Individual skill
find-hidden-subdomains
Enumerate an organisation's subdomains and sibling domains from Certificate Transparency logs and passive DNS, without sending traffic to the target. Covers crt.sh and CT log queries, certificate SAN fields, subfinder and amass, and newly issued TLS certificates. Use when looking for staging, dev, admin or VPN hosts, mapping the full hostname footprint of a domain, or spotting infrastructure a company forgot it had. Applies to attack-surface mapping, vendor and supply-chain security review, brand-infringement discovery, and M&A technical diligence. Reference at useosint.com/skills/find-hidden-subdomains.
skills/find-hidden-subdomains/SKILL.md
Source commit 06243a5620b0c9c97502edd4ee9e31995a3bdccd
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- Matches the observed repository HEAD
- Yes
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- Observed behavior
- Not checked yet
- Last checked
- Nov 6, 2026
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Repository context
- Repository stars (not skill ratings)
- 61
- Repository last updated
- Aug 3, 2026
- License
- MIT
Observed agent integration: No specific integration evidence. Standard SKILL.md installation does not imply verified compatibility.
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SKILL.mdcore · 12,869 bytesSKILL.mdcore · 12,869 bytesSKILL.mdcore · 12,869 bytesSKILL.mdcore · 12,869 bytesSKILL.mdcore · 12,869 bytesSKILL.mdcore · 12,869 bytesSKILL.mdcore · 12,869 bytesSKILL.mdcore · 12,869 bytesSKILL.mdcore · 12,869 bytesSKILL.mdcore · 12,869 bytesSKILL.mdcore · 12,869 bytesSKILL.mdcore · 12,869 bytesreference/name-pattern-triage.mddoc · 8,405 bytesreference/subdomain-sources.mddoc · 7,660 bytes
Read excerpt
# Find hidden subdomains
The hosts an organization forgot about are the ones worth finding, and most of
them announced themselves the moment someone requested a TLS certificate.
Certificate Transparency turns that into a searchable, permanent, historical
index — free and completely passive. The beginner's mistake is treating a CT hit
as a live host: most of what you pull back does not resolve, and that is
information too, not noise.
## Which source first
| You hold | Start with | Why |
|---|---|---|
| An apex domain | crt.sh wildcard query | Broadest free coverage, includes long-dead names |