CT Subdomain Finder & New Certificate Alerts — know every host on your domain

Every TLS certificate is published in public Certificate Transparency (CT) logs. SiteVerdict turns those logs into a clean inventory of your subdomains and alerts you the day a new host or certificate appears — forgotten staging servers, shadow-IT SaaS, a vendor issuing certificates for your brand, or an attacker's phishing host on a dangling DNS record.

• 🔎 Full subdomain inventory from CT: first/last seen, latest expiry, issuers, certificate count
• ✳️ Wildcard aware: *.example.com kept as its own row; hosts covered by a wildcard are marked
• 🟢 Live check by DNS (A/AAAA/CNAME) — no web requests to your hosts
• ⚠️ Takeover hints: CNAMEs to S3, Azure, Heroku, GitHub Pages, Shopify, Zendesk and 40 other services that no longer resolve
• 🔔 Monitor mode: new hosts flagged isNew, sent to Slack or your webhook
• 🐢 Polite to free services: one crt.sh query per domain, long timeouts, careful retries and a 12-hour cache; or use your own Cert Spotter API key

Who it is for

1. Security teams and pentesters (authorised scope) — attack-surface discovery and continuous monitoring of your own domains.
2. IT and DevOps — find forgotten hosts and certificates before they expire or get hijacked.
3. Agencies and MSPs — a weekly "new hosts" digest for every client domain.

Example output

{
  "recordType": "subdomain",
  "domain": "sample-shop.example",
  "host": "help.sample-shop.example",
  "isNew": true,
  "wildcard": false,
  "coveredByWildcard": true,
  "resolves": false,
  "cname": "help-sample-shop.s3-website.eu-west-2.amazonaws.com",
  "takeoverHint": "CNAME points to help-sample-shop.s3-website.eu-west-2.amazonaws.com (s3-website) which does not resolve - possible dangling record / subdomain takeover risk. Verify and remove the record if unused.",
  "activeCert": true,
  "firstSeen": "2026-01-05T00:00:00.000Z",
  "latestExpiry": "2027-02-05T00:00:00.000Z",
  "issuers": ["Amazon"]
}

Each domain also gets a domain-summary row: certificates, subdomains, valid-cert count, resolving count, wildcards, new hosts and takeover hints.

Before: "Do we have anything else on our domain?" — After: a sorted list of 140 hosts, 3 of them new this week and 1 dangling CNAME to fix today.

How to use

1. Enter your domains (e.g. example.com).
2. Keep crt.sh (free) or choose Cert Spotter and paste your API key for faster, more reliable results.
3. For monitoring: turn on Monitor mode (+ Only output new subdomains), add a Slack/webhook URL and schedule daily. The first run is the baseline.

Pricing

Event	Price (proposed)
Domain scanned (domain-scanned) — all subdomains, DNS checks and takeover hints for one domain	$0.015

Examples: 10 domains ≈ $0.15; daily monitoring of 5 domains ≈ $2.25 a month. Failed CT queries (e.g. crt.sh down) and sample mode are free.

Benchmark (honest summary)

Compared with subdomain / CT Actors on the Apify Store by documented features (we did not run them):

	SiteVerdict	Typical CT/subdomain finders
CT source	crt.sh (cached, polite) or Cert Spotter with your key	crt.sh only, or brute-force DNS wordlists
New-host monitoring + Slack/webhook	✅	Rare
Wildcard handling	✅ separate rows + "covered by wildcard"	Usually stripped or mixed
DNS liveness + dangling-CNAME takeover hints	✅	Rare
Brute force / active probing	❌ never (passive only)	Often
Price	$0.015 per domain	$0.02 per domain or per-row pricing

Caveat: crt.sh is a free community service and is sometimes too slow for very large domains (e.g. big tech). Those runs are reported as failed (not charged); use Cert Spotter with your own key.

FAQ

Is this legal? CT logs are public by design (RFC 6962/9162) and we only read them. DNS lookups are ordinary public queries. We never probe, scan ports or brute-force. Only monitor domains you own or are authorised to assess.

Why are some hosts not resolving? CT shows every name that ever had a certificate. Old hosts disappear from DNS; that is useful history (and the source of takeover risk).

Will internal hostnames leak? Only names already published in public CT logs appear — which is exactly why you should know about them.

Limits

• Max 200 domains per run; up to 20,000 subdomains per domain; up to 5,000 DNS checks per domain (default 500).
• crt.sh: one query per domain, 10 s between domains by default, up to 3 attempts with 20–40 s backoff; responses up to 60 MB.
• Cert Spotter: up to 50 pages per domain; without a key the free quota is small.
• Passive only: no HTTP requests to discovered hosts.

Sources & compliance

• crt.sh, operated by Sectigo — public CT search, https://crt.sh/ (read 11 Oct 2026). No published terms forbid automated JSON queries; we follow its etiquette: one query per domain, caching, honest User-Agent SiteVerdict/ct-subdomain-monitor with contact aman.thakran@bharatfare.com.
• Cert Spotter API by SSLMate — https://sslmate.com/help/reference/ct_search_api_v1 and terms https://sslmate.com/terms (read 11 Oct 2026); used with the user's own key.
• Takeover service list: service names compiled from the public "can-i-take-over-xyz" project (MIT) — names only, no code.
• No personal data is collected. Certificates sometimes contain email-like SANs; these are dropped. UK GDPR/PECR: none of the output is personal data unless your hostnames contain personal names.
• Secrets (Cert Spotter key, webhook URLs) are encrypted inputs, never logged or output.
• Not legal advice; takeover hints are signals to verify, not proof.