Secure by Design
Applying OWASP Principles to Core Banking APIs
In modern banking, APIs are the new core. They connect institutions, fintechs, and customers — powering everything from instant payments to KYC checks and account aggregation. But with that connectivity comes exposure. A single misconfigured endpoint can compromise entire systems, leak customer data, or violate compliance frameworks like PSD2 or PCI DSS.
To prevent that, we don’t just test for security — we design for it. This is the foundation of the “Secure by Design” principle, guided by the OWASP (Open Web Application Security Project) standards.
Why Secure by Design Matters in Banking
In fintech, every API call carries sensitive information: account balances, transaction histories, personal identifiers. Unlike consumer web apps, these systems operate in regulated, zero-tolerance environments where a security breach equals both financial and reputational loss. Traditional “add-on” security — handled post-development — doesn’t work here.
Instead, security must be built in from the first line of code.
That’s what OWASP principles help achieve: practical, repeatable patterns that make APIs inherently secure, auditable, and compliant.
1. Authentication & Authorization: No Token, No Entry
Every request must be authenticated and authorized. In banking systems, this means adopting OAuth 2.0 and OpenID Connect (OIDC) as default. Avoid homegrown tokens or custom session handlers.
Example of enforcing token validation in Spring Boot:
1 @Configuration
2 @EnableWebSecurity
3 public class SecurityConfig {
4
5 @Bean
6 SecurityFilterChain filterChain(HttpSecurity http) throws Exception {
7 http
8 .authorizeHttpRequests(auth -> auth
9 .requestMatchers("/api/internal/**").hasRole("ADMIN")
10 .requestMatchers("/api/payments/**").authenticated()
11 .anyRequest().denyAll())
12 .oauth2ResourceServer(OAuth2ResourceServerConfigurer::jwt);
13 return http.build();
14 }
15 }
✔ OWASP Principle: Broken Authentication — mitigated through standard, auditable token flows.
✔ Banking Context: Mandatory for PSD2-compliant API access via third-party providers (TPPs).
2. Data Protection in Transit and at Rest
Every API must enforce TLS 1.2+ encryption. Period. But secure banking APIs also require payload-level encryption, especially when sensitive PII or transaction details pass between services.
Example: Encrypting payloads with AES before sending over HTTPS.
1 Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
2 cipher.init(Cipher.ENCRYPT_MODE, secretKey, new GCMParameterSpec(128, iv));
3 byte[] encrypted = cipher.doFinal(transactionJson.getBytes(StandardCharsets.UTF_8)); ✔ OWASP Principle: Sensitive Data Exposure
✔ Banking Context: PCI DSS mandates encryption of cardholder data both in transit and storage.
3. Input Validation and Sanitization
One of the top OWASP vulnerabilities is injection attacks — often through JSON or query parameters. APIs must validate every input explicitly, rejecting unknown or malformed fields.
Example:
1 @PostMapping("/transfer")
2 public ResponseEntity transfer(@Valid @RequestBody TransferRequest request) {
3 // Only predefined fields allowed
4 }
5 And in TransferRequest.java:
1 @NotNull
2 @Pattern(regexp = "\\d{10,16}")
3 private String accountNumber;
4 ✔ OWASP Principle: Injection
✔ Banking Context: Prevents malicious payloads from corrupting core financial logic or triggering fraud modules.
4. Rate Limiting and Throttling
In open banking, APIs are public-facing. That means exposure to brute-force and denial-of-service (DoS) attacks. Rate limiting protects both the system and regulatory uptime SLAs.
Example with Spring Cloud Gateway:
1 spring:
2 cloud:
3 gateway:
4 routes:
5 - id: payment-api
6 uri: http://payments-service
7 filters:
8 - name: RequestRateLimiter
9 args:
10 redis-rate-limiter.replenishRate: 50
11 redis-rate-limiter.burstCapacity: 100
12 ✔ OWASP Principle: Denial of Service
✔ Banking Context: Essential for maintaining SLA thresholds during peak load or attack attempts.
5. Logging, Monitoring, and Traceability
Security is not just prevention — it’s also detection and response. Every request and exception must be logged with context, without ever leaking sensitive data.
Example using Spring AOP for audit logging:
1 @AfterReturning(pointcut = "execution(* com.bank.api..*(..))", returning = "result")
2 public void logApiResponse(JoinPoint joinPoint, Object result) {
3 log.info("API Call: {} returned {}", joinPoint.getSignature().getName(), result);
4 }
5 Integrate these logs into ELK or Grafana Loki pipelines for pattern-based anomaly detection.
✔ OWASP Principle: Insufficient Logging and Monitoring
✔ Banking Context: Enables audit trails required by regulators.
6. Secure Configuration and Secrets Management
Hardcoding secrets is a rookie mistake with catastrophic potential. Banking APIs should store credentials only in vaulted services like AWS Secrets Manager, HashiCorp Vault, or Azure Key Vault.
Example configuration:
1 export DB_PASSWORD=$(aws secretsmanager get-secret-value --secret-id corebank-db-pass) ✔ OWASP Principle: Security Misconfiguration
✔ Banking Context: Compliant with ISO 27001 and PCI DSS Key Management Requirements.
From Secure Code to Secure Culture
Secure API design isn’t a one-time task — it’s a mindset that evolves with the system. At OceanoBe, our engineers embed OWASP checks directly into CI/CD pipelines, running automated vulnerability scans and enforcing code review gates before deployment.
1 stages:
2 - security_scan
3 - deploy
4
5 security_scan:
6 script:
7 - mvn verify -DskipTests
8 - owasp-dependency-check.sh --scan ./target
9 This continuous feedback loop ensures vulnerabilities are caught early — before they reach production.
Trust is built through transparency and security
By applying OWASP principles early in the design process, developers can create APIs that don’t just comply with regulations — they inspire confidence in partners, regulators, and users alike.
At OceanoBe, “Secure by Design” isn’t another headline. It’s how we write, test, and deliver code — line by line.
