TaintedHeader

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

Risk

The risk of a header injection depends hugely on your environment.

If your webserver supports something like XSendFile / X-Accel, an attacker could potentially access arbitrary files on the systems.

If your system does not do that, there may be other concerns, such as:

<?php

header($_GET['header']);

Make sure only the value and not the key can be set by an attacker. (e.g. header('Location: ' . $_GET['target']);)

Verify the set values are sensible. Consider using an allow list. (e.g. for redirections)