Cryptocurrency mining
Sites that use user devices to mine cryptocurrencies can have lower rankings in the search. When the users try to open such sites, they see a warning.
The code intended for mining can be installed by the resource owner, or by hackers who hacked someone else's site.
If your site is hacked, use the recommendations in the How to cure an infected site and How to protect your site from infection sections.
You can speed up the removal of the danger mark by sending the site for re-check via Yandex.Webmaster. In most cases, it takes a few days.