Since application programming interfaces (APIs) power everything from mobile apps to backend systems, ensuring they are secure is critical. This article delves into the essential traits of secure APIs and outlines proven techniques for testing them effectively.
Why API security is critical
Application programming interfaces are the backbone of modern applications, connecting mobile apps, web interfaces, third-party integrations, and cloud services. But that comes with risks. Without proper security, an API becomes a vulnerable access point for malicious actors. Consequences can include:
- Data breaches: Inadequately protected APIs can expose sensitive user information, including personal details and financial data, making it vulnerable to theft and misuse.
- Escalation of privileges: Attackers can exploit vulnerabilities to impersonate users or gain access to administrative-level functions.
- Service disruption: Weak API security can be exploited to launch denial-of-service (DoS) attacks or trigger chain reactions that disrupt interconnected systems and services.
- Compliance violations: Unsecured APIs can lead to non-compliance with standards such as GDPR, HIPAA, or PCI-DSS, which can result in fines and legal consequences.
API security checklist
Although absolute security is unattainable, well-architected APIs significantly minimize potential threats.
Mandatory authentication
A secure API almost always requires authentication, even for read-only endpoints (unless a service is intentionally launched publicly). Any API that exposes sensitive data or core business logic without proper authorization mechanisms poses a serious security concern. Implementing authentication is essential to ensure that only verified users or systems can interact with the API’s features. Common authentication techniques include OAuth 2.0, JSON Web Tokens (JWT), and API keys configured with restricted access rights. Whenever tokens or keys are used, they need to be securely stored and managed.
Using HTTPS only
Transporting data via HTTPS secured with TLS (Transport Layer Security) is a standard baseline for protecting APIs and applications. Without it, sensitive information, potentially including user credentials, tokens, and logins, can be intercepted in transit and compromised. Where possible, APIs should use HSTS to enable HTTPS in all APIs and completely reject unencrypted requests to eliminate risks.
Rate limiting
This serves as a key safeguard against brute-force attempts, compromised credentials, and misuse of API functionality. Secure APIs should define request limits for each user or token and enforce exponential rejection or blocking policies when exceeded. This helps maintain service availability and protect against denial-of-service (DoS) attacks.
Maintaining audit logs
A secure API ecosystem maintains logs of authentication attempts, resource access, and other similar events. Audit logs are useful for identifying unusual activity and serve as a valuable resource during security incidents. For sensitive APIs, logs should be immutable, time-stamped, and centrally stored with appropriate access controls.
How to check API security
Even if a company follows secure coding practices to minimize the risk of vulnerabilities and uses runtime protection to reduce attack attempts, testing is the only way to verify that API security mechanisms actually work properly. It should be regular and multi-layered.
Using automated API security testing tools
By their nature, APIs provide unified access to backend systems and applications, making dynamic testing (DAST, black-box) the primary approach to checking security. To make testing an integral part of API and application development, it is a good practice to start by incorporating DAST tools into DevOps pipelines.
Modern DAST platforms like Invicti (formerly Netsparker) are capable of scanning APIs in real-time within active environments and can effectively emulate attacker behavior. The solution is compatible with OpenAPI/Swagger and other specifications, supports various authentication methods (including OAuth), and allows the use of custom headers, making it a strong fit for complex, large-scale enterprise systems.
Performing manual penetration testing
An experienced pentester can uncover issues such as improper authorization at the function level or privilege escalation paths that automated tools cannot detect. This is particularly critical for APIs, where the interaction of roles, permissions, and business logic across multiple systems can produce unexpected behaviors – sometimes exposing complex vulnerabilities that require human insight to identify and address.
Applying access control
Every API should have access control rules. Below is a list of important characteristics:
- Only authenticated users access secure APIs
- Sensitive actions are safeguarded against cross-site request forgery (CSRF) and replay-based exploits
- Users are restricted to performing only those operations that align with their assigned roles
- Access decisions are applied server-side, not client-side
Security assessment checklist
Transport Layer Security
- Using HTTPS on all APIs
- Turning off support for outdated TLS versions (anything prior to 1.2) enhances protocol-level security
- Using trusted cipher suites and valid certificates
- Enforcing HSTS (HTTP Strict Transport Security) helps block downgrade attacks by ensuring browsers only use secure connections
Data validation layers
- Server-side input sanitization (never trusting the client)
- Applying strict validation of content types (e.g., enforcing application/json when only JSON input is expected) helps prevent unexpected or malicious payloads
- Validating the format, length, and data type of parameters
- Applying schema validation (e.g., JSON Schema) where possible
Obfuscating error messages
- Avoiding detailed error messages that reveal internal structures (like database schema)
- Returning generic error codes (such as 401 Unauthorized or 403 Forbidden) without revealing internal endpoint logic helps prevent information leakage
- Monitoring for excessive logging or debug modes that may have been accidentally enabled in production
Conclusion
It is important to check API security through comprehensive testing. By starting with a secure architecture – incorporating strong authentication, encrypted transport, access restrictions, and audit logging – teams can further strengthen their defenses by integrating regular DAST scans and manual testing to greatly minimize the risk of breaches.
As a leader in application and API security, Invicti DAST combines API discovery, testing with issue confirmation, and vulnerability management into a single platform that integrates into development and AppSec workflows.
If you would like to receive a free temporary license to test the Invicti solution or a demo, please contact us via e-mail or in any other way convenient for you.
