Safeguard 1.5: Use a Passive Asset Discovery Tool

To meet the safeguard 1.5: use a passive asset discovery tool requirement, you must deploy a passive discovery capability that continuously observes network and environment signals to identify assets that may not appear in your active scans or CMDB. Operationalize it by defining scope, data sources, ownership, review cadence, and a remediation workflow that turns “newly seen” assets into managed inventory entries ¹.

Key takeaways:

• Passive discovery complements active scanning by finding “unknown” or transient assets through observation, not probing ².
• Your audit risk is usually evidence, not tooling: show ownership, cadence, exceptions, and tracked remediation ².
• Treat every newly discovered asset as a ticketable event: identify, classify, assign owner, and bring it under management.

Safeguard 1.5 sits inside CIS Control 1 (Inventory and Control of Enterprise Assets) and addresses a common operational gap: assets exist on your networks and in your cloud environments that your “official” inventory does not capture. Passive asset discovery closes that gap by watching for assets through telemetry you already generate (network traffic, logs, platform events), instead of relying only on active probes that can miss segmented, ephemeral, or sensitive systems ¹.

For a Compliance Officer, CCO, or GRC lead, the fastest path is to translate the requirement into a control you can run: define what environments are in scope, pick the passive sources you will treat as authoritative signals, specify what “discovered” means, and set an operational workflow that turns discoveries into accountable inventory records. Examiners and customers tend to focus on two things: whether passive discovery is actually running across the right segments, and whether you consistently act on the results. This page gives you a requirement-level runbook: steps, artifacts, and the questions you will get in audits.

Regulatory text

Framework requirement (excerpt): “CIS Controls v8 safeguard 1.5 implementation expectation (Use a Passive Asset Discovery Tool).” ¹

Operator interpretation: You need a passive asset discovery mechanism that observes your environment to identify enterprise assets. The expectation is not “buy a tool” in the abstract; it is to continuously detect assets that communicate or appear in telemetry and to feed those findings into your inventory and control processes ².

Plain-English interpretation

Passive asset discovery means: collect signals that already exist (network, logs, cloud control plane events) and use them to detect devices, systems, and workloads you did not explicitly register. It is how you find:

• unmanaged devices on corporate networks
• systems that were stood up outside standard provisioning
• short-lived cloud instances/containers that never made it into the CMDB
• “shadow IT” gateways, lab gear, or partner-connected equipment

A practical definition you can adopt:

• Passive discovery is “always on” observation.
• A “discovery” is an asset seen in telemetry that is not already a known, owned inventory item.
• The outcome is a reconciled inventory plus remediation tickets for exceptions.

Who it applies to

Entity types: Enterprises, service organizations, and technology organizations implementing CIS Controls v8 ¹.

Operational context where it matters most:

• You have multiple network segments, remote offices, or OT/IoT-adjacent networks where active scanning is limited.
• You run hybrid cloud and cannot rely on one inventory source.
• You have M&A activity, contractors, or third parties connecting devices to your networks.
• You have strong change control “on paper,” but recurring surprises in incident response (unknown hosts, unknown IPs, unknown SaaS-connected endpoints).

Teams typically involved (assign named owners):

• Security Operations (telemetry, detection engineering, triage)
• Network Engineering (SPAN/TAP, NetFlow, DHCP/DNS sources)
• Cloud Platform team (cloud logs, asset feeds)
• IT Asset Management / Endpoint Engineering (inventory reconciliation)
• GRC (control definition, evidence, exceptions)

What you actually need to do (step-by-step)

Step 1: Write the control card (make it executable)

Create a one-page control card that states:

• Objective: Identify enterprise assets through passive observation and reconcile them to inventory ².
• Owner: a role with operational authority (often SecOps or IT Asset Management).
• Scope: networks, cloud accounts/subscriptions/projects, and data centers.
• Trigger events: “new asset seen,” “asset reappears,” “inventory mismatch,” “unknown MAC/OUI,” “unknown hostname.”
• Cadence: continuous detection, plus a scheduled review of the exception queue.
• Exception rules: research networks, approved guest networks, lab segments, third-party managed segments, approved monitoring exclusions.

This is the fastest way to prevent the common audit failure: “We have a tool, but no one can explain who runs it or what ‘good’ looks like.”

Step 2: Define passive data sources (and what each proves)

Pick sources that are hard to spoof and broadly available. Document which sources count as “asset presence” for your environment:

• Network-layer: NetFlow/IPFIX, SPAN/TAP sensor telemetry, IDS/NSM sensor logs.
• Infrastructure services: DHCP leases, DNS query logs, NAC/802.1X events.
• Identity/security logs: VPN concentrator logs, wireless controller associations.
• Cloud control plane: cloud audit logs and asset inventory feeds from cloud providers (where available to you).

Create a simple “source-to-asset-fields” mapping table:

Passive source	What it can discover	Minimum fields to capture	Common gap
DHCP	New endpoints	MAC, IP, lease time, VLAN/segment	No user/owner
DNS logs	Systems reaching out	hostname, source IP, query time	NAT hides device
NetFlow	“Talkers” on network	src/dst IP, ports, timestamps	No hostname
VPN logs	Remote endpoints	user, device ID (if present), source IP	BYOD ambiguity

Your goal is not perfection. Your goal is repeatable discovery and reconciliation.

Step 3: Implement reconciliation logic (inventory matching rules)

Define matching rules that your operators can execute and your auditors can follow:

• Match on stable identifiers first (device ID, instance ID, MAC address).
• Fall back to hostname and IP with time-window logic.
• If a discovery cannot be matched confidently, classify it as “unattributed” and route it for investigation.

Operational rule you should enforce: no discovery ages out without a disposition (owned and inventoried, approved exception, or removed/blocked).

Step 4: Build the workflow from “seen” to “owned”

Treat passive discoveries like security findings with SLA-backed handling:

1. Detection: tool flags a new/unknown asset.
2. Triage: analyst or asset manager validates signal quality and de-duplicates.
3. Enrichment: pull context (segment, switch/AP, cloud account, tags, recent authentications).
4. Disposition decision:
   • Legitimate and managed: add/update inventory record; map to owner and business service.
   • Legitimate but unmanaged: onboard to endpoint management; apply baseline configuration standards.
   • Not authorized: isolate/quarantine, block, or remove; open an incident if needed.
   • Approved exception: document exception, expiration, compensating controls.
5. Close the loop: update inventory source of truth and keep a link to the ticket.

If you use Daydream for GRC execution, this is where it helps: store the control card, define the evidence bundle, and track remediation items to validated closure with due dates so you can prove sustained operation during audits ².

Step 5: Establish control health checks (prove it keeps working)

Define a recurring control health check that answers:

• Is the passive sensor/log feed still connected and ingesting?
• Are discovery rules firing?
• Are tickets being created and closed with disposition codes?
• Are exception approvals current and expiring as expected?

This is where teams often fail: they can show a dashboard screenshot, but not whether anyone acted on it.

Required evidence and artifacts to retain

Store evidence in a system your audit team can retrieve without heroics. Minimum bundle:

• Control card/runbook for Safeguard 1.5 (owner, scope, cadence, exceptions) ².
• Architecture/data-flow evidence: diagram or written description of passive sources and collection points.
• Configuration evidence: list of enabled data sources, sensors, or log integrations; detection rules enabling discovery.
• Output evidence ²:
  • export/report of newly discovered assets
  • reconciliation results (matched vs unmatched)
  • sample tickets with full lifecycle (open → disposition → inventory update)
• Exception register entries tied to specific segments/assets, with approval and review dates.
• Control health check results and remediation tracking ².

Auditors do not need every alert. They need a defensible sampling story and proof of operation over time.

Common exam/audit questions and hangups

Expect these, and pre-answer them in your control card and evidence:

• “What is your definition of an asset for this control?” (enterprise assets in observed scope)
• “Which networks and cloud environments are covered, and which are excluded?”
• “Show me the last set of newly discovered assets and what you did about them.”
• “How do you prevent duplicates across DHCP/DNS/flow logs?”
• “Who can approve an exception, and how do exceptions expire?”
• “How do you know passive discovery is still running, not just configured?”

The hangup is usually scope and follow-through. A passive tool that finds assets but doesn’t drive inventory updates will not satisfy the operational intent ².

Frequent implementation mistakes (and how to avoid them)

Mistake	Why it fails in audits	Fix
Treating passive discovery as a one-time project	Requirement implies ongoing detection 2	Add health checks, ticket workflow, recurring review
No defined “source of truth” inventory	Discoveries never reconcile	Declare the inventory system and matching rules
Over-scoping on day one	Team drowns in noise	Start with critical segments, then expand with clear milestones
Exceptions live in email	No traceability	Maintain an exception register with expirations and approvals
No ownership	Findings don’t convert to actions	Name an operator owner and backup; define escalations

Risk implications (why operators care)

Passive asset discovery reduces the chance that unknown systems become your blind spots for:

• vulnerability management (assets not scanned don’t get patched)
• incident response (unknown hosts slow containment)
• access control (unmanaged devices bypass endpoint and identity controls)
• third-party connections (partner devices can appear on internal segments)

From a compliance standpoint, the risk is control design failure (no defined operation) or control operating failure (no evidence of action). Your evidence package should make both hard to allege.

Practical 30/60/90-day execution plan

Use a phased plan without promising calendar time-to-complete. The goal is controlled rollout and auditable operation.

First 30 days (stand up and define)

• Publish the Safeguard 1.5 control card: scope, owner, workflow, exceptions ².
• Inventory your available passive sources (DHCP, DNS, flows, cloud logs) and pick initial “authoritative” feeds.
• Stand up a basic “newly seen asset” queue and ticket type with required fields (segment, identifier, disposition).
• Define matching rules to your inventory source of truth.

Days 31–60 (operate and tune)

• Run discovery continuously; start reconciliation and disposition at a steady cadence.
• Tune noise: suppress known benign ranges, fix mis-parsing, improve enrichment.
• Produce your first evidence packet: report export + sample tickets + updated inventory entries.
• Implement health checks for ingestion and rule execution ².

Days 61–90 (expand coverage and harden)

• Expand to additional segments/accounts that were out of scope.
• Formalize exception approvals and expirations; migrate any informal exclusions into the exception register.
• Add reporting that shows trend lines qualitatively (e.g., “new assets per week”) without publishing unsourced numbers.
• Run an internal control effectiveness review and log remediation items to closure ².

Frequently Asked Questions

Does passive asset discovery replace active scanning?

No. Passive discovery finds assets through observation; active scanning validates exposure and configuration. Treat passive discovery as the “find what you missed” layer that feeds your inventory and scanning scope ².

What counts as an acceptable “passive asset discovery tool”?

Any capability that passively observes your environment and produces identifiable asset records from telemetry can qualify. The audit test is whether it reliably detects unknown assets in scope and whether you act on the findings ².

How do we handle encrypted traffic if we rely on network telemetry?

Use metadata sources that remain visible, such as DHCP, DNS logs, flow records, NAC, and cloud control plane logs. Your control should document which sources you use and what fields they provide.

Our environment is mostly cloud. What is “passive” there?

In cloud, “passive” usually means consuming control plane and platform logs and asset feeds rather than probing workloads. Define which cloud accounts are in scope and how discoveries map to your inventory ¹.

What evidence do auditors usually accept without debate?

A written control card, documented data sources, and a sample set of discoveries traced through tickets to inventory updates and dispositions. Add proof the feed is live (ingestion/health check output) ².

How should we treat third-party connected devices on our networks?

Treat them as enterprise assets for discovery purposes if they appear on your networks, then disposition them as third-party owned with an accountable internal sponsor. Record the exception or contract controls that govern them, and enforce segmentation where required.

Footnotes

1. CIS Controls v8; CIS Controls Navigator v8

2. CIS Controls v8