Security changes in a site developed in Joomla 1.5

I own a website that was programmed in Joomla 1.5.

Now, did the migration of this site to a new server. However, security testing found some gaps that need to be corrected before the site goes into production on the new server.

There are two major gaps:

1 - The application allows you to redirect the user to any web page.
Reference: https://www.owasp.org/index.php/Top_10_2010-A10

2 - The application must address adequately the data entries
(URL Encode) that are performed by the user.

Reference: https://www.owasp.org/images/b/ba/AppsecEU09_CarettoniDiPaola_v0.8.pdf

So I need a professional able to solve these two problems in the short term, but that is not an option to upgrade the version of Joomla (we are already developing a new version of the site in another environment).