[VIM] Dynamic Evaluation Vulnerabilities in PHP applications

[Steven M. Christey]

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Dynamic Evaluation Vulnerabilities in PHP applications
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Following is a brief introduction to a growing class of serious
vulnerabilities in PHP applications.  They can allow execution of
arbitrary code or arbitrary functions, or read/write access of
arbitrary internal variables.

It seems that only a handful of researchers are currently looking for
these issues, including Stefan Esser, retrogod, and Gulftech.
However, these serious problems are probably present in many
applications, especially large or complicated ones.  In addition,
researchers can trigger these vulnerabilities but incorrectly label
them as XSS if they do not perform sufficient diagnosis.

Note that these types of vulnerabilities are not unique to PHP.  Other
interpreted languages can have similar issues.  For example, Perl,
Python, and Javascript have eval functions.  A recent myspace XSS
issue used eval injection in Javascript [1], and eval injection has
been reported in some Python applications (CVE-2005-2483,
CVE-2005-3302) and Perl (CVE-2002-1750, CVE-2003-0770, CVE-2005-1527,
CVE-2005-2837).


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Eval Injection
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Terminology note: this term is not common, but it is used in CVE.  As
of this writing, there is no commonly used alternative.

An eval injection vulnerability occurs when an attacker can control
all or part of an input string that is fed into an eval() function
call.  Eval will execute the argument as code.  The security
implications for this are obvious.  This issue has been known for
years [2], but it is still under-researched.

Example:

  $myvar = "varname";
  $x = $_GET['arg'];
  eval("\$myvar = \$x;");

  What happens if arg is set to "10 ; system(\"/bin/echo uh-oh\");" ?

Basic detection:

  - With source code: since it is a standard PHP function, it is easy
    to grep for potentially dangerous calls to eval().  However, the
    researcher must further investigate whether inputs can be
    controlled by an attacker.

  - Without source code: if verbose errors are available, an invalid
    input might trigger an error message related to a parsing error.
    Using an input of "phpinfo" might be useful.  However, you might
    have to play with the inputs to match the syntactic requirements
    of the statement that is finally fed into the eval, just like you
    sometimes need to do in XSS or SQL injection.

Eliminating the problem:

  - avoid eval() whenever possible

  - use only whitelists of acceptable values to insert into eval()
    calls.  The whitelist might need to change depending on where in
    the program you are.


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Dynamic Variable Evaluation
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Terminology note: there is no common term for this kind of issue.

PHP supports "variable variables," which are variables or expressions
that evaluate to the names of other variables [3].  They can be used
to dynamically change which variable is accessed or set during
execution of the program.  This powerful and convenient feature is
also dangerous.

If the variable names are not controlled, an attacker can read or
write to arbitrary variables, depending on the application.  The
consequences depend on the program.  In some cases, even critical
variables such as $_GLOBALS can be modified [4].

Example:

  $varname = "myvar";
  $$varname = 10;
  echo $myvar;

This will set $myvar, and print the string "10"!

It seems likely that this issue will occur more frequently as PHP
developers modify their programs so that they do not require
register_globals.

A number of applications have code such as the following:

  $safevar = "0";
  $param1 = "";
  $param2 = "";
  $param3 = "";
  # my own "register globals" for param[1,2,3]
  foreach ($_GET as $key => $value) {
    $$key = $value;
  }

If the attacker provides "safevar=bad" in the query string, then
$safevar will be set to the value "bad".

Detection Examples:

  $$varname

  ${$varname}

  ${$var . $name}

  ${arbitrary expression}

Eliminating the problem:

  - use only whitelists of acceptable variable names.  The whitelist
    might need to change depending on where in the program you are.


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Dynamic Function Evaluation
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Terminology note: there is no common term for this kind of issue.

Variable variables can also be used to dynamically reference
functions:

  $funcname = "myfunction";

  $$funcname("Arg1", "Arg2");

This effectively calls myfunction("Arg1", "Arg2") !

Detection Examples:

  $$fname();

  ${$var1 . $var2} ("arg");

  ${"varname"} ();

Eliminating the problem:

  - use only whitelists of acceptable function names.  The whitelist
    might need to change depending on where in the program you are.


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References
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[1] Myspace.com - Intricate Script Injection
    Justin Lavoie
    http://marc.theaimsgroup.com/?l=bugtraq&m=114469411219299&w=2

[2] A Study In Scarlet: Exploiting Common Vulnerabilities in PHP Applications
    Shaun Clowes
    http://www.securereality.com.au/studyinscarlet.txt

    This classic paper briefly mentioned the risk of eval

[3] PHP: Variable variables
    http://us3.php.net/manual/en/language.variables.variable.php

[4] $GLOBALS Overwrite and it's Consequences
    Stefan Esser
    http://www.hardened-php.net/globals-problem

    This paper talks specifically about dynamic variable evaluation
    and the impact on superglobals such as $_GLOBALS.  Esser was one
    of the first (if not the first) researchers to use the term "eval
    injection".


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Sample Vulnerable Program
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<html>
<body>
<h1>Dynamic Evaluation Vulnerabilities in PHP Applications - Examples</h1>
<table border=2>
<tr>
<td>Dynamic variable evaluation (a "variable variable")
<td><a href="?test=1&varname=myvar">?varname=myvar</a>
<tr>
<td>Dynamic function evaluation
<td><a href="?test=2&myfunc=phpinfo">?myfunc=phpinfo</a>
<tr>
<td>Eval injection
<td><a href="?test=3&ev=do_this();">?ev=do_this();</a>
</table>
<p>

<?php
// error_reporting(8);
// ini_set('display_errors', 1);
// ini_set('display_startup_errors', 1);

 function do_this () { echo "Do this!<br>"; }

 $test = $_GET['test'];
 if ($test == 1)
 {
   echo "<b>=== Implicit variable evaluation in \$myvar ===</b><br>\n";
   echo "Parameter varname = " . $_GET['varname'] . "<br>\n";
   $myvar = "unchangeable value";
   echo "before: \$myvar = \"" . $myvar . "\"<br>\n";
   $varname = $_GET['varname'];
   echo "EXECUTE: \$\$varname = \"new value\";<br>\n";
   $$varname = "new value";
   echo "after: \$myvar = \"" . $myvar . "\"<br>\n";
 }
 elseif ($test == 2)
 {
   echo "<b>=== Implicit function evaluation in \$myfunc ===</b><br>\n";
   $myfunc = $_GET['myfunc'];
   echo "EXECUTE: \$myfunc();<br>\n";
   ${"myfunc"}();
   $myfunc();
 }
 elseif ($test == 3)
 {
   echo "<b>=== Eval Injection in \$ev ===</b><br>\n";
   $ev = $_GET['ev'];
   echo "EXECUTE: eval(\$ev);<br>\n";
   echo "actual statement will be: eval($ev)<br><br><br>\n";
   eval($ev);
 }
?>