TaintedCookie

Potential cookie injection. This rule is emitted when user-controlled input can be passed into a cookie.

Risk

The risk of setting arbitrary cookies depends on further application configuration.

Examples of potential issues:

Session Fixation: If the authentication cookie doesn't change after a successful login an attacker could fixate the session cookie. If a victim logs in with a fixated cookie, the attacker can now take over the session of the user.
Cross-Site-Scripting (XSS): Some application code could read cookies and print it out unsanitized to the user.

php

<?php

setcookie('authtoken', $_GET['value'], time() + (86400 * 30), '/');

If this is required functionality, limit the cookie setting to values and not the name. (e.g. authtoken in the example)

Make sure to change session tokens after authentication attempts.