REST (Representational State Transfer) has become the standard for building web APIs that allow different software systems to communicate and share data over HTTP. As REST APIs provide capabilities to access and manipulate remote resources and data, they can be vulnerable to security threats if not properly secured.
When implementing a REST API, developers need to consider various security principles and best practices to ensure the confidentiality, integrity, and availability of the API. Here are some of the key security aspects to consider:
Use HTTPS
Using HTTPS with SSL/TLS encryption should be the first security measure for any REST API. HTTPS provides bidirectional encryption between the client and server, preventing man-in-the-middle attacks where traffic can be intercepted and manipulated.
Encrypting traffic protects sensitive data such as authentication credentials, request parameters, and response payloads as they are transmitted over the network. It also provides integrity checks to prevent tampering with API requests/responses.
Authentication and Authorization
Proper authentication and authorization mechanisms need to be implemented to control access to the API. Some options for REST API authentication include:
- Basic authentication – Clients provide username and password with each request which is validated by the server
- Token-based authentication – Clients pass an access token with each request, which is verified by the server
- API keys – Unique API key is assigned to each client which identifies and authenticates them
- OAuth 2.0 – Allows limited access to the API without exposing user credentials
Once authenticated, proper authorization controls should be in place to limit client actions based on their privileges. For example, restricting delete operations to admin users only.
Input Validation
All input to the API including headers, path parameters, query strings and request bodies should be validated and sanitized. This prevents issues like SQL injections, cross-site scripting (XSS), denial of service (DoS) etc.
Validation should check for type, length, format, and range as applicable. Input sanitization like stripping malicious code from user input also helps strengthen security.
Rate Limiting
To prevent abuse and denial of service attacks, rate limiting should be implemented on the API. Limits can be applied on parameters like:
- Number of requests per IP address
- Number of requests per API key
- Number of concurrent connections
- Number of requests per time period (second, minute, hour)
Requests exceeding the limit can be throttled or rejected with appropriate error responses.
TLS Protocol Settings
It is recommended to disable old, insecure versions of TLS (like TLS 1.0 and TLS 1.1) and only allow connections over TLS 1.2 and above. Ciphers should also be limited to strong encryption algorithms like AES and ECC.
Certificate validation is also important, requiring client certificates or validating server certificates prevents man-in-the-middle attacks.
Minimize Attack Surface
The REST API should expose the minimum endpoints, data and functionality required externally. Any internal endpoints, debug information, headers etc. that are not absolutely necessary should be removed or disabled on production deployments.
Unused HTTP methods like PUT, DELETE etc. should be disabled if not required. The API framework itself should also be hardened by removing unnecessary modules, error handlers etc.
Logging and Monitoring
Extensive logging of API requests/responses, errors, performance metrics and security events provides visibility and is critical for detecting and investigating issues.
A monitoring system can track KPIs like API uptime, request rates/latency, error rates and set alerts as needed. Unauthorized requests can also be flagged in real-time.
API Keys and Secrets Management
API keys, tokens and other secrets used for authentication should be properly secured. Generating strong random keys, rotating them periodically, revoking compromised keys help improve security.
Secrets and keys should never be checked into source code repositories. A secrets management system provides secure storage and lifecycle management of API credentials.
API Definition and Documentation
Having a strict API definition that properly documents all the endpoints, request/response schemas, error codes etc. helps avoid ambiguity. It serves as a contract between the API provider and consumers.
The API definition should include security provisions like the supported authentication methods, required headers, rate limits etc. Good documentation is crucial for end users to securely integrate with the API.
Security Testing
Comprehensive testing of the API helps identify vulnerabilities or gaps before launch. Different types of security testing to consider for REST APIs include:
- Static Analysis – Review app config, source code, infrastructure as code etc. for security defects
- DAST – Detect vulnerabilities by actively attacking the running API
- SAST – Scan source code for security issues
- Pen Testing – Simulate attacks to exploit vulnerabilities in the API authorization, authentication, session management, encryption, business logic and input validation.
Continuous testing and scanning at regular intervals is recommended to detect issues early.
Deployment and Infrastructure Security
The infrastructure used to host the REST API also needs to be properly hardened. Measures like:
- Firewall policies restricting network access to API resources
- OS hardening to remove unused features/services, enable logging, disable unused users etc.
- Keeping servers/apps patched and updated
- Isolating API servers into DMZs or private subnets
- Restricting and auditing SSH/RDP access
- WAF/RASP to filter malicious traffic
- DDoS protection
aid in protecting the API server environment itself.
Role Based Access Control (RBAC)
Implementing RBAC with principles of least-privilege and separation of duties helps prevent unauthorized access and privilege escalation. Different teams like developers, testers, ops can be provided fine-grained access only to resources they need.
OWASP API Security Top 10
The OWASP API Security Top 10 provides a standard checklist of the most critical API vulnerabilities. Addressing the top risks highlighted here goes a long way in building a secure REST API:
| OWASP API Security Risks | Description |
|---|---|
| Broken Object Level Authorization | Improper authorization checks allowing access to other user’s data or system functions |
| Broken User Authentication | Flawed authentication mechanisms exposing passwords, keys or session tokens |
| Excessive Data Exposure | Overly detailed error messages or verbose debugging information |
| Lack of Resources & Rate Limiting | Allows enumeration attacks or DoS from excessive requests |
| Broken Function Level Authorization | Overly privileged roles and functions or incorrect access controls |
| Mass Assignment | API allows setting arbitrary object properties without validation |
| Security Misconfiguration | Insecure default settings, unnecessary HTTP methods/headers exposed |
| Injection | Lack of input validation leading to injection attacks |
| Improper Assets Management | Lack of binary signatures, weak crypto keys |
| Insufficient Logging & Monitoring | Inadequate event tracking and alerting |
API Security Guidelines
General API security best practices include:
- Encrypt traffic using HTTPS and TLS
- Authenticate and authorize API clients
- Validate, sanitize and limit incoming data
- Enable CORS carefully according to application needs
- Never expose internal error details
- Enforce rate limits and concurrent connection limits
- Log API requests, errors and audit trails
- Harden infrastructure and follow security hygiene practices
- Continuously monitor the API for anomalies
- Perform security tests like DAST, SAST regularly
- Version APIs and maintain backwards compatibility
- Document security provisions clearly for end users
API Security Architecture
A typical API security architecture consists of the following components:
- Firewall/WAF to filter allowed traffic to API endpoints
- API Gateway acting as single entry point to all microservices and implementing security controls
- Authentication service for validating API keys, JWT tokens, OAuth etc.
- Access control service for authorization and role/policy checks
- API management service for security, lifecycle management and developer portals
- Encryption mechanisms like SSL/TLS
- Security monitoring systems like SIEM
- High availability infrastructure to prevent downtime
These systems handle security at different layers and work together to provide end-to-end protection for the API stack.
Security Standards for REST APIs
Some standards and guidelines to incorporate for secure REST APIs include:
- OAuth 2.0 – For delegated authorization over HTTPS
- JSON Web Tokens (JWT) – Securely transmitting claims between parties as a JSON object
- OpenID Connect (OIDC) – Authentication protocol built on OAuth 2.0
- HTTPS TLS 1.2+ with PFS ciphers – Secure encrypted connections
- ISO 27001 – Information security standard
- NIST Cybersecurity Framework – Security best practices
- CSA STAR – Cloud security controls
- PCI DSS – Payment card industry data security
Conclusion
Securing REST APIs requires a layered defense with controls at the protocol, network, application, codes, and policies level. A combination of up-to-date standards, security testing, infrastructure hardening and continuous monitoring are necessary to manage risks. Adopting these API security principles and best practices will help organizations maintain robust protection for their REST APIs.
