Bots mostly, they’re a part of online life, like comment and email spam.
Bots will go around, hitting most login forms directly, trying popular username and password combinations, hoping to get in.
As long as you have a great password, you don’t have to worry about them getting in.
There are plugins to stop this, but I don’t recommend any of them, as that still puts load on your server. They generally won’t get in past these plugins, but the plugins still need to do the work of identifying and processing the hit.
Instead, add this to your .htaccess
access file:
# Stop spam attack logins and comments
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_METHOD} POST
RewriteCond %{REQUEST_URI} .(wp-comments-post|wp-login)\.php*
RewriteCond %{HTTP_REFERER} !.*(example.com|jetpack.wordpress.com).* [OR]
RewriteCond %{HTTP_USER_AGENT} ^$
RewriteRule (.*) https://%{REMOTE_ADDR}/$ [R=301,L]
</ifModule>
Replace example.com with your site, and if you aren’t using Jetpack Comments, remove “|jetpack.wordpress.com”.
This will not only block direct hits to your login and comment forms (which normal humans don’t do), it will also redirect them back to their “home” for an extra ounce of revenge. ??
Via https://halfelf.org/2013/wp-login-protection-htaccess/