API Penetration Testing to Address the Wide Range of API Vulnerabilities

List of Top 10 API Vulnerabilities

What is API?

Before going into the details of “API Penetration Testing,” let us first understand what API means and its significance. API stands for Application Programming Interface, API essentially facilitates communication between computers and more specifically between independent computer programs or software. Technically termed a “Software Interface,” API is important in software and application development. APIs play a significant role in the development of Mobile applications, web applications, and other computer programs. 

API Documentation and API Specifications

The word API is often used interchangeably with two related, but distinct attributes called API Documentation and API Specifications. API document is described as a standard reference manual for developers that outlines the rules of how to use an API. API document tells a developer how to use the API. The API specifications, on the other hand, are a resource that explains how a particular API behaves, how to link with different APIs and what results to expect from it. 

Unlike User Interfaces, APIs are not used directly by average humans [other than computer programmers], though they engage with APIs without being aware. The immense functionality of the APIs does come with a price  – SECURITY. This is where the topic under consideration in this article, “API Penetration Testing,” comes into the picture.

What is API Penetration Testing?

API Penetration Testing is a comprehensive methodology that tests the overall security features of the APIs. It is an ethical hacking process deliberately deployed to ascertain and evaluate the security aspects of the API. During API Penetration Testing, the possible vulnerabilities in the API are exploited by ethical hacking methods and reported to the developers. Developers then fix these issues strengthening the API further to prevent unauthorized access and data breach.

API Abuse

It is one of the most common threats to web applications. API Abuse is the wrong handling of the APIs with the intent of gaining unauthorized access, scraping critical business data, and launching DDoS attacks on the server. API abuse can severely disrupt the smooth functioning of your IT infrastructure. And hence API Penetration Testing is of paramount importance to avert the possible abuse of APIs.

API Penetration Testing is a very complex task that calls for a sound understanding of various tools needed to check the vulnerabilities in the APIs. Here are a few tools that cyber security service providers employ.

Burp Suite – Offers scalable automated scanning as well as manual testing. Used by cyber security experts for manual penetration testing. Rated 4.3 out of 5 by Peerspot.com.

Postman – Tests HTTP requests with the help of a user-friendly graphical UI. Cybersecurity experts use it to obtain different responses and then validate them. Postman API testing tool Allows you to create different testing environments and is fast too.

ZAP Proxy – This API testing tool understands API formats like JSON & XML and is used by cyber security experts to scan APIs. Peerspot.com has rated OWASP ZAP at 7.2 out of 10.

Top 10 API Vulnerabilities

Top 10 API Vulnerabilities

API Vulnerability 1 – Excessive Data Exposure

This situation arises when the API returns sensitive data through techniques like API Response and Request calls. An API abuse can manifest when developers and programmers might inadvertently expose objects, disregarding individual security. This data may contain personally identifiable information like email ids, phone nos., etc. The best way to prevent API abuse in this fashion is to never rely on the client to filter sensitive data.

API Vulnerability 2 – Security Misconfiguration

This scenario consists of various parameters such as misconfigured HTTP headers, incomplete ad-hoc configurations, insecure APIs, open cloud storage, insecure default configuration, error messages showing sensitive information, etc. The abuse of this API vulnerability can be avoided by adopting the best practice of following an API life-cycle that repeatedly hardens the process, resulting in the fast and smooth deployment of a properly locked environment.

API Vulnerability 3 – SQL Injection [SQLi]

The method is a form of code injection used by hackers to target data-driven applications. SQL injections, Command injections, and NoSQL injections exploit the security weaknesses in the applications & programs. These cyber attack methods send data from unknown sources to an interpreter through a query or command. Injection vulnerabilities are common in SQL, LDAP (Lightweight Directory Access Protocol), NoSQL queries, OS commands, Object-Relational Mapping (ORM), and XML parsers. These vulnerabilities can be easily identified when a programmer scrutinizes the source code.

Attackers often deploy scanning & fuzzing techniques to exploit the vulnerabilities in the applications and programs to gain unauthorized access to sensitive information. The key to avoiding SQL injection is to keep the data distinct from commands and queries. Maintaining a single, actively maintained library for data validation is also recommended.

API Vulnerability 4 – Broken Object Level Authorization

Here the APIs have a propensity of exposing the endpoints that handle object identifiers, which can result in a wide attack surface Level Access Control issue. To avoid this scenario, developers must inspect all the functions that access a data source with user input. Put in place a fail-proof authorization mechanism that takes into account the hierarchy & user policies.

API Vulnerability 5 – Broken User Authentication

This issue is related to an incorrectly implemented authentication mechanism. This weakness allows attackers to compromise authentication tokens or exploit the flaw in implementation by assuming other users’ identities – temporarily or permanently. This issue compromises the system’s ability to identify the client/user, and overall API security. The API abuses arising from this vulnerability can be avoided by checking the flows to authenticate the APIs. Developers must know all the flows to authenticate to the API.

API Vulnerability 6 – Lack of Resources & Rate Limiting

It is time and again observed that APIs fail to impose a cap on the quantity or size of the resources a client/user can request. The lack of such restriction has an impact on the performance of the API server, which can lead to a Denial of Service (DoS) situation. These unchecked restrictions can also inadvertently facilitate attack methods such as brute force, which takes advantage of the authentication weakness. To avoid the API abuse with this technique, it is suggested to use Dockers to restrict the number of processes, restarts, CPU, memory, and file descriptors.

API Vulnerability 7 – Broken Function Level Authorization

A big organization has several levels of hierarchy that can be categorized into several groups with specific roles and functions. These types of authorization flaws become apparent when there is no clear distinction between regular & administrative functions. It is recommended that the application have an authorization module that is easy to analyze and is invoked from all the business functions.

API Vulnerability 8 – Mass Assignment

Mass Assignment vulnerability occurs when there is no proper filtering based on the allowlist of the data [like JSON] provided by the client, bound with data models. In this vulnerability, the attackers modify the object’s properties, which they are not authorized to do. They employ methods such as guessing the object properties, reading the document, investigating endpoints of other APIs, and exploiting the additional object properties provided in request payloads. To avoid the APIs being abused in this fashion it is recommended to prevent the practice of using functions that bind the client’s input to code variables or internal objects.

API Vulnerability 9 – Improper Assets Management

Compared to traditional web applications, APIs have a tendency of exposing more endpoints. And hence thorough documentation of APIs gains paramount importance. Proper host, API versioning & API inventory also help in preventing API abuse pertaining to deprecated API versions and also exposed debug endpoints. To prevent API abuse it is recommended to maintain an inventory of all API hosts. And also, thorough documentation of the essential features needs to be undertaken. Pay more attention to the API Environment. Assess which users have network access to the API host and the API version.

API Vulnerability 10 – Insufficient Logging & Monitoring

The harmful combination of lack of proper monitoring & logging and improper integration with incident response encourages hackers to attack newer systems persistently. The attackers can abuse the API vulnerability to extract data or destroy it. Study shows that the time taken to detect a data breach is more than 200 days, and in most cases, the data breach is detected by external parties and not by the internal monitoring processes. To prevent this type of API abuse, it is recommended to maintain a log of events that include input validation errors, incidents of denied access & failed authentication attempts. 


Saeel Relekar is a Certified Ethical Hacker and an Information Security Analyst at Suma Soft. Suma Soft is a market leader in Cyber Security Services and provides comprehensive security solutions to safeguard enterprises.