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Your organization’s real internet-facing attack surface is not just the systems on its asset list. It is everything a remote observer can still discover and reach—including forgotten staging hosts, old subdomains, exposed databases, and remote-access services. Certificate records and DNS can help uncover those assets, but discovery is only the start: each name must be checked for current resolution and reachability, and active probing should be limited to systems you own or are authorized to test.
Why the internet may remember more than your inventory
Projects end, teams change, and temporary systems get overlooked. A hostname can remain in public DNS or point to a reachable service after the people who created it have moved on. An internal asset list records what an organization believes it owns; an outside-in view shows what can still be found from the internet. Those two lists can differ.
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That difference matters because an undocumented asset can escape routine maintenance and review. Its presence does not prove that it is vulnerable or compromised, but it is a reason to identify who owns it and decide whether it should remain reachable.
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Certificate Transparency logs
Certificate Transparency (CT) makes certificate issuance publicly verifiable and monitorable. Its public logs can show which certificates were issued, when, and for which domain names. That makes CT a useful source of clues about hostnames an organization may no longer remember. See the Certificate Transparency project overview.
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A CT entry is historical evidence that a certificate was issued for a name; it is not proof that the hostname still resolves, that its destination remains under your control, or that a service is running today. Nor does every certificate necessarily enumerate every covered subdomain: a wildcard certificate can cover names without listing each one individually. A small number of names in CT therefore does not establish that the organization has a small internet-facing footprint.
DNS and live checks
Discovery sources produce candidates, not a verified current inventory. Check whether each hostname currently resolves, then determine whether a service is reachable from the scanning location. The Attack Surface Monitor repository describes combining CT and DNS discovery, retaining names that resolve, and conducting active probing only after domain control has been verified: Attack Surface Monitor on GitHub.
Coverage depends on the sources used and the vantage point of the scan. Names absent from CT and DNS may be missed, while an external scan cannot automatically see systems that are reachable only from inside a private network. Internal-only assets require an appropriately placed scan.
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What exposure monitoring can tell you
The Attack Surface Monitor repository describes checks for reachable databases such as PostgreSQL, MySQL, MongoDB, Redis, and Elasticsearch; remote-access services such as RDP, VNC, and Telnet; and open ports. Its stated workflow compares scans so newly discovered hosts or ports can be reported, while closed ports or removed assets can be resolved from findings.
Reachability is not a verdict about security. An open port or exposed service does not by itself show that a system is exploitable or compromised. Exposure discovery answers what can be reached; vulnerability assessment asks whether a weakness can be exploited. The repository explicitly distinguishes those questions and recommends pairing exposure monitoring with a vulnerability scanner when exploitability assessment is needed.
How to investigate a finding safely
- Confirm scope and authorization. Scan only systems you own or have explicit permission to test. The Attack Surface Monitor repository requires proof of domain control—through a DNS TXT record or an HTTP file—before active probing. Check applicable cloud-provider rules as well; some providers may require notice before security testing.
- Validate the hostname. Treat CT and other passive results as leads. Check current DNS resolution rather than assuming a historical certificate name still points to an active asset.
- Check reachability from the right place. Establish what responds from the external scanning location. If the asset should be private or internal-only, use a separately authorized scan from an appropriate internal vantage point.
- Identify the owner and purpose. Match the reachable host or port to a team, service, or business purpose. If no owner or purpose can be established, escalate through your organization’s asset and security processes.
- Decide whether exposure is intentional. Close or restrict services that should not be public, or document why a service must remain reachable. Use a vulnerability assessment separately if you need to evaluate exploitability.
- Compare subsequent scans. Review newly appearing hosts and ports, and verify that removed assets or closed ports no longer appear. A changing inventory is more useful than a one-time list.
Attack Surface Monitor: what its documented editions include
Attack Surface Monitor describes itself as self-hosted attack-surface discovery and exposure monitoring. Its repository says asset lists and findings remain on the user’s network. The following is the matrix documented in the repository snapshot; pricing, limits, and features can change, so confirm current terms in the repository before adopting a tier.
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| Documented feature | Free | Pro | Team |
|---|---|---|---|
| Monitored domains | 1 | 10 | Unlimited |
| Scan schedule | Weekly fixed scans | Custom scans and scan-now | Custom scans and scan-now |
| History retention | 14 days | 1 year | Unlimited |
| Support | Community | Priority | |
| Price documented in repository snapshot | Free, with no stated time limit | $29/month | $99/month |
| Trial documented in repository snapshot | Not stated | 14 days | 14 days |
| Notification channels | Differ by tier; consult the repository for the current matrix | Differ by tier; consult the repository for the current matrix | Differ by tier; consult the repository for the current matrix |
The repository documents the free edition as a way to monitor one domain with weekly scans and 14 days of history. Paid tiers add domain capacity, scan controls, longer retention, and different support and notification options. These are vendor-published descriptions, not independent performance findings.
When an outside-in scan is not enough
- Names not present in discovery sources: Passive discovery is not exhaustive; assets absent from CT and DNS may not be found.
- Wildcard certificates: A wildcard can cover subdomains without individually naming them in CT, so CT-derived counts may understate the number of relevant names.
- Internal-only services: A scan from the public internet sees only what is reachable from its deployment location. It does not replace an internal scan.
- Exploitability and compromise: Exposure monitoring identifies reachability, not whether a service can be exploited or has been compromised.
Use outside-in monitoring as one view of your assets, not as a guarantee that every system has been found or assessed.
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