Accountability should sit with the security architecture and application ownership functions together, because segmentation failures usually come from a mix of design choices, undocumented dependencies, and poor change coordination. In regulated environments, the control owner must be able to show why trust was granted, who approved it, and when it will be revisited.
How accountability shifts when segmentation is part of a regulated boundary
Segmentation failures during an acquisition or audit are rarely owned by a single team in practice, even though auditors often want a clear control owner. The accountable parties are usually the security architecture function, which defines the boundary and trust model, and the application or service owner, which understands the real dependencies that may cross it. When those two functions are not aligned, segmentation is often treated as a network diagram exercise rather than a governance decision about who can legitimately reach what. In a regulated setting, that distinction matters because the organisation must be able to justify the trust path, not just describe it.
For control assurance, the more useful question is not only who manages the firewall rule, but who can defend the business justification for the exception, the approval trail, and the review date. That is why segmentation accountability belongs to the people who can explain both the technical boundary and the operational need. For a control reference, see NIST SP 800-53 Rev 5 Security and Privacy Controls. In practice, many security teams discover the ownership gap only when an acquisition due diligence request or audit evidence pack exposes undocumented east-west trust.
Why segmentation ownership breaks down during due diligence
Segmentation is often assumed to be a network security problem, but acquisitions and audits expose it as an accountability problem. The control can fail even when the firewall is technically configured correctly, because the real issue is whether the allowed communication path was deliberately approved, still needed, and traceable to a business owner. If an integration was inherited from a merger, or a legacy service depends on a route nobody formally owns, the segmentation model becomes hard to defend.
In assurance terms, the weak point is not only enforcement but evidencing intent. A reviewer needs to see who accepted the trust relationship, which system or application required it, and how the decision will be revisited when the environment changes. That is why NHI Management Group treats segmentation as part of broader control governance, not a standalone network task. The NIST Cybersecurity Framework 2.0 is useful here because it reinforces governance, inventory, and control accountability rather than treating segmentation as a one-off technical setting.
Where teams get it wrong is assuming that a diagram or a ticket equals ownership. In regulated acquisitions, those artefacts may support the answer, but they do not replace a named decision-maker who can defend the boundary.
When the answer changes, and when it does not
Tighter segmentation often increases coordination overhead, requiring organisations to balance stronger boundary control against merger speed and audit deadlines. That tradeoff becomes visible when an acquired environment still needs temporary trust to keep critical services running, or when an audit finds that a valid connection lacks a current owner because the original approver left the business.
There is an important distinction between operational responsibility and accountability. Network or platform teams may implement the rule set, but accountability should remain with the security architecture function and the application owner together. If the issue is a shared platform, the accountable model may also include a service owner or product owner, but the principle does not change: the person or function that accepted the risk must be identifiable.
There is no universal consensus that every segmentation exception must be owned by the same role in every organisation. The defensible approach is to align ownership to the system of record for the trust decision, then make sure the audit trail shows approval, expiry, and periodic revalidation. In practice, acquisitions and audits expose weakness fastest when legacy exceptions outlive the people who approved them, so ownership without review is only partial accountability.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
NIST CSF 2.0, CIS Controls v8 and NIST SP 800-63 set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| NIST CSF 2.0 | GV.OV — Oversight | Segmentation accountability depends on clear governance and oversight of control ownership. |
| ID.AM — Asset Management | Inherited connections during acquisitions expose gaps in system and dependency visibility. | |
| Recommendation — Assign oversight for segmentation exceptions and verify ownership stays current. Maintain an accurate inventory of cross-segment dependencies before approving trust paths. | ||
| CIS Controls v8 | 6 — Access Control Management | Segmentation failures often reflect unmanaged exceptions and unclear access ownership. |
| 12 — Network Infrastructure Management | Segmentation is enforced through network boundary design and rule governance. | |
| 5 — Account Management | Ownership must identify who approves and reviews trust relationships over time. | |
| Recommendation — Review and remove unnecessary cross-zone access paths under a named owner. Document and validate network segmentation rules against approved business need. Tie each segmentation exception to an accountable approver and review cycle. | ||
| NIST SP 800-63 | 4 — Identity Proofing | Acquisition and audit accountability rely on trusted approval and evidence of who accepted risk. |
| Recommendation — Use strong identity proofing for approvers who authorize enduring trust exceptions. | ||
Practitioner Guidance
What to verify: Confirm that every cross-zone or cross-segment path has a named business owner, a named technical approver, and a current reason for existence. If any of those three elements is missing, treat the segmentation control as incomplete rather than merely documented.
What to prioritise: Focus first on exceptions, inherited connections, and temporary merger bridges, because those are the paths most likely to lack durable accountability. Stable internal flows are usually easier to evidence; undocumented trust is where audit findings and post-acquisition surprises tend to cluster.
Decision rule: If the team cannot explain why trust exists in plain terms, the connection should be treated as a risk acceptance problem, not just a firewall rule problem. If the justification is valid but stale, require reapproval before relying on it in an audit.
Practitioner takeaway: Segmentation accountability is strongest when ownership is tied to the approved trust decision, not to the team that happened to implement the last change.
Related resources from NHI Mgmt Group
- Who is accountable when endpoint compliance fails during an audit?
- Who is accountable for identity continuity when access fails during an outage?
- Who is accountable when a shared-device access process fails compliance or audit review?
- Who is accountable when password recovery fails during an incident?
Deepen Your Knowledge
Reviewed and updated by the NHIMG editorial team on September 7, 2026.
NHI Mgmt Group — the #1 independent authority on Non-Human Identity, IAM, and Agentic AI security. nhimg.org
