Some body changed files in server.

Hello communnity member,

I am having problem with the drupal installation, As i am drupal developer since last 5 years. I am not in trouble faced.

Some of the website`s index.php, update.php, authorise.php is getting changed in two hours only. though i have set the permission to 444, it is changed to 777 and website get in terminated state. 

I had checked all the security checks in website. and Also search for virus/malware as well.

Please help me to get rid of this issue. What is resolution for this query.


Drupal version: