API Security: Unknown Structure

TL;DR

Secure APIs with unknown structures by focusing on input validation, authentication/authorisation, rate limiting, and monitoring. Treat all data as untrusted, regardless of source (device or server). Implement a Web Application Firewall (WAF) and regularly scan for vulnerabilities.

Securing Unknown Structure APIs

1. Treat All Input as Untrusted: Regardless of whether the API request comes from an end-user device or your own server, assume all data is potentially malicious. Never trust client-provided data without validation.
   • Input Validation: This is critical. Define expected data types, lengths, formats, and ranges for every possible input parameter. Reject anything that doesn’t match.
   • Sanitisation: Remove or encode potentially harmful characters from inputs. Be careful with sanitisation – it’s often better to reject invalid input than try to fix it.
2. Authentication and Authorisation: Verify the identity of the caller (authentication) and ensure they have permission to access the requested resources (authorisation).
   • API Keys: Simple but less secure. Good for basic identification, but easily compromised.
   • OAuth 2.0/OpenID Connect: Industry standard for delegated authorisation. Allows users to grant limited access without sharing credentials.
   • JSON Web Tokens (JWT): A compact, URL-safe means of representing claims between parties. Use a strong signing algorithm (e.g., RS256).
3. Rate Limiting: Prevent abuse and denial-of-service attacks by limiting the number of requests from a single source within a given timeframe.
   • Implement at multiple levels: Consider rate limits per IP address, user account, and API key.
   • Example (using Nginx):

     limit_req_zone $binary_remote_addr zone=mylimit:10m rate=5r/s;
     server {
         ...
         location /api/ { 
             limit_req zone=mylimit burst=10 nodelay; 
             ... 
         }
     }

4. Web Application Firewall (WAF): Deploy a WAF to filter malicious traffic before it reaches your API.
   • Cloud-based WAFs: AWS WAF, Cloudflare, Azure Web Application Firewall. Easy to set up and maintain.
   • Open Source WAFs: ModSecurity (requires more configuration).
   • Configure rules: Protect against common attacks like SQL injection, cross-site scripting (XSS), and remote code execution (RCE).
5. Logging and Monitoring: Record all API requests and responses for auditing and security analysis.
   • Log relevant data: IP address, timestamp, user ID, requested resource, status code, request body.
   • Monitor for anomalies: Unusual traffic patterns, error rates, or suspicious activity.
   • Alerting: Set up alerts to notify you of potential security incidents.
6. Regular Security Scanning: Identify vulnerabilities in your API code and infrastructure.
   • Dynamic Application Security Testing (DAST): Scan the running API for vulnerabilities. Tools like OWASP ZAP, Burp Suite.
   • Static Application Security Testing (SAST): Analyse the source code for potential security flaws.
   • Penetration testing: Hire a professional to simulate real-world attacks against your API.
7. Data Encryption: Protect sensitive data in transit and at rest.
   • HTTPS/TLS: Use HTTPS for all API communication. Ensure you have a valid SSL/TLS certificate.
   • Encryption at Rest: Encrypt sensitive data stored in databases or other storage systems.