Reliance on Cookies without Validation and Integrity Checking in a Security Decision

Description

Attackers can easily modify cookies, within the browser or by implementing the client-side code outside of the browser. Attackers can bypass protection mechanisms such as authorization and authentication by modifying the cookie to contain an expected value.

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

The following code excerpt reads a value from a browser cookie to determine the role of the user.

Cookie[] cookies = request.getCookies();
for (int i =0; i< cookies.length; i++) {
  Cookie c = cookies[i];
  if (c.getName().equals("role")) {
    userRole = c.getValue();
  }
}

Example Two

The following code could be for a medical records application. It performs authentication by checking if a cookie has been set.

$auth = $_COOKIES['authenticated'];
if (! $auth) {
  if (AuthenticateUser($_POST['user'], $_POST['password']) == "success") {
    // save the cookie to send out in future responses
    setcookie("authenticated", "1", time()+60*60*2);
  }
  else {
    ShowLoginScreen();
    die("\n");
  }
}
DisplayMedicalHistory($_POST['patient_ID']);

The programmer expects that the AuthenticateUser() check will always be applied, and the "authenticated" cookie will only be set when authentication succeeds. The programmer even diligently specifies a 2-hour expiration for the cookie.

However, the attacker can set the "authenticated" cookie to a non-zero value such as 1. As a result, the $auth variable is 1, and the AuthenticateUser() check is not even performed. The attacker has bypassed the authentication.

Example Three

In the following example, an authentication flag is read from a browser cookie, thus allowing for external control of user state data.

Cookie[] cookies = request.getCookies();
for (int i =0; i< cookies.length; i++) {
  Cookie c = cookies[i];
  if (c.getName().equals("authenticated") && Boolean.TRUE.equals(c.getValue())) {
    authenticated = true;
  }
}

See Also