Unable to upload files or add users. Receiving HTTP 403 Forbidden error

Users were unable to upload files or add users to SharePoint sites. Moreover they received  HTTP 403 Forbidden error message when trying to upload or add users.

Fix:

This is a known issue and needs the hot fix https://support.microsoft.com/kb/934793/ applied on the server!

In some cases, another possible cause is: anonymous access might removed from READ permissions of IIS virtual directories. Fire IIS console and fix it.