pfSense: pfSense-SA-20_03.webgui: XSS vulnerability in the WebGUI

A Cross-Site Scripting (XSS) vulnerability was found in
services_captiveportal_mac.php, a component of the pfSense software WebGUI, on
version 2.4.4-p3 and earlier.

The page did not encode the username or delmac parameters in its output, leading
to a possible XSS.

To take advantage of this XSS, the following prerequisites must be met:

1. There must be at least one Captive Portal zone defined
2. The attacker must know the name of a valid Captive Portal zone
3. There must be at least one pass-through MAC entry defined

Due to the lack of proper encoding on the affected parameters susceptible to
XSS, arbitrary JavaScript could be executed in the user's browser. The user's
session cookie or other information from the session may be compromised.