Horizontal Expansion

Expanded Definition

Horizontal expansion describes the widening of an identity’s access footprint across additional applications, systems, teams, or environments. In NHI governance, the risk is not simply that access grows, but that older permissions often remain active as new ones are added, creating overlapping entitlements and cross-functional reach. That pattern can affect human users, service accounts, API clients, and agent identities when role changes are not matched by timely revocation.

Definitions vary across vendors on whether horizontal expansion is treated as a role-design issue, an access-review issue, or an identity lifecycle issue. NHI Management Group treats it as an operational exposure pattern that becomes visible when entitlement accumulation outpaces governance. This is closely related to least privilege and Zero Trust ideas in the NIST Cybersecurity Framework 2.0, but the term itself is more descriptive than normative. The most common misapplication is assuming a transfer or project move automatically removes prior access, which occurs when joiner-mover-leaver workflows do not include systematic entitlement cleanup.

Examples and Use Cases

Implementing controls against horizontal expansion rigorously often introduces friction for teams that need fast access changes, requiring organisations to weigh speed of delivery against entitlement discipline.

• A developer moves from one product squad to another and keeps write access to the former squad’s repositories, CI/CD pipelines, and internal admin tools.
• A service account originally tied to one application is reused across multiple environments, then accumulates API permissions that were never removed after migration.
• An AI agent is granted access to a new ticketing system and analytics platform while retaining older tool permissions that are no longer needed for its current workflow.
• A contractor transitions into a permanent role, but the temporary project access persists alongside the new role-based entitlements.
• During periodic access review, reviewers confirm the new access request but fail to question dormant legacy access that still spans other teams.

These patterns are often documented in broader NHI lifecycle guidance, including the Ultimate Guide to NHIs, where offboarding, rotation, and visibility are central themes. The same access sprawl can also appear in service-to-service architectures that rely on identity federation and shared credentials rather than scoped, purpose-built privileges.

Why It Matters in NHI Security

Horizontal expansion matters because it turns a legitimate access change into a broader blast radius. When identities retain legacy access across multiple systems, incident response becomes harder, privilege boundaries blur, and auditors struggle to explain why an identity can still reach teams it no longer supports. In NHI environments, this is especially dangerous because machine identities often operate continuously and at scale, so one overlooked entitlement can propagate across automation, data pipelines, and administrative workflows.

NHI Management Group research shows that 97% of NHIs carry excessive privileges, increasing unauthorised access and broadening the attack surface, a reminder that entitlement growth is already a systemic issue in many enterprises. The Ultimate Guide to NHIs also notes that only 5.7% of organisations have full visibility into their service accounts, which makes horizontal expansion difficult to detect before it is exploited. Practitioners should connect this term to access governance, entitlement hygiene, and lifecycle cleanup, not just IAM administration. Organisational teams typically encounter the damage only after a breach review or audit finding reveals that a moved user or agent still had cross-team access, at which point horizontal expansion becomes operationally unavoidable to address.

Related resources from NHI Mgmt Group

• What is the difference between secure collaboration and uncontrolled access expansion?
• When should organisations prioritise workload IAM over vault expansion?
• What should IAM leaders prioritise after a year of remote work expansion?
• What should identity teams ask before approving AI platform expansion?