You've deployed FIDO2 passkeys. Your vendor promised interoperability. Your pilot users love not typing passwords. Then a contractor with a different device manufacturer can't authenticate, or your metadata service rejects a perfectly valid authenticator, or worse, someone finds a way to force a fallback to a weaker authentication method because your policy configuration assumed all authenticators would behave identically.
These failures don't happen because teams skip testing entirely. They happen because interoperability testing for passkeys is fundamentally different from testing a single authentication flow, and most teams don't realize what they're missing until production.
Why These Mistakes Keep Happening
Passkey deployments rely on interoperability across devices, browsers, identity providers, authenticators, and relying parties. This isn't a vendor integration problem you can solve with one API contract. It's a standards compliance issue where every participant must implement the same specification correctly, including edge cases your internal QA environment will never surface.
Most teams test the happy path: register a passkey on Chrome, authenticate on Chrome, and call it done. But FIDO2 certification exists because the unhappy paths, cross-platform authentication, metadata validation failures, unsupported credential types, are where security gaps and user lockouts occur.
The FIDO Alliance runs structured interoperability events because implementers need to test against other vendors' implementations, not just their own test harnesses. Your internal staging environment can't replicate what happens when your server meets an authenticator from a manufacturer you've never heard of, using a browser build that interprets the WebAuthn spec slightly differently.
Mistake 1: Skipping Conformance Self-Validation Before Integration Testing
Why it happens: Teams assume that if their passkey flow works in their environment, they're ready to test interoperability. They jump straight to testing against partner systems or production-like scenarios without validating that their implementation conforms to the FIDO2 specification.
The consequence: You waste integration testing time debugging failures that are actually your own non-conformance issues. Worse, you might pass integration tests with one vendor's stack and fail with another, creating a false sense of security. When you deploy, users with certain device-browser combinations get locked out, and you can't tell if it's your server, their authenticator, or the relying party logic.
The fix: Run conformance testing self-validation before any interoperability event. The FIDO Alliance requires this step for certification candidates, and you should require it internally even if you're not seeking certification. Conformance tests validate that your implementation correctly handles the FIDO2 specification's requirements, credential creation options, assertion responses, attestation formats, and extension processing. Use the Alliance's conformance tools or equivalent test suites that cover the specification's mandatory and optional features. Document which optional features you support, because "we support FIDO2" is meaningless without specifics.
Mistake 2: Testing Only Within Your Vendor Ecosystem
Why it happens: Your identity provider, authenticator vendor, and browser all come from partners with existing relationships. Testing within that ecosystem is frictionless, and it works. Expanding testing to other vendors requires coordination, access to hardware you don't own, and time your sprint schedule doesn't have.
The consequence: A product that works only inside one vendor's environment doesn't deliver the portability and phishing resistance that FIDO standards are intended to support. When your environment inevitably expands, new device types, BYOD policies, contractor access, M&A integration, you'll discover interoperability failures in production. Users will fall back to passwords or legacy MFA, defeating the entire point of deploying passkeys.
The fix: Test your server implementation against authenticators from at least three different manufacturers. Test your authenticator (if you're building one) against servers from vendors you don't partner with. Participate in FIDO Alliance interoperability events if you're building products; if you're deploying commercial products, require your vendors to show current certification and ask them which interoperability events they've passed. Don't accept "FIDO2 compatible" marketing language, ask for the certification listing in the Alliance's metadata service.
Mistake 3: Ignoring Metadata Validation and Attestation Handling
Why it happens: Metadata validation feels like bureaucratic overhead when your authenticators are already working. Teams focus on the user-facing authentication flow and treat attestation as a checkbox feature they'll "configure later." They don't realize that inconsistent metadata handling is a common failure mode in interoperability testing.
The consequence: Your relying party can't verify authenticator properties during registration and authentication. You can't enforce policies like "only FIPS 140-2 validated authenticators" or "reject authenticators with known vulnerabilities." Worse, you might accept attestations from authenticators that have been compromised or don't meet your security requirements. A failed metadata lookup can also cause user lockouts during registration, creating support tickets that blame "the passkey system" instead of your incomplete implementation.
The fix: Implement proper metadata service integration from day one. Use the FIDO Alliance metadata service to verify authenticator properties. Build policy logic that handles metadata lookup failures gracefully, decide whether to reject unknown authenticators or allow them with reduced trust. Test your metadata validation against authenticators that aren't in your metadata cache yet. Document your attestation policy: do you require direct attestation, accept self-attestation, or allow none? Each choice has security implications, and "we accept everything" is a policy decision, not a default state.
Mistake 4: Deploying Without Downgrade Attack Protection
Why it happens: Teams implement passkeys as an additional authentication option alongside existing methods. They assume users will naturally choose the strongest option and don't consider that an attacker might force a fallback to a weaker method. Policy configuration for authentication method priority is often an afterthought, not a security control.
The consequence: Proofpoint warned of downgrade attack risks for FIDO passkeys. An attacker who can't bypass passkey authentication might be able to force your system to fall back to SMS OTP, security questions, or password reset flows. Your phishing-resistant authentication becomes a decorative feature that attackers simply route around. You've deployed passkeys without actually improving your security posture against targeted attacks.
The fix: Configure authentication policies that prevent downgrade attacks. If a user has enrolled a passkey, require passkey authentication, don't offer password or SMS OTP as alternatives for normal login. Implement account recovery flows that don't undermine passkey protection (use backup passkeys, hardware tokens, or in-person verification instead of email reset links). Test your policy logic: can an attacker who controls the user's email force a fallback to a weaker method? Can they manipulate client hints or browser capabilities to trigger legacy authentication paths? Your policy configuration is as critical as your FIDO2 implementation.
Mistake 5: Treating Certification as a Launch Gate Instead of Continuous Validation
Why it happens: Teams view FIDO certification as a pre-launch milestone. Once certified, they assume their implementation remains compliant as they add features, update libraries, and integrate with new systems. They don't plan for regression testing or re-validation after changes.
The consequence: Your certified implementation drifts from the specification as you ship updates. A library upgrade changes attestation handling. A new feature adds credential management logic that doesn't properly handle edge cases. You're no longer interoperable with authenticators or servers that expect spec-compliant behavior, but you won't know until users report failures. The FIDO Alliance schedules multiple interoperability events per year because implementations need ongoing validation, not one-time certification.
The fix: Build regression testing into your deployment pipeline. Maintain a test suite that validates FIDO2 conformance after every change to authentication logic, credential management, or policy configuration. If you're a vendor, participate in multiple interoperability events per year, don't just certify once and assume you're done. If you're deploying commercial products, require your vendors to demonstrate ongoing certification and ask when they last participated in interoperability testing. Treat certification as a continuous validation process, not a checkbox.
Prevention Checklist
Before you deploy passkeys to production:
- Run conformance self-validation tests against your server, authenticator, or client implementation
- Test interoperability with at least three different vendor implementations you don't control
- Implement and test metadata service integration with proper attestation validation
- Document your attestation policy and supported FIDO2 extensions
- Configure authentication policies that prevent downgrade attacks
- Test account recovery flows to ensure they don't bypass passkey requirements
- Verify your vendors have current FIDO Alliance certification (check the metadata service listing)
- Build regression tests that validate conformance after code changes
- Plan for re-validation after major updates or library upgrades
- Test failure scenarios: unsupported authenticators, metadata lookup failures, cross-platform authentication
- Document which optional FIDO2 features you support and which you explicitly don't
Interoperability testing isn't optional overhead. It's the difference between deploying a passkey feature and deploying phishing-resistant authentication that actually works when your environment expands beyond your initial pilot users.
