Aqui todo dia são bloqueados 2, 3 ips.

Já havia recebido alguns ataques, fiz tudo o que recomendaram (Só falta instalar o CSF-ConfigServer Security & Firewall), mas por precaução, adicionei estas linhas no .htaccess do whmcs:

# Block out any script trying to set a mosConfig value through the URL

RewriteCond %{QUERY_STRING} mosConfig_[a-zA-Z_]{1,21}(=|\%3D) [OR]

# Block out any script trying to base64_encode data within the URL

RewriteCond %{QUERY_STRING} base64_encode[^(]*\([^)]*\) [OR]

# Block out any script that includes a <script> tag in URL

RewriteCond %{QUERY_STRING} (<|%3C)([^s]*s)+cript.*(>|%3E) [NC,OR]

# Block out any script trying to set a PHP GLOBALS variable via URL

RewriteCond %{QUERY_STRING} GLOBALS(=|\[|\%[0-9A-Z]{0,2}) [OR]

# Block out any script trying to modify a _REQUEST variable via URL

RewriteCond %{QUERY_STRING} _REQUEST(=|\[|\%[0-9A-Z]{0,2})

# Return 403 Forbidden header and show the content of the root homepage

RewriteRule .* index.php [F]

########### End - Rewrite rules to block out some common exploits

Afinal, se estão tão fissurados assim em invadir o WHMCS, segurança nunca é demais...

Otima dica