How to block Lets Encrypt Validation Server
Operated by Let's Encrypt. The Let's Encrypt validation server is a bot that verifies domain ownership as part of the process for issuing free SSL/TLS certificates. This bot visits websites to complete domain validation challenges required before certificate issuance as part of their automated system.
nginx
In the server block, then reload.
# In the server block. 403 rather than 444: a closed connection tells the
# operator nothing, and an agent that gets a status code can log it.
if ($http_user_agent ~* "(Lets Encrypt Validation Server)") {
return 403;
}
Apache
In .htaccess or the vhost.
BrowserMatchNoCase "Lets Encrypt Validation Server" bad_bot
<RequireAll>
Require all granted
Require not env bad_bot
</RequireAll>
Cloudflare
Security → WAF → Custom rules.
# Security → WAF → Custom rules, action: Block (http.user_agent contains "Lets Encrypt Validation Server")
WordPress
A child theme's functions.php, or a small plugin.
// functions.php of a child theme, or a small plugin. Runs before WordPress
// builds the page, so a blocked agent costs one PHP process and no queries.
add_action('init', function () {
$agent = $_SERVER['HTTP_USER_AGENT'] ?? '';
foreach (['Lets Encrypt Validation Server'] as $needle) {
if (stripos($agent, $needle) !== false) {
status_header(403);
exit('Blocked: Lets Encrypt Validation Server');
}
}
});
A rule on the user agent is a rule on a string it chose
The server rules match on a header the requester sets, which stops the agent that says who it is and not the one that copies somebody else's name. Botscope verifies identity against DNS and published address ranges, records which check decided each request, and shows you the ones that lied.