How should security teams secure public APIs by default when multiple teams contribute to the same surface?

Security teams should assume every exposed API endpoint is part of the attack surface and must be protected by default. That means authenticating and authorising by default, using least privilege for roles, scanning source code and documentation early, and adding runtime and penetration testing. The goal is to reduce reliance on any single control and catch gaps before release.

Protect the API surface first, not just the endpoint list

When multiple teams contribute to the same API surface, the failure mode is usually inconsistent defaults, not a single missing control. Treat every exposed route as production attack surface, then standardise the baseline so authentication, authorisation, and logging happen by default across teams. That reduces the chance that one team’s shortcut becomes everyone’s exposure.

Default-secure API design also needs consistent review of how endpoints are described and consumed. If source code, documentation, and gateway policy drift apart, teams will ship assumptions that never get enforced at runtime. A useful baseline is to align design review, code review, and traffic enforcement so the same rule set governs build-time and live requests.

That approach is reinforced by the broader guidance in CISA Secure by Design, which emphasises secure defaults and reducing reliance on users or downstream teams to “turn security on” later. For API-specific abuse patterns, the OWASP API Security Top 10 is the right lens for broken authorisation, excessive resource exposure, and other common surface-level failures.

Make ownership and control inheritance explicit across teams

Shared API surfaces fail when nobody can answer who owns authentication policy, who approves scopes, who maintains schema changes, and who tests the live contract. The practical fix is to define control inheritance up front: platform teams may provide the gateway, but each product team still owns the security of its routes, data access, and action-level permissions.

Least privilege has to be applied at the API action level, not only at the application or team level. If one service or role can call every route because it is easier to manage, the shared surface becomes a privilege multiplier. Security teams should prefer narrow roles, explicit scopes, and separate permissions for read, write, admin, and bulk operations.

This is also where implementation guidance matters. The OWASP Cheat Sheet Series is useful for concrete control patterns around authentication, session handling, and input handling, while OWASP Web Security Testing Guide helps teams verify that those controls still work once multiple services, gateways, and clients are involved.

Build verification into release, then keep checking the live surface

For shared APIs, “secure by default” only holds if teams test before release and after deployment. Static checks in source and documentation catch missing auth paths early, but they do not prove runtime behaviour. Teams should combine code scanning, contract review, and penetration testing with runtime checks that confirm the gateway, service, and backend all enforce the same decision.

The most common blind spot is believing a central gateway is enough. If a backend endpoint can still be reached directly, or if a new route bypasses the policy layer, the shared surface is only protected where the control happens to exist. Runtime validation should therefore focus on auth bypass, broken object-level access, excessive data returned per request, and any route that behaves differently when called outside the intended path.

For teams that need a governance anchor, NIST Cybersecurity Framework 2.0 provides the broad control lifecycle for govern, identify, protect, detect, respond, and recover, which fits API programmes that need repeatable ownership and verification. If the surface includes identity-bearing credentials, the risk is amplified by long-lived keys and overprivileged integrations, a pattern captured in NHI Management Group’s Ultimate Guide to Non-Human Identities.

Risk and Threat Considerations

Shared API surfaces increase the odds of inconsistent authorisation, shadow endpoints, and accidental overexposure. Attackers do not need every team to fail, only one route, one privilege gap, or one stale test assumption that lets them reach data or actions meant to stay restricted.

Failure mechanism: A team adds a new endpoint, relaxes scopes for convenience, or ships a direct backend path that is not covered by the central policy layer. Over time, the surface fragments and the “default secure” posture becomes dependent on manual review.

Impact: The result can be broken object-level authorisation, excessive data access, privilege escalation across services, and a much larger blast radius if a client credential, token, or integration key is abused.

Standards & Framework Alignment

This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.

OWASP Agentic AI Top 10 and OWASP Non-Human Identity Top 10 address the attack and risk surface, while CIS Controls v8 and NIST CSF 2.0 set the governance and control requirements practitioners need to meet.

Framework	Control / Reference	Relevance
CIS Controls v8	6 — Access Control Management	Shared APIs need least-privilege scopes and consistent access decisions.
	16 — Application Software Security	The question centers on securing APIs through code review and testing before release.
	8 — Audit Log Management	Runtime validation depends on logging API actions and detecting bypass or misuse.
Recommendation — Apply CIS Control 6 to enforce narrow API permissions and review privileged routes. Apply CIS Control 16 to test API code, contracts, and releases before deployment. Apply CIS Control 8 to log API access and investigate anomalous requests quickly.
NIST CSF 2.0	PR.AC — Identity Management, Authentication and Access Control	API security by default depends on enforced authentication and authorisation.
	GV.OE — Organizational Context	Multiple contributing teams require clear ownership and security accountability for the surface.
	DE.CM — Continuous Monitoring	Runtime and penetration testing require ongoing monitoring of live API behavior.
Recommendation — Enforce PR.AC controls so every API request is authenticated and authorised by default. Assign clear API ownership so each team is accountable for its routes and permissions. Use DE.CM to monitor live API behavior and detect auth bypass or policy drift.
OWASP Agentic AI Top 10	A1 — Tool and Resource Access Control	Shared APIs need tightly bounded action and tool access where clients or automations call them.
Recommendation — Restrict API actions to the minimum tool or resource access required by each caller.
OWASP Non-Human Identity Top 10	NHI-02 — Authentication and Credential Security	API surfaces often rely on keys and tokens that must be protected and validated by default.
	NHI-03 — Authorization and Least Privilege	The question explicitly requires default authorisation and least privilege across shared APIs.
	NHI-05 — Secrets Exposure and Leakage	Source scanning and documentation review are meant to catch exposed keys and credentials early.
Recommendation — Protect API keys and tokens with strong authentication, rotation, and validation controls. Constrain each API identity to the minimum permissions needed for its routes and actions. Scan code and docs for exposed secrets before release and remove them immediately.

Practitioner Guidance

What to prioritise: Put route-level authorisation, ownership, and test coverage ahead of cosmetic API hardening. If a control only exists in one layer, treat that as a design gap, not a compensating control.

What to verify: Confirm that every new or changed endpoint has an owner, an auth decision, a minimum-privilege scope, and a negative test that proves unauthorised requests fail in production-like conditions.

Common mistake: Teams often secure the gateway and assume the backend inherits the same protection. In practice, you need evidence that the policy is enforced where the request is actually executed, not only where it first arrives.

Practitioner takeaway: For shared APIs, the right default is not “secure once at the edge,” but “secure everywhere the request can succeed,” with ownership and verification distributed as carefully as the code.