I disabled all plugins but no luck. Looks like the site has been hacked at the server level and is not something that can controlled at WordPress level. Generally, the hosting support would know how to fix this since they would have encountered such issues before. We are not the best people to look into this since we have never dealt with such a situation before.
Can you pls contact the hosting support to resolve this? Worst case, you will need to do a clean install of the entire site after taking an backup of wordpress contents using export/import tool. Hope this helps.