The period when source and target identity environments are connected during migration. It is often operationally necessary, but it creates a bridge for access, trust, and attack paths unless tightly segmented, logged, and reviewed throughout the cutover.
Expanded Definition
A coexistence window is the controlled overlap between source and target identity environments during migration, when both sides must remain functional long enough to preserve availability while trust is being transferred. In NHI and IAM work, this period usually includes duplicated credentials, parallel policy enforcement, synchronized logging, and explicit cutover criteria. It is not simply a project phase; it is a security state that changes how access is issued, validated, and revoked.
Definitions vary across vendors on how broad the window should be, but the security requirement is consistent: the overlap should be as short as operationally possible and tightly segmented. That aligns with the risk-based discipline described in the NIST Cybersecurity Framework 2.0, where migration activity must not dilute visibility or control. The most common misapplication is treating the coexistence window like a simple cutover timeline, which occurs when teams leave old credentials trusted after new controls are live.
Examples and Use Cases
Implementing a coexistence window rigorously often introduces short-term complexity, requiring organisations to weigh continuity of service against duplicate trust paths, extra logging, and tighter operational coordination.
- During an IdP migration, service accounts authenticate against both environments until token issuance is fully switched and legacy tokens are retired.
- While moving machine-to-machine access into a new vault, secrets are mirrored temporarily so production jobs do not fail mid-cutover, then rotated after validation. The Ultimate Guide to NHIs is especially relevant here because it highlights how weak rotation and poor visibility amplify migration risk.
- In a merger, two IAM stacks may coexist while entitlements are reconciled and stale access is identified before decommissioning the legacy directory.
- When moving API authentication from static keys to short-lived credentials, both methods may be accepted briefly so integrations can be reconfigured safely, consistent with the monitoring emphasis in NIST Cybersecurity Framework 2.0.
NHIMG research shows why this window must be tightly governed: 97% of NHIs carry excessive privileges, and 80% of identity breaches involved compromised non-human identities such as service accounts and API keys. Those conditions make temporary dual-trust states especially dangerous if segmentation and revocation are not enforced from the first day of overlap. The Ultimate Guide to NHIs also notes that only 5.7% of organisations have full visibility into their service accounts, which means a migration can expose identities that were already poorly governed. Organisations typically encounter the operational cost of a coexistence window only after the first failed cutover or unexpected access incident, at which point the term becomes operationally unavoidable to address.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
OWASP Non-Human Identity Top 10 address the attack and risk surface, while NIST CSF 2.0 and NIST Zero Trust (SP 800-207) set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| OWASP Non-Human Identity Top 10 | NHI-02 | Migration overlap often exposes secret sprawl and weak revocation controls. |
| NIST CSF 2.0 | PR.AC-4 | Coexistence windows must preserve least privilege during parallel identity operations. |
| NIST Zero Trust (SP 800-207) | SC-7 | Zero Trust requires explicit trust boundaries even when source and target systems coexist. |
Maintain access segmentation and review entitlements continuously throughout migration overlap.
