I have found some unknown "Package manager extension" installed on the site automatically. If I go to that Extension site I have found that non of there extension is installed on my site, then how the installer is installed on my site ? Is there auto install extension option in the admin panel or this is the malware attack ?
Is there auto install extension option
No. It could be a left over from a previous extension installation. Many extension providers add a "common", "base" or "all" module to their extensions, and it could be that you removed the single modules but not this generic one.
But if it does not look familiar at all, it's best to assume the worst case and that's that your site has been hacked and the attacker installed this extension.
If you have compilation disable then let say yes!
Magento will run all the installer located on
If the version of the module is match by the
But if you don't want to do this things, then try to enable
More details you will find here http://alanstorm.com/magento_setup_resources