I'll try to explain my problem as clear as possible.
The server of a company I support runs many websites. This server runs Windows Server 2012 with Microsoft SQL Server 2014.
Almost all of the websites are running a proprietary web application, made by the same company.
Only some of the websites are affected by a regular mass editing of (almost) every TEXT, NTEXT and NVARCHAR(MAX) fields in their respective databases.
HTML with malicious or spam links is added to every record of the table, only in the field with the type specified above.
The server has already been scanned with several tools and all the main password (administrator, sa of SQL Server) has been changed. I also tried to run the SQL Profiler to try to identify the mass update query, but without success.
As I can imagine for now, this may be an SQL Injection attack that uses a vulnerability in the software that runs those websites, but why only some of them? Other websites with the same version never got this problem.
As you know, is there something else I can try, or I'm missing? Reply without problems if you may need other data.