Open APIs are interfaces that allow applications and systems to exchange data and actions in a structured way. In financial services, they support modern digital experiences and partner integration, but they also expand the identity and access surface that security and compliance teams must govern carefully.
What Open APIs Are in Security Terms
Open application programming interface expose structured programmatic interfaces so approved systems can exchange data, invoke actions, and integrate services. The security significance is not the API label itself, but the trust, access, and control boundary it creates.
For practitioners, an open API is a governed access surface: every exposed endpoint, object, action, and token flow becomes part of the control plane that must be authenticated, authorised, logged, and bounded.
Why Open APIs Expand the Security Surface
Open APIs make integration easier by allowing third parties, internal teams, and automated clients to consume services without manual intervention. That same convenience increases exposure because more actors, more credentials, and more machine-to-machine transactions must be trusted correctly.
In practice, the biggest security shifts are usually around authorisation scope, data exposure, and the reliability of upstream assumptions. A well-designed API can be safe to publish, but only when its business objects, actions, and response data are constrained to the minimum the caller should receive.
Common Control Areas for Open APIs
Open APIs are usually governed through layered controls rather than one gate. Authentication establishes who or what is calling, authorisation decides what each caller can do, and rate limiting, input validation, and schema enforcement reduce abuse and accidental breakage.
Good API security also depends on inventory and lifecycle discipline. Undocumented, orphaned, or version-sprawled endpoints often become the easiest path to data leakage or broken access control because they fall outside normal review and monitoring.
For API testing and verification, the most relevant control thinking often aligns with OWASP API Security Top 10, OWASP ASVS, and PCI DSS v4.0 where payment and financial-data interfaces are involved.
Open APIs in Financial Services and Compliance
In financial services, open APIs often support account aggregation, payments, onboarding, and partner ecosystems, so they sit close to regulated data and transaction flows. That makes governance more than a technical exercise, because an API exposure can create customer harm, fraud opportunity, or compliance findings if controls do not match the sensitivity of the underlying function.
Security teams therefore need to treat API design, third-party onboarding, consent, and access review as connected concerns. If the interface is open to external consumption, then the service owner must be able to explain who can call it, which data they can retrieve, and how misuse is detected and contained.
Risk and Threat Considerations
Open APIs are attractive to attackers because they often present high-value data and business actions through predictable, machine-readable paths. Weak object-level authorisation, excessive token scope, exposed test endpoints, and poor inventory management can turn a convenience layer into a direct abuse path.
Failure mechanism: Callers exploit gaps between authentication and authorisation, or abuse overly broad permissions, to read data, invoke restricted functions, or automate abuse at scale. Missing telemetry and inconsistent version control make these failures harder to see before they are exploited.
Impact: The result can be account takeover, data exposure, transaction abuse, partner compromise, fraud, or regulatory scrutiny. In the financial sector, the blast radius can extend beyond a single application if the same integration pattern is reused across multiple services.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
OWASP API Security Top 10 addresses the attack surface, OWASP ASVS sets the technical controls, and PCI DSS v4.0 defines the regulatory obligations.
| Framework | Control / Reference | Relevance |
|---|---|---|
| OWASP API Security Top 10 | API1 — Broken Object Level Authorization | Open APIs commonly fail when callers can access objects they should not. |
| API5 — Broken Function Level Authorization | Open APIs expose callable actions that must be restricted by role or scope. | |
| API9 — Improper Inventory Management | Open APIs need complete inventory and version control to avoid shadow exposure. | |
| Recommendation — Enforce object-level checks on every API request before returning data or actions. Verify function-level permissions for each API operation instead of trusting the caller context. Maintain a current API inventory and retire unused or forgotten endpoints promptly. | ||
| OWASP ASVS | V8 — Authorization | Open APIs require strong authorization checks around exposed data and business actions. |
| V10 — OAuth and OIDC | Open APIs frequently rely on delegated access and token-based authentication patterns. | |
| Recommendation — Apply explicit authorization checks for every API resource, object, and function. Use OAuth and OIDC correctly to scope and validate API access tokens. | ||
| PCI DSS v4.0 | 7.2 — Restrict access to system components and cardholder data by business need to know | Financial open APIs must restrict access to sensitive data and functions by business need. |
| 8.6 — Use of system and application accounts and related authentication factors | Open APIs often depend on application accounts and their authentication controls. | |
| Recommendation — Limit API access to the minimum data and functions required for the business purpose. Control API service accounts and their authentication factors tightly and review them regularly. | ||
Practitioner Guidance
Why practitioners should care: An open API is not just a connector, it is a policy boundary. The security question is whether the interface exposes only the data and actions that the calling system truly needs, and whether that decision stays correct as the API evolves.
What to watch for: Review whether every exposed endpoint has an owner, an authentication method, a documented purpose, and a current authorisation model. Legacy versions, partner exceptions, and “temporary” test routes are common places where control drift begins.
Practitioner takeaway: Treat open APIs as continuously governed products, not static technical endpoints, because their risk profile changes as integrations, permissions, and consumers change.
Related resources from NHI Mgmt Group
- Why do open banking APIs create more authorization risk than many other application interfaces?
- Static Application Security Testing
- Why do exposed admin interfaces create more risk than ordinary application pages?
- Who is accountable for fixing open redirect risk across application teams and authentication owners?
