Strange errors

You have developed a complex web application and deployed it to production. The application is a new hybrid of wiki and a forum. Users can use it to brainstorm on ideas and write a document together. Suddenly users start contacting your support desk. They are all reporting ”that your application looks strange.” It suddenly contains extra URLs that link to external websites that are mixed with the original website’s layout.

What could be the problem?
How will you solve it?