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

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.

Example Two

This simple script asks a user to supply a list of numbers as input and adds them together.

def main():

  sum = 0
  numbers = eval(input("Enter a space-separated list of numbers: "))
  for num in numbers:

    sum = sum + num

  print(f"Sum of {numbers} = {sum}")
main()

The eval() function can take the user-supplied list and convert it into a Python list object, therefore allowing the programmer to use list comprehension methods to work with the data. However, if code is supplied to the eval() function, it will execute that code. For example, a malicious user could supply the following string:

__import__('subprocess').getoutput('rm -r *')

This would delete all the files in the current directory. For this reason, it is not recommended to use eval() with untrusted input.

A way to accomplish this without the use of eval() is to apply an integer conversion on the input within a try/except block. If the user-supplied input is not numeric, this will raise a ValueError. By avoiding eval(), there is no opportunity for the input string to be executed as code.

def main():

  sum = 0
  numbers = input("Enter a space-separated list of numbers: ").split(" ")
  try:

    for num in numbers:

      sum = sum + int(num)

    print(f"Sum of {numbers} = {sum}")
  except ValueError:

    print("Error: invalid input")


main()

An alternative, commonly-cited mitigation for this kind of weakness is to use the ast.literal_eval() function, since it is intentionally designed to avoid executing code. However, an adversary could still cause excessive memory or stack consumption via deeply nested structures [REF-1372], so the python documentation discourages use of ast.literal_eval() on untrusted data [REF-1373].

See Also