Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection')

The software receives input from an upstream component, but it does not neutralize or incorrectly neutralizes code syntax before using the input in a dynamic evaluation call (e.g. "eval").


Description

This may allow an attacker to execute arbitrary code, or at least modify what code can be executed.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

edit-config.pl: 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 goes here


}

sub config_file_set_key {

  my ($fname, $key, $arg) = @_;

  # code to set key to a particular file goes here


}

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 is super-efficient code, especially if you have to invoke


  # any one of dozens of 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.

See Also

Validate Inputs

Weaknesses in this category are related to the design and architecture of a system's input validation components. Frequently these deal with sanitizing, neutralizing a...

SFP Secondary Cluster: Tainted Input to Command

This category identifies Software Fault Patterns (SFPs) within the Tainted Input to Command cluster (SFP24).

Comprehensive CWE Dictionary

This view (slice) covers all the elements in CWE.

CWE Cross-section

This view contains a selection of weaknesses that represent the variety of weaknesses that are captured in CWE, at a level of abstraction that is likely to be useful t...

Weaknesses Introduced During Implementation

This view (slice) lists weaknesses that can be introduced during implementation.


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