Api Testing

What is an api? An api is server side code or program that allows one softwar, program or application to communicate with another software, program or application. Usually the api code written in one language spits data in a specific format like json or xml and this data is used by another program or application.

Discovering Api Endpoints

• Burp will automatically find apis or we can crawl using burp.

• Sometimes api endpoints can be placed on separate subdomains eg: https://api-dev.example.com

• Try finding the api documentation via google dorks or some other techniques.

• Common api documentation endpoints /api, /swagger/index.html, /openapi.json

• IF you identify api resource eg: /api/swagger/v1/users/123 make sure to investigate /api/swagger/v1, /api/swagger, /api

• Also read javascript files for valid api endpoints.

• JS Link Finder can be a good tool to find api endpoints.

Testing api endpoints

• Identify supported HTTP Methods like GET, POST, PUT, DELETE, OPTIONS because a single api endpoint eg: /api/tasks can be used to GET list of tasks, POST tasks and DELETE tasks.

• Identify the content type

• Maybe in javascript files you will find apis that are never used by the application and we may be able to use them?

• We can fuzz for additional functons for example we have identified PUT /api/user/update now we can fuzz for other functions such as delete, add, etc.

• Maybe there exists mass assignment vulnerability where we identify one parameter in an api request and use same parameter in another request? For example in the following api endpoint /api/checkout the response was

{"chosen_discount":{"percentage":0},"chosen_products":[{"product_id":"1","name":"Lightweight \"l33t\" Leather Jacket","quantity":1,"item_price":133700}]} so what what if during the checkout /api/purchase in the response {"chosen_products":[{"product_id":"1","quantity":1}]}

• we add item_price to 000