Improper Control of Dynamically-Identified Variables

Description

Many languages offer powerful features that allow the programmer to access arbitrary variables that are specified by an input string. While these features can offer significant flexibility and reduce development time, they can be extremely dangerous if attackers can modify unintended variables that have security implications.

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

This code uses the credentials sent in a POST request to login a user.

//Log user in, and set $isAdmin to true if user is an administrator

function login($user,$pass){
  $query = buildQuery($user,$pass);
  mysql_query($query);
  if(getUserRole($user) == "Admin"){
    $isAdmin = true;
  }
}

$isAdmin = false;
extract($_POST);
login(mysql_real_escape_string($user),mysql_real_escape_string($pass));

The call to extract() will overwrite the existing values of any variables defined previously, in this case $isAdmin. An attacker can send a POST request with an unexpected third value "isAdmin" equal to "true", thus gaining Admin privileges.

See Also