How are people discovering random subdomains on my server?

if you use Let’s Encrypt (ACME protocol) AFAIK you can find all domains registered in a directory that even has a search, no matter if it’s wildcard or not.

It was something like this https://crt.sh/ but can’t find the site exactly anymore

LE: you can also find some here https://search.censys.io/

Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:

7 acronyms in this thread; the most compressed thread commented on today has 16 acronyms.

[Thread #990 for this comm, first seen 11th Jan 2026, 01:25] [FAQ] [Full list] [Contact] [Source code]

Chrome: Sees new website domain

Google: 👀

For anyone who needs to read it: At the end of the day this is obscurity, not security; however obscurity is a good secondary defense because it buys time.

I too would be interested to learn how this leaked

Do post again if you figure it out!

If you have browser with search suggestions enabled, everything you type in URL bar gets sent to a search engine like Google to give you URL suggestions. I would not be surprised if Google uses this data to check what it knows about the domain you entered, and if it sees that it doesn't know anything, it sends the bot to scan it to get more information.

But in general, you can't access a domain without using a browser which might send that what you type to some company's backend and voila, you leaked your data.

Maybe that particular subdomain is getting treated as the default virtual host by Apache? Are the other subdomains receiving scans too?

I don't use Apache much, but NGINX sometimes surprises on what it uses if the default is not specifically defined.

https://crt.sh/

When a CA issues an SSL/TLS certificate, they're required to submit it to public CT logs (append-only, cryptographically verifiable ledgers). This was designed to detect misissued or malicious certificates.

Red and Blue team alike use this resource (crt.sh) to enumerate subdomains.

A long time ago, I turned a PC in my basement into a web server. No DNS. Just a static IP address. Within 15 minutes, the logs showed it was getting scanned.

SSL encrypts traffic in-transit. You need to set up auth/access control. Even better, stick it behind a Web Application Firewall.

Or set up a tunnel. Cloudflare offers a free one: https://developers.cloudflare.com/cloudflare-one/networks/connectors/cloudflare-tunnel/

We're always watching.

Crawlers typically crawl by ip.

Are u sure they just not using ip?

U need to expressly configure drop connection if invalid domain.

I use similar pattern and have 0 crawls.

You say you have a wildcard cert but just to make sure: I don't suppose you've used ACME for Letsencrypt or some other publicly trusted CA to issue a cert including the affected name? If so it will be public in Certificate Transparency Logs.

If not I'd do it again and closely log and monitor every packet leaving the box.

I believe that some DNS servers are configured to allow zone transfers without any kind of authentication. While properly configured servers will whitelist the IPs of secondaries they trust, for those that don't, hackers can simply request a zone transfer and get all subdomains at once.

https://www.namesilo.com/blog/en/dns/passive-dns-explained-domain-history

Are you sure they're hitting the hostname and not just the IP directly?

Did you yourself make a request to it or just set it up and not check it? My horrifying guess it that if you use SNI in a request every server in the middle could read the subdomain and some system in the internet routing is untrustworthy.

You need to look at the DNS server used by whatever client is resolving that name. If it's going to an external recursive resolver instead of using your own internal DNS server then you could be leaking lookups to the wider internet.

Do you use an external DNS when accessing your subdomain? I can only guess that it's the DNS leaking it.

Maybe using subfinder?

All the obvious things have been mentioned.

The only way to identify the problem is to share the exact steps youve followed and then others can reproduce.

Based on what youve told us, no one knows how the subdomain is leaked. Without meaning to be derisive, that suggests that something youve told us isn't quite correct.

Scans from where? Is it exposed to the internet? What does the scan traffic look like?

@BonkTheAnnoyed@lemmy.blahaj.zone are you generating certificates for each of the random subdomains?

My guess would be NSEC zone walking if your DNS provider supports DNSSEC. But that shouldn't work with unregistered or wildcard domains

The next guess would be during setup, someone somewhere got ahold of your SNI (and/or outgoing DNS requests). Maybe your ISP/VPN service actually logs them and announce it to the world

I suggest next time, try setting up without any over-the-internet traffic at all. E.g. always use curl with the --resolve flag on the same VM as Apache to check if it's working

If you do a GET / request against the IP (typically http too) does it yield a redirect to your proper fqdn? It shouldn't return anything and remain stealthy as you likely dont want to expose anything directly on IP connections and rely solely on your vhosts.

You need better logging. Try doing a packet capture with tcpdump then decrypt the HTTPS traffic. Because what you've described so far, especially before the edit makes no sense.

If you don't have a DNS record pointing the subdomain to the IP address of the server, it shouldn't be possible to resolve the IP for random Internet users. If this VHOST only exists in your Apache config file and nowhere else, it is private.

Reverse DNS? Or vuln scans just hitting IPs. Don't need DNS for that.

https://subdomainfinder.c99.nl/

I can't say I know the answer but a few ideas:

• did you access it with a browser? Maybe it snitches on you or some extension does?
• did you try to resolve it with a public DNS server at any point (are you sure nothing forwarded the request to one)?

You could try it again, create the domain in the config and then do absolutely nothing. Don't try to confirm it works in any way. If you don't see the same behaviour you can do one of the above and then the other and see when it kicks in. If it gets picked up without you doing anything..then pass!

I run my webservers behind a pfsense firewall with ssl offloading(using a wildcard cert) with a static IP and use Haproxy to have sub-domain's go to individual servers. Even though I've seen my fair share of scans, I only ever expose port 443 and keep things updated.

Recently though someone on here mentioned routing everything over Tailscale via a VPS. I didn't want to pay for a VPS and frankly can't even find one that is reasonably priced in the US(bandwidth limits mainly), so I threw Tailscale onto my pfsense, setup split-dns on Tailscale's admin panel with my domain name, and then reconfigured Haproxy to listen on my Tailscale interface. Even got IPv6 working(huge pain due to a bug it seems). Oh and setup pfblocker.

My current plan is I'm going to run my webservers behind Tailscale and keep my game servers public and probably segment those servers to a different vlan/subnet/dmz/whatever. And maybe just have a www/blog landing page that is read only on 443 and have it's config/admin panel accessible via my tailscale only.

Anyway, back on topic. I run my game servers and I don't advertise them out anywhere(wildcard cert) and do whitelist only, yet I still see my minecraft servers get hit constantly on port 25565.

So not much you can do except minimize exposure as much as possible.

Have you sent the URL across any messaging services? Lots of them look up links you share to see if it's malware (and maybe also to shovel into their AI). Even email services do this.

Inb4 some lucky dude just ran sublist3r or wfuzz on your subdomain and got a hit

If you do a port scan on your box, what services are running? Maybe something like email or diagnostics is exposed to the internet and announcing subdomains?

Following this thread!

Stupid question, but are you somehow publicly exposing your vhost config (or a bak file of it)? Or do you see logs of someone bruteforcing the subdomain?

Did you generate a DNS A record for the subdomain?

if there's no dns entry do you mean you are getting scans to your ip with these random subdomain headers? so someone would need both pieces of information? curious