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Ask HN: How did the internet discover my subdomain?

287 points govideo | 1 comments | 06 Mar 25 22:34 UTC | HN request time: 0.289s | source

I have a domain that is not live. As expected, loading the domain returns: Error 1016.

However...I have a subdomain with a not obvious name, like: userfileupload.sampledomain.com

This subdomain IS LIVE but has NOT been publicized/posted anywhere. It's a custom URL for authenticated users to upload media with presigned url to my Cloudflare r2 bucket.

I am using CloudFlare for my DNS.

How did the internet find my subdomain? Some sample user agents are: "Expanse, a Palo Alto Networks company, searches across the global IPv4 space multiple times per day to identify customers' presences on the Internet. If you would like to be excluded from our scans, please send IP addresses/domains to: scaninfo@paloaltonetworks.com", "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_7; en-us) AppleWebKit/534.20.8 (KHTML, like Gecko) Version/5.1 Safari/534.20.8", "Mozilla/5.0 (Linux; Android 9; Redmi Note 5 Pro) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/76.0.3809.89 Mobile Safari/537.36",

The bots are GET requests which are failing, as designed, but I'm wondering how the bots even knew the subdomain existed?!

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yatralalala ◴[07 Mar 25 12:49 UTC] No.43289743[source]▶
Hi, our company does this basically "as-a-service".

The options how to find it are basically limitless. Best source is probably Certificate Transparency project as others suggested. But it does not end there, some other things that we do are things like internet crawl, domain bruteforcing on wildcard dns, dangling vhosts identification, default certs on servers (connect to IP on 443 and get default cert) and many others.

Security by obscurity does not work. You can not rely on "people won't find it". Once it's online, everyone can find it. No matter how you hide it.

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1. jiggawatts ◴[08 Mar 25 21:38 UTC] No.43303762[source]▶
That’s all absolutely true, but I have found that wildcard DNS zones with wildcard certificates tend to get zero un-solicited traffic as long as the client devices are not browsers.

I.e.: if the host is listening only to some specific host header but registered with a wildcard prefix, then drive-by attackers have no trivial way to guess the prefix.

I would never rely on this for security, but it does help cut down on the “spam” in the request logs so that I can focus on the real errors.

This works best for API endpoints not used by browsers or embedded into web pages.

It’s also my current preferred setup for Internet-facing non-production sites. Otherwise they get so much attack traffic that the real log entries might be less than 0.1% of the total.