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Best Practices for
REST API Testing

REST API testing is a prerequisite for the security and stability
of interconnected services.

Learn How to Secure Your REST APIs

There are many different ways to test REST APIs. To help you find the solution that is best for your project(s), we have summarized all relevant information about REST API testing for you to either download or check out below.

API Testing Guide

What to Expect on This Page

What Is a REST API?

REST (Representational State Transfer) is a highly popular web API type because it offers flexible, fast, and simple communication between RESTful web applications. Compared to other API formats, REST is by far the most used, as over 80% of public web APIs are RESTful. Although stateful REST APIs are theoretically compatible with any protocol or data format, they mostly communicate through HTTP, using JSON, XLT, HTML, XML, or simple text. Out of these data formats, JSON is the most common as it is compatible with most languages.

Their adaptability makes REST APIs especially useful for services that are growing in complexity. Thanks to their ability to process commands from multiple users and different data formats, REST APIs are highly popular in various industries, such as ecommerce or IoT. 

REST APIs use five HTTP methods to request a command:

GET: Retrieve a resource
POST: Create a new resource
PUT: Update an existing resource
PATCH: Modify an existing resource
DELETE: Delete an existing resource

Below, you can see an example of a POST request:

POST/WebGoat/register.mvc HTTP1.1
Connection:keep-alive
Content-Length:75
Cache-Control:max-age=0
Upgrade-Insecure-Requests:1
Content-Type:application/x-www-form-urlencoded
User-Agent:Mozilla/5.0(X11; Linux x86_64)AppleWebKit/537.36(KHTML, like
Gecko)Chrome/92.0.4515.159 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.0,image/avif,image/webp,image/
apng,*/*
;q=0.8,application/signed-exchange;v=b3;q=0.9

Accept-Encoding:gzip, deflate
Accept-Language:en-US,en;q=0.9

username=cifuzz&password=nnMjJa721x>&mathingPassword=nnMjJa72lx&agree=agree

REST vs SOAP

The main difference between REST and SOAP (Simple Object Access Protocol) is that to be RESTful, an API has to simply meet a specific set of characteristics. Meanwhile, SOAP is an actual protocol, built to enable applications to communicate across languages and platforms. REST APIs are generally seen as more flexible and faster than SOAP protocols. Although SOAP protocols slightly decrease the speed of web services, they provide several features such as improved security, atomicity, consistency isolation, and durability (ACID). SOAP interfaces can process multiple protocol types (HTTP, SMTP TCP, etc.). However, SOAP return messages are always sent in XML. Thus, while REST APIs enable flexible high-speed communication, SOAP web services are slightly slower, but offer more built-in functionality.

REST vs gRPC

gRPC (Remote Procedure Call) is a Google-developed open-source data interchange mechanism that uses the HTTP/2 protocol. gRPC APIs exchange data using the Protocol Buffers binary format (Protobuf), which imposes standards that developers must follow when creating or using gRPC web APIs. While REST APIs are mainly useful for microservice architectures and third-party apps, gRPC is often applied in IoT systems, browserless mobile apps, applications with multiplexed streams.

Types of REST API Tests

API tests are not only done for security, but also for other reasons such as performance, functionality, and stability. Which testing approach is the right one for your REST APIs, strongly depends on what you are trying to achieve. However, most modern testing tools can be used for more than one form of testing. Generally, REST API testing approaches, include:

Unit Testing: Testing the functionality of individual operations
Integration Testing: Testing the interaction between multiple software modules 
Functional Testing: Ensuring that REST APIs behave exactly as it should
Load Testing: Measuring how many calls REST APIs can handle
Reliability Testing: Ensuring that REST APIs produce consistent results and connections
Security Testing: Validating REST API encryption methods and access control

The Challenges of REST API Testing

Securing REST APIs is a challenging task, as they are highly complex: They are difficult to reach, produce countless parameter combinations, and constantly communicate with a vast number of other systems. Looking for security vulnerabilities in REST APIs manually is like looking for a needle in a haystack. To deal with the complexity of REST APIs, many dev teams are adopting automated testing methods. Below, you can find an overview of the top 6 challenges of REST API testing.

1. Securing REST API Parameter Combinations

As presented below, REST APIs consist of various different parameters such as the request methods, request URI and query parameter - just to name a few. These parameters can take up countless combinations that have to be tested. Specific parameter combinations can otherwise lead to erroneous program states.

Query Parameter

 

2. Validating REST API Parameters

Validating REST API parameters is highly challenging. If they are not validated properly, issues such as wrong string/data types and parameter data outside the predefined value range can come up.

3. Maintaining the Data Formatting Schema

The data formatting schema specifies how REST APIs handle responses and requests. The challenge in maintaining data formatting is that whenever new parameters are added, they have to be included in the schema.

4. Testing REST API Call Sequences

Testers need to ensure that REST API calls are called in the correct order to prevent errors. In REST APIs this is especially important since they are generally multithreaded.

5. REST API Testing Set-Up

Setting up automated testing cycles is the part of REST API testing that requires the most manual effort. Especially for large projects enterprise testing platforms will help you speed up the initial set-up dramatically.

6. Error Reporting for REST APIs

Especially with black-box testing tools, error reporting for REST APIs is tricky, as the amount of tested parameter combinations is unknown. The best way to monitor and report REST API tests is with coverage-guided testing approaches, as they can provide meaningful coverage and error reports.

Find out more about common REST API testing challenges.

Enhancing REST API Security Through Testing Automation

Security testing is a particularly important part of REST API testing, as the implications of an exploited security vulnerability are usually not limited to the functionality and usability of a program. Due to the complexity and connectivity of REST APIs, conducting effective security tests that can detect API endpoints and cover all relevant parameter combinations is a tough nut. Manual testing is often too time-consuming and tends to neglect edge cases and vulnerabilities that stem from the communication between services. 

To test microservice architectures with all their dependencies, it is considered a best practice to automate your testing efforts as much as possible. The main reasons why test automation is so beneficial for REST APIs are:

System Complexity: REST APIs and backend services are often integrated into a layered architecture, which makes it difficult to cover all relevant test cases. Automated testing solutions enable developers to deal with this complexity by identifying endpoints and testing relevant parameter combinations more efficiently than it would be possible with manual testing methods.

Missing GUI: Since REST APIs do not have a GUI, all REST API tests must be performed at the message level, making it even more difficult for developers to conduct manual tests. In many cases, it is easier for a tester to write a script that automates REST API tests than it would be to write them manually.

Structured Inputs/Outputs: REST APIs generally underlie highly standardized protocols that mainly process HTTP, JSON, and XML files. Therefore, they provide a fairly stable interface to the tested program. Since the structure of the inputs and outputs are partly predefined, automating REST API tests is usually a viable option. 

Automating your API tests will save you time and increase the functionality, reliability, and security of your application. So, automate your testing, if you can! But also, don’t avoid manual testing completely. Your team should always be able to run manual tests, to validate if the automated tests are still working, as they are supposed to. As always, you need to find the mix that fits your use case best.

How to Automate Security Testing for REST APIs

Due to their complex structure, automated testing is one of the most effective ways to ensure the security and stability of REST APIs. But not all automated testing approaches are equally effective. The fastest way to implement test automation would be with black-box testing tools, such as Burp or OWASP ZAP, potentially enhanced with some additional system tests.

Although these black-box approaches are somewhat automated, they leave plenty of room for improvement, as they still require testers to have prior knowledge about the system under test to be effective. Black-box tests are great to test a program from an attacker's perspective. They generate test inputs randomly, from static corpora, from OpenAPI imports or based on heuristics. However, such inputs often fail to reach complex vulnerabilities and edge cases, since they do not take code coverage into account. For example, a black-box testing tool would take the API request from above and try out countless different parameter settings in hopes of identifying a request that breaks something.

Automated white-box testing is far more effective at finding buggy REST API requests: Since they use information about the source code, white-box approaches can automatically exclude irrelevant parameter settings from the corpus. Through information about code coverage, they can find crashing REST API requests much faster and much more accurately. White-box automation also enables better reporting by providing code-coverage visibility. The advantages of this approach are especially useful to secure vast microservice environments that are connected through APIs, and projects that are expanding in size.

Fuzz Testing for REST APIs

The most efficient way to implement automated white-box testing is with feedback-based fuzzing. During feedback-based fuzz testing, code instrumentation is used to measure the test progress within individual microservices and APIs. Through this instrumentation, the fuzzer collects information about test inputs, which it then uses to create further inputs that traverse even more code paths. Modern fuzzers can be customized to scan for specific bug classes and to remove blockers. Since this technology can be integrated into any build system, it enables developers to continuously test their REST APIs for security vulnerabilities and stability issues. Click here to learn more about fuzz testing.


Show Me Some Code

In this recorded live coding session, I will demonstrate how to use fuzzing to automate your security testing for REST APIs, on an intentionally insecure web application (WebGoat) that is usually used for educational purposes.

Use Case: How to Automate Your REST API Testing in 5 Easy Steps
Click below to watch the full live coding session on YouTube:Watch Full Session

What Kind of Web Vulnerabilities Can You Find With Fuzzing

In the video, I am using the Code Intelligence Testing Platform, to build automated REST API testing cycles. The Code Intelligence platform, which is available as SaaS and On-Premise, enables runtime error detection, advanced REST and gRPC API scans, and OWASP vulnerability detection. Here, is an overview of some bug classes you can find with this fuzzing platform. Click here to see the full list. 

 

Category

Reference

Severity

 

Broken Access

OWASP A01:2021

critical

 

Cryptographic Failures

OWASP A02:2021

critical

 

Injections (XSS, SQL, …)

OWASP A03:2021

critical

 

Insecure Design

OWASP A04:2021

critical

 

Security Misconfiguration

OWASP A05:2021

critical

 

Vulnerable and Outdated Components

OWASP A06:2021

critical

 

Identification and Authentication Failures

OWASP A07:2021

critical

 

Software and Data Integrity Failures

OWASP A08:2021

critical

 

Security Logging and Monitoring Failures

OWASP A09:2021

critical

 

Server-Side Request Forgery

OWASP A10:2021

critical

 

Improper Neutralization of Input During Web Page Generation

CWE-79

high

 

Sensitive Cookie with Improper SameSite Attribute

CWE-1275

medium

 

Sensitive Cookie Without 'HttpOnly' Flag

CWE-1004

medium

 

Sensitive Cookie in HTTPS Session Without 'Secure' Attribute

CWE-614

medium

 

Insufficient Logging

CWE-778

medium

 

Logging of Excessive Data

CWE-779

medium

 

Exposure of Sensitive Information to an Unauthorized Actor

CWE-200

high

 

Generation of Error Message Containing Sensitive Information

CWE-209

high

 

Failed Assertion

CWE-617

low

 

Uncaught Exception

CWE-248, CWE216

low

  Logic Issues

CWE-840

low

  Infinite Loops

CWE-835

low

  Denial of Service (DoS) 

CWE-400

low

How to Get Started With REST API Testing

I hope you now have a broad overview of REST API testing and the different approaches to it. If you are curious about how fuzz testing can help you build more secure web apps, you can check out the step-by-step REST API testing guide I have created. Follow these instructions to make sure that your test inputs cover all REST API endpoints and parameter combinations that are relevant to the security of your applications.Thomas

About Daniel Teuchert

Daniel Teuchert is a Senior Security Engineer at Code Intelligence. As member of the customer success team, he supports dev teams to automate their security testing, and to implementing new fuzzing technologies. He holds a master's diploma in IT Security in addition to OSWE and OSCP offensive security certifications.
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