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.
robots.txt
What the agent asks to be told. It obeys this or it does not — nothing enforces it.
User-agent: Let& Disallow: /
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|Let&)") {
return 403;
}
Apache
In .htaccess or the vhost.
BrowserMatchNoCase "Lets Encrypt Validation Server" bad_bot
BrowserMatchNoCase "Let&" 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") or (http.user_agent contains "Let&")
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', 'Let&'] as $needle) {
if (stripos($agent, $needle) !== false) {
status_header(403);
exit('Blocked: Lets Encrypt Validation Server');
}
}
});
robots.txt is a request. The rules below it are the wall.
Lets Encrypt Validation Server publishes a token, so the directive above will work for as long as it keeps honouring it — and nothing except its own good manners makes it. 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.