The blog section of my website is currently hacked. When I do a site: I can find a lot of new pages with Japanese text(randomly creating directories) that have been indexed. I have gone through the guidelines provided by Google on Japanese keyword hack. We are unable to find the root cause of the hack and we are still investigating the issue. There are 30,000+ odd junk urls and manually removing URL via Search Console is a temporary solution and cumbersome as well. Can anyone suggest
The following may help you fix this:
Method 1: It appears deep infection. If you got last stable blog backup with files and DB, you may wish to restore and do prevention measures.
Method 2: If it is not custom build or WordPress then you may like to install WordPress and there are some free plugins available which you can use to scan the blog directory and locate infection and manually clean those.
Prevention: Ensure blog is updated. Also, ensure you got right file and directory permissions.
Removing Junk URLs: Once you will clean the blog, most likely these junk URL will go to a 404 and then you can submit a reconsideration request to Google Webmaster to remove any red flag.
Your prime focus should be removing the infection and ensure the blog is safe then come back to junk urls and 404 pages.