REST API (Representational State Transfer API) is one of the most commonly used solutions for communication between systems in web and mobile applications. Due to its simplicity, scalability, and universality, REST API has become a standard in modern projects. Unfortunately, improper implementation can lead to serious privacy breaches and exposure of user data.
This article explains how such incidents occur and how they can be effectively prevented.
The most common reasons for data exposure through REST API are:
1. Lack of proper authentication and authorization
* One of the most common mistakes is allowing access to API resources without verifying the identity of the user. Errors in this area include:
+ Failing to require login for sensitive endpoints
+ Using simple API tokens without expiration dates
+ Inadequate access control at the resource level (lack of RBAC or ABAC)
* Result: An unauthorized person can gain access to other users’ data
2. Overly detailed responses from the API
* Sometimes, in response to REST API requests, entire user objects are returned, including:
+ Email addresses
+ Phone numbers
+ User IDs in the system
+ Roles and permissions
* For example, instead of returning only name and avatar_url, the system returns the entire user record.
* Result: Even public data can be used for social engineering or spamming attacks
3. Exposing user identifiers (enumeration)
* When REST API uses predictable identifiers (e.g., /users/1001, /users/1002), an attacker may try to “guess” IDs of other users.
* This type of attack is known as user enumeration – a significant threat, especially in systems without proper authentication.
* Solution: Instead of numeric IDs, use random UUIDs (e.g., /users/7f3a9d54-21b8…)
4. Lack of security headers
* Omitting crucial HTTP headers can lead to XSS and CSRF attacks. Examples of missing security features include:
+ Content-Security-Policy
+ X-Content-Type-Options
+ Access-Control-Allow-Origin (in CORS configuration)
* Result: Possible session takeover or execution of malicious code from an external site
5. Exposing configuration files
* A poorly configured server may inadvertently make public files like .env, config.php, or settings.json, containing:
+ API keys
+ Database access credentials
+ JWT secrets
* Result: Full access for the attacker to the production environment.
Good practices for securing REST API:
1. Strong authentication and authorization mechanisms
* Implement standards like OAuth 2.0, JWT, or OpenID Connect.
* Use RBAC (role-based access control).
* Regularly rotate tokens and sessions.
2. Minimizing data in responses
* Return only necessary information (data minimization).
* Mask sensitive data.
* Use intermediary layers (DTOs) to avoid exposing entire database models.
3. Securing user identifiers
* Replace numeric IDs with random UUIDs.
* Verify if a user has access to the resource before returning data.
4. Implementing security headers
* Add Content-Security-Policy, Strict-Transport-Security, Referrer-Policy, and other necessary headers.
* Configure CORS to allow only trusted domains to communicate with your API.
5. Protecting configuration files
* Do not store sensitive data in directories accessible from the browser.
* Use environment variables and dedicated secret managers (e.g., HashiCorp Vault, AWS Secrets Manager).
In conclusion, REST API security is not an afterthought – it’s a foundation for every e-commerce platform, portal, or mobile app that handles user data. To minimize risk:
* Ensure strong authentication and authorization.
* Minimize data in API responses.
* Secure identifiers and configuration files.
* Regularly perform penetration testing and security audits.
Secure API = Secure Users = Trustworthy System.
