What is API Security? Understanding and Preventing API Vulnerabilities
Learn how APIs work, why they are prime targets for attackers, and how to protect against common API vulnerabilities like BOLA, injection attacks, and broken authentication.
Application Programming Interfaces (APIs) have become the backbone of modern software, enabling everything from mobile apps to cloud services to communicate and share data. However, this connectivity comes with significant security risks. API vulnerabilities are now the leading cause of data breaches, with incidents like the recent 700Credit breach—which exposed 5.8 million records through an API flaw—demonstrating the devastating consequences of inadequate API security.
In this comprehensive guide, you will learn what APIs are, how they work, the most common API security vulnerabilities, and practical steps to protect your applications and data. Whether you are a developer, security professional, or simply want to understand how your personal information might be at risk, this guide will give you the knowledge you need.
What is an API?
An Application Programming Interface (API) is a set of rules and protocols that allows different software applications to communicate with each other. Think of an API as a waiter in a restaurant: you (the client application) tell the waiter (the API) what you want, the waiter takes your request to the kitchen (the server), and then brings back your food (the data or service response).
APIs are everywhere in modern technology. When you check the weather on your phone, the app uses an API to fetch data from a weather service. When you log into a website using your Google or Facebook account, APIs enable that authentication. When a dealership runs a credit check, they use APIs to communicate with credit reporting services like 700Credit.
Types of APIs
Understanding the different types of APIs helps contextualize their security challenges:
Why API Security Matters
APIs have become the primary attack surface for modern applications. According to security researchers, API attacks have increased by over 600% in recent years. There are several reasons why APIs are such attractive targets:
OWASP API Security Top 10
The Open Web Application Security Project (OWASP) maintains a list of the most critical API security risks. Understanding these vulnerabilities is essential for anyone involved in building, securing, or using API-connected services.
1. Broken Object Level Authorization (BOLA)
BOLA is the most common and dangerous API vulnerabilityVulnerability🛡️A weakness in software, hardware, or processes that can be exploited by attackers to gain unauthorized access or cause harm., ranking #1 on the OWASP API Security Top 10. This is exactly the type of vulnerability that enabled the 700Credit data breach.
BOLA occurs when an API fails to verify that the user making a request has permission to access the specific object (data record) being requested. In the 700Credit case, the API did not validate that consumer reference IDs belonged to the requesting party, allowing attackers to access any customer record by simply changing the ID number in their requests.
Example of vulnerable code:
// VULNERABLE: No authorization check
app.get("/api/customer/:id", (req, res) => {
const customer = database.getCustomer(req.params.id);
return res.json(customer); // Anyone can access any customer!
});Secure implementation:
// SECURE: Verify user has access to requested resource
app.get("/api/customer/:id", authenticate, (req, res) => {
const customer = database.getCustomer(req.params.id);
// Verify the authenticated user has permission
if (customer.dealerId !== req.user.dealerId) {
return res.status(403).json({ error: "Access denied" });
}
return res.json(customer);
});2. Broken Authentication
APIs with weak or improperly implemented authentication mechanisms allow attackers to impersonate legitimate users or bypass authentication entirely. Common issues include:
3. Injection Attacks
When APIs accept user input without proper validation and sanitization, attackers can inject malicious code. This includes SQL injection, NoSQL injection, command injectionCommand Injection🛡️A security vulnerability that allows attackers to execute arbitrary operating system commands on the host system through a vulnerable application., and LDAP injection. Proper input validation and parameterized queries are essential defenses.
4. Excessive Data Exposure
APIs often return more data than necessary, relying on the client application to filter out sensitive fields. Attackers who intercept API responses can access data that was never intended to be exposed. APIs should implement response filtering at the server level, returning only the minimum data required.
5. Lack of Rate Limiting
Without rate limiting, APIs are vulnerable to brute force attacks, denial of service, and mass data scraping. In the 700Credit breach, the attackers were able to exfiltrate 20% of customer data over five months—proper rate limiting and anomaly detection could have detected and stopped this activity much sooner.
API Security Best Practices
Protecting APIs requires a comprehensive approach that addresses authentication, authorization, data protection, and monitoring. Here are the essential practices every organization should implement:
Implement Strong Authentication
Use industry-standard authentication protocols like OAuthOAuth🛡️An open standard authorization protocol that allows applications to access user resources without exposing passwords, using tokens instead of credentials. 2.0 with OpenID Connect. Implement short-lived access tokens (15-60 minutes) with refresh tokenRefresh Token🛡️A long-lived credential used to obtain new access tokens without requiring the user to re-authenticate, enabling persistent application access. rotation. Never use API keys as the sole authentication method for sensitive operations. Consider mutual TLS (mTLS) for high-security scenarios.
Enforce Object-Level Authorization
Every API endpoint that accesses data must verify that the authenticated user has permission to access the specific resource being requested. This check should happen on every request, not just at login. Implement attribute-based access control (ABAC) or role-based access control (RBAC) consistently across all endpoints.
Validate All Input
Never trust client input. Validate data types, lengths, formats, and ranges for all input parameters. Use allowlists rather than blocklists when possible. Sanitize input to prevent injection attacks. Implement schema validation for request bodies.
Minimize Data Exposure
Return only the data fields required for each operation. Never expose sensitive data like Social Security Numbers unless absolutely necessary. Implement field-level encryptionEncryption🛡️The process of converting data into a coded format that can only be read with the correct decryption key. for highly sensitive data. Use data masking for logging and debugging.
Implement Rate Limiting and Throttling
Set appropriate rate limits based on expected usage patterns. Implement different limits for different operations (higher for reads, lower for writes). Use exponential backoff for rate limit violations. Monitor for unusual patterns that might indicate an attack.
Monitor and Log API Activity
Comprehensive logging enables detection of attacks and forensic analysis after incidents. Log all API requests including timestamps, user identifiers, endpoints accessed, and response codes. Implement real-time alerting for suspicious patterns such as unusual access volumes or failed authentication attempts.
API Security in Third-Party Integrations
The 700Credit breach originated through a compromised integration partner, highlighting the critical importance of securing API connections with third parties. When you expose APIs to partners or consume APIs from vendors, you extend your security perimeter.
Organizations should implement dedicated API gateways for partner connections, use separate credentials for each integration partner, monitor partner API usage patterns, and require immediate notification of security incidents affecting partner systems. Contractual requirements for security standards and breach notification are essential components of third-party API security.
API Security Testing
Regular security testing is essential for identifying vulnerabilities before attackers do. Key testing approaches include:
Key Takeaways
API security is not optional in today's connected world. As the 700Credit breach demonstrates, a single API vulnerability can expose millions of sensitive records. Remember these key points:
