completely useless. I don’t see an serious use case for this “feature”.

    • Davey Shafik says:

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

      • EllisGL says:

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

  4. Timothy says:

    man… and I thought I wrote sinister code! Thanks for sharing, Davey.

  5. Matt says:

    Lookie:

    http://pastebin.com/f6662eb57

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

    • Davey Shafik says:

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

  6. Žilvinas says:

    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.

Twitter

Is 40 IRC channels on auto join too many? >.

@dshafik [7 minutes ago]

@eryno I paused the DVR at least twice with like "BUT THAT MEANS…"

@dshafik [34 minutes ago]

@eryno it was so brain bending I don't think it's possible to spoil it. Also: I would hate myself for depriving a fan of the surprise.

@dshafik [35 minutes ago]

Books & Things

Search

Archives

Tags