An Application Programming Interface (API) is a set of protocols that allow software components to interact. The intermediary interface is commonly used for streamlining development by enabling software teams to reuse code. APIs also abstract functionality between systems by decoupling applications from the infrastructure they run on. Though APIs’ benefits and use cases in modern business continue to rise, inherent security challenges present various security risks.

This article delves into various risks associated with API vulnerabilities while learning common API security best practices to implement robust security mechanisms.

What is API Security?

An API represents a set of services that allow one program to communicate with another external or internal program. When we talk about API security, we typically refer to securing an application’s backend services, including its database, user management system, or other components interacting with the data store.

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API security encompasses the adoption of multiple tools and practices to protect the integrity of a tech stack. A robustly secured API covers both the APIs an organization uses and the services that use them. This includes preventing malicious actors from accessing sensitive information or taking actions on your behalf that you did not intend them to perform. Unfortunately, while APIs are a crucial part of modern applications, they are a common target of attackers to access sensitive information.

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It is crucial to understand how third-party applications funnel data through the interface when using APIs. Furthermore, with APIs increasingly becoming an attack vector, API security measures help security teams to assess security risks and have a comprehensive plan to protect them.

Risks of API Vulnerabilities.

As APIs are publicly accessible, they are common targets to steal sensitive information, including application logic, user credentials, credit card numbers, etc. In addition, vulnerabilities in an API endpoint are also exploited by malicious actors to gain unauthorized access to a system or network for other forms of attacks such as cross-site scripting and code injections. The Online Web Application Security Project (OWASP) issues risk-based recommendations on the top 10 vulnerabilities to secure web API. These include:

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• Broken User Authentication – Basic authentication presents a unique challenge in APIs since multi-factor authentication and credential-based logins are often considered impractical for API calls. As APIs rely on session tokens embedded into the calls to authenticate clients, APIs with insufficient authentication, such as faulty access token implementation, allow hackers to assume the identity of legitimate users. On the other hand, long-lived tokens also allow the attacker to persist, compromising the system indefinitely.
• Broken Object Level Authorization – In APIs, object-level authorization is a code-level control mechanism used to validate object access. For APIs with broken object-level authorization vulnerabilities, an external user can substitute the ID of their own resource with the ID of another user’s resource. This allows attackers to access the specified user’s resource, leading to unauthorized access of sensitive data.
• Lack of Resource and Rate Limiting – When the API does not limit the number and frequency of requests from a particular client, they could make numerous calls per second. The API client can also request access to multiple resources and records at once, overloading the application server to service multiple requests instantly. This can lead to Denial-of-Service attacks since a client making too many requests at once hinders the ability of the server to process requests. Lack of rate-limiting also encourages hackers to perform brute-force attacks on authentication endpoints.
• Mass Assignment – The mass assignment vulnerability occurs in APIs that automatically pipes user input to objects or program variables. While this feature simplifies code development, some users can initialize and over-write server-side variables, compromising the application. Attackers mostly exploit this by guessing and providing additional object properties when crafting requests. They can also read the application’s documentation or identify weak API endpoints that allow them to modify server-side objects.
• Security Misconfigurations – Multiple security misconfigurations pose a threat to APIs. These include:

• Verbose error messages – Some APIs send descriptive error messages containing stack traces and system information, keeping the user informed on how the application works under the hood.
• Misconfigured HTTP Headers expose security gaps that hackers can use to exfiltrate data and perform deeper sophisticated attacks. 
• Unnecessary HTTP methods and services – If administrators fail to close down unnecessary services, malicious attackers can modify posted resources using different HTTP methods. 
• Insecure default configurations – APIs connect with third-party dependencies, many of which are insecure by default and require an enhanced security posture to deal with a widened attack surface.

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API Security – Best Practices to Consider

The following web API security best practices can help mitigate API attacks:

Use throttling and rate-limiting

Throttling involves setting a temporary state that allows the API to evaluate every request and is often used as an anti-spam measure or to prevent abuse or denial-of-service attacks. There are two primary considerations when implementing the throttling feature: how much data should be allowed per user, and when should the limit be enforced?

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On the other hand, rate-limiting helps administer REST API security by avoiding DoS and Brute force attacks. In some APIs, developers set soft limits, which allow clients to exceed request limits for a brief duration. Setting timeouts is one of the most straightforward API security best practices as it can handle both synchronous and asynchronous requests. Request queue libraries enable the creation of APIs that accept a maximum number of requests then put the rest in a waiting queue. Each programming language comes with a queue library directory to implement request queues. 

Scan for API Vulnerabilities

Use HTTPS/TLS for REST APIs

Restrict HTTP methods

Implement sufficient input validation

Use an API gateway