The OWASP API Security Top 10 (2023): A Practical, Actionable Guide for Security Teams
APIs are the backbone of modern software—powering web applications, mobile apps, microservices, and backend integrations. But as organizations scale API adoption, the attack surface expands with it. The OWASP API Security Top 10 (2023) remains the leading framework to understand the most critical API security risks and the defensive priorities that engineering, product, and security teams must align on.
This guide provides a clear, practitioner-focused breakdown of all 10 OWASP API risks—what they mean, how to defend against them, and how to validate controls safely in staging environments. It also includes a consolidated action plan with measurable, CI/CD-ready recommendations.
API1:2023 — Broken Object Level Authorization (BOLA)
What it is
APIs often expose object identifiers (IDs) directly. If the server does not strictly validate ownership or entitlements, an attacker can manipulate IDs to access unauthorized objects—a common and highly exploitable vulnerability.
Defensive Focus
- Enforce server-side ownership checks for every object access path.
- Keep entitlement logic centralized and consistent across services.
- Log all failed authorization attempts for anomaly detection.
Safe Testing Hint
Use two valid test accounts and verify both cannot access or modify each other’s objects.
Action
Integrate object-level authorization checks into code reviews, schema design, and automated tests.
API2:2023 — Broken Authentication
What it is
Weak, incorrect, or incomplete authentication enables account compromise, token replay, or impersonation. This includes improper JWT validation (e.g., accepting alg:none, ignoring invalid signatures, missing claims).
Defensive Focus
- Use short-lived access tokens with rotation.
- Validate all token claims: issuer, audience, expiry, signature, and key ID.
- Prefer asymmetric signing (RS256/ES256).
- Enforce MFA where appropriate.
Safe Testing Hint
Ensure tokens with invalid signatures, expired timestamps, or missing claims are rejected with proper error handling.
Action
Audit your authentication and token-handling configuration; enable centralized revocation and rotation.
API3:2023 — Broken Object Property Level Authorization (BOPLA)
What it is
Clients can read or modify object fields (properties) they should not have access to—such as privilege flags, internal identifiers, or system-controlled fields.
Defensive Focus
- Implement field-level allowlists based on role and context.
- Enforce strict schema validation on input and output.
- Prevent mass assignment vulnerabilities by avoiding blind object binding.
Safe Testing Hint
Verify that protected fields, when sent by a client, are ignored or rejected and never affect server-side state.
Action
Define allowlists/schema contracts for every API update endpoint.
API4:2023 — Unrestricted Resource Consumption
What it is
APIs without resource limits are vulnerable to performance degradation or denial-of-service. Attackers may exploit unlimited payload size, concurrency, or expensive queries.
Defensive Focus
- Rate limiting and quotas.
- Timeout enforcement.
- Payload size caps (≤ 10 MB for public APIs is typical).
- Query cost/depth controls for GraphQL or complex queries.
Safe Testing Hint
Run controlled stress tests to verify rate limits, timeouts, and maximum payload sizes behave as configured.
Action
Apply gateway-level and application-level controls for rate limiting, payload caps, and concurrency.
API5:2023 — Broken Function Level Authorization
What it is
Privileged endpoints (e.g., admin operations) are exposed to users without proper authorization checks.
Defensive Focus
- Maintain a comprehensive endpoint inventory mapped to required privileges.
- Use centralized policy engines (OPA, Cerbos, Casbin).
- Log and alert on attempted access to privileged functions.
Safe Testing Hint
Confirm that non-admin users receive 403s when calling admin-scoped endpoints and no state change occurs.
Action
Integrate function-level authorization policies into your CI/CD pipeline.
API6:2023 — Unrestricted Access to Sensitive Business Flows
What it is
Critical workflows such as payments, password resets, registration, or ticketing can be abused at scale if they lack friction controls such as rate limiting, anti-automation, or step-up authentication.
Defensive Focus
- Per-flow throttling (e.g., password reset limits).
- Behavioral bot detection.
- CAPTCHA or proof-of-work where applicable.
- Step-up authentication for high-risk operations.
Safe Testing Hint
Simulate scaled but legitimate traffic to ensure throttles and anti-automation mechanisms activate correctly.
Action
Implement flow-specific safeguards, not only endpoint-level protections.
API7:2023 — Server-Side Request Forgery (SSRF)
What it is
APIs that fetch remote URLs using user input can be abused to reach internal systems, cloud metadata endpoints, or private infrastructure.
Defensive Focus
- Block private IP ranges (RFC 1918/4193) and cloud metadata endpoints.
- Maintain outbound allowlists.
- Isolate egress points.
- Disable open redirects.
Safe Testing Hint
Ensure all outbound requests go only to approved domains and internal addresses trigger explicit blocks.
Action
Enforce outbound allowlists at both application and network layers.
API8:2023 — Security Misconfiguration
What it is
Insecure defaults, verbose error messages, misconfigured CORS, and exposed admin tools increase attack surface.
Defensive Focus
- Disable debug/verbose logs in production.
- Restrict CORS to known origins.
- Apply security headers (HSTS, CSP where appropriate).
- Use automated configuration scanning tools.
Safe Testing Hint
Verify that debug flags are off, stack traces aren’t exposed, and response headers meet baseline security standards.
Action
Adopt hardened configuration baselines and automated scanning in CI.
API9:2023 — Improper Inventory Management
What it is
Shadow APIs, untracked endpoints, deprecated versions, or forgotten services remain exposed without monitoring or maintenance.
Defensive Focus
- Maintain a centralized API inventory with runtime discovery.
- Assign clear ownership for every service.
- Define versioning and sunset policies.
Safe Testing Hint
Compare static API inventories with actual runtime traffic to identify undocumented endpoints.
Action
Reconcile your API inventory monthly and decommission legacy endpoints systematically.
API10:2023 — Unsafe Consumption of APIs
What it is
Trusting input from third-party APIs or upstream systems without validation can introduce injection, cross-site scripting, or data integrity issues.
Defensive Focus
- Validate and sanitize all external data.
- Enforce strict content-type and schema validation.
- Use output encoding for client-rendered content.
- Treat upstream inputs as untrusted, even from partners.
Safe Testing Hint
Inject controlled malformed or edge-case responses from upstream services in staging to validate sanitization.
Action
Build a robust validation framework for all external and third-party data flows.
The Complete OWASP API Security Action Plan (2023)
API Code | Risk | Priority | Action (Concise & Measurable) |
API1:2023 | BOLA | High | Add server-side ownership checks; enforce in CI/CD tests |
API2:2023 | Broken Authentication | High | Reject alg:none; enforce RS256; enable rotation & revocation |
API3:2023 | BOPLA | High | Apply field allowlists + strict schema validation |
API4:2023 | Resource Consumption | High | Rate limiting (≤100 req/min/user), ≤10 MB payload caps, timeouts |
API5:2023 | Function-Level Authorization | High | Map admin endpoints; enforce via policy engine; block non-admin calls |
API6:2023 | Sensitive Business Flows | Medium | Add per-flow throttling + bot detection |
API7:2023 | SSRF | Medium | Block private IP ranges; outbound domain allowlists |
API8:2023 | Security Misconfiguration | Medium | Harden configs; enforce CSP/HSTS; disable debug |
API9:2023 | Inventory Management | Medium | Build runtime inventory; retire legacy endpoints |
API10:2023 | Unsafe Consumption | Low | Validate & sanitize 3rd-party data; enforce CSP; schema checks |
Priority Logic
High
Directly affects confidentiality, integrity, or authentication; typically easy to exploit.
Medium
Context-dependent or mitigated by layered defences but still material risks.
Low
Lower likelihood or impact but necessary for strong security hygiene.
Final Thoughts
The OWASP API Security Top 10 (2023) is more than a checklist—it is a strategic framework for aligning engineering, product, and security teams around the highest-impact API risks. Implementing these controls early in the SDLC and validating them continuously through CI/CD pipelines dramatically reduces organizational exposure.
Adopt a structured API security program, assign ownership, automate validation, and regularly review threat models. As API ecosystems continue to grow, disciplined governance and proactive defences will determine how resilient your digital platforms truly are.
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