What is a virus on my site?

Question

Drupal

Caught an interesting virus. What do I have.
Drupal 7 site with domain module.
So when I change cities on the site, the first thing I see is an advertising page.

Reloading the page of the site, the advertisement disappears. As I understand it somewhere in cookies it is written. Then it pops up again.

Answer 1

see js injections in files that are responsible for displaying views of the current page
Or some other inclusions of foreign code

Answer 2

If there are third-party js (metrics, ads, bootstraps, jiveries) disable them and see how it goes without them.
If nothing has changed, then look for changes in the modules: https://www.drupal.org/project/hacked
If nothing has been found, look at the blocks and the topic.
And update the engine and all modules to the latest version is required.

Answer 3

You can try this balalaika, it helped https://revisium.com/ai/