Prioritise the applications that matter most to security and user friction, then use phased rollout and compatibility testing to isolate where legacy constraints are unavoidable. The goal is to prevent old systems from becoming permanent exceptions that preserve password dependence. If the legacy layer cannot be modernised immediately, govern its fallback tightly and separately.
How to triage legacy systems without letting them set the password policy for everything
When passkeys meet older platforms, the practical question is not whether every application can move at once. It is which systems carry the most security risk, which create the most user friction, and which can be isolated quickly so compatibility gaps do not become permanent exceptions. A good plan protects the modern path first, then contains the legacy path.
The strongest approach is usually to rank applications by business criticality, exposure, and sign-in pain. That ranking determines where phased rollout, testing, and exception handling should focus first, so the organisation reduces password dependence without blocking the entire programme on one brittle dependency.
A useful companion for teams planning that prioritisation is the Passwordless and Passkeys Guide, which sets out how passkeys and FIDO2 change the sign-in model and what a controlled rollout should look like. For workforce estates, the broader Workforce Identity Security Guide helps teams decide where sign-in hardening matters most.
Why legacy constraints should be treated as bounded exceptions
Legacy support gaps are manageable when they are treated as an implementation constraint, not as a reason to preserve password flows indefinitely. The risk is that a temporary fallback becomes a standing control, especially for high-friction or high-importance applications where teams defer change because the path of least resistance is familiar.
That is why separate governance matters. A legacy fallback should be narrowly scoped, visible, and revisited on a timetable, rather than blended into the standard authentication estate. Where the fallback is unavoidable, the control objective is to prevent it from quietly becoming the default route for users, admins, or sensitive transactions.
The Identity Security Programme Guide is useful here because it frames these exceptions as programme decisions with ownership, not just technical glitches. For teams also managing service and workload access, the Cloud Workload Identity Guide reinforces the same principle of reducing static credentials where the platform allows it.
What good rollout sequencing looks like when old systems lag behind
Passkey rollout should start where the organisation gets the most value from reduced phishing exposure and reduced help desk load. That usually means high-volume workforce applications, externally reachable systems, and any workflow where password resets, OTP fatigue, or account recovery already create measurable friction.
Then test compatibility by application family, browser, device class, and user group. If a legacy system fails passkey support, the team should decide whether the app can be modernised, fronted by a compatible sign-in layer, or held behind a tightly controlled fallback. The important judgment is to keep the exception list short enough that it can be owned and retired.
For procurement and vendor selection, the IAM and Identity Provider Buyer’s Guide is a practical reference for comparing rollout fit, while NIST SP 800-63 Digital Identity Guidelines is the clearest external reference for phishing-resistant authentication, authenticator assurance, and controlled recovery.
Risk and Threat Considerations
Legacy authentication often keeps passwords, recovery flows, or weak step-up paths alive longer than intended, which leaves the most exposed systems carrying the least modern protection. The main risk is not just that passkeys are unavailable, but that the organisation normalises weaker sign-in for the same users and applications that matter most.
Failure mechanism: Teams allow compatibility exceptions to spread, so the fallback path becomes the path of least resistance for users, admins, or support staff. That keeps password reuse, phishing, reset abuse, and account recovery exposure in play even after the modern sign-in path exists.
Impact: The result is a larger attack surface, weaker user experience, and a longer tail of legacy exceptions that are harder to retire. In practice, this can preserve the very credential dependence passkeys are meant to reduce.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
NIST SP 800-63, CIS Controls v8 and NIST SP 800-53 Rev 5 set the technical controls, while ISO/IEC 27001:2022 defines the regulatory obligations.
| Framework | Control / Reference | Relevance |
|---|---|---|
| NIST SP 800-63 | Digital Identity Guidelines | Passkeys and recovery flow decisions are governed by digital identity assurance guidance. |
| Recommendation — Apply phishing-resistant authentication and controlled recovery guidance to the highest-risk apps first. | ||
| CIS Controls v8 | CIS-5 — Account Management | Legacy fallback governance depends on tightly managing accounts and sign-in paths. |
| Recommendation — Restrict fallback accounts and review them on a fixed retirement schedule. | ||
| NIST SP 800-53 Rev 5 | IA-2 — Identification and Authentication (Organizational Users) | Workforce authentication choices and fallback handling map directly to user authentication control. |
| Recommendation — Enforce stronger user authentication on the applications that matter most. | ||
| ISO/IEC 27001:2022 | A.5.15 — Access control | Separate, bounded fallback access is an access-control decision for legacy systems. |
| A.8.5 — Secure authentication | Passkeys are a secure authentication measure replacing weaker legacy sign-in methods. | |
| Recommendation — Document and enforce distinct access rules for legacy authentication exceptions. Adopt secure authentication methods where application compatibility allows. | ||
Practitioner Guidance
What to prioritise: Put the highest-risk and highest-friction applications first, because those are the systems where passkeys deliver the most security gain and the fastest user benefit. If a low-value legacy app consumes disproportionate effort, hold it to a tighter fallback standard rather than letting it shape the broader programme.
Decision rule: If a system can support passkeys with reasonable change, modernise it early; if it cannot, isolate the fallback, time-box the exception, and require explicit ownership for retirement. Do not accept “legacy” as a permanent control design.
What to verify: Confirm that compatibility testing covers the actual user journeys, including recovery and help desk flows, not just primary sign-in. A passkey rollout that works in the lab but fails in account recovery is not operationally complete.
Practitioner takeaway: The goal is not universal instant support, it is to stop legacy limitations from becoming a durable reason to keep password-dependent access where the business needs stronger authentication most.
Related resources from NHI Mgmt Group
- Why do legacy systems make AI governance harder for IAM teams?
- Why do legacy PHP systems create security debt for IAM teams?
- How should security teams use DES when they still have to support legacy systems?
- How should security teams replace legacy IAM and IGA systems without disrupting access governance?
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Reviewed and updated by the NHIMG editorial team on October 6, 2026.
NHI Mgmt Group — the #1 independent authority on Non-Human Identity, IAM, and Agentic AI security. nhimg.org