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What is an API Call and How Does it Work?

Have you ever wondered how your weather app knows it’s going to rain tomorrow, or how your social media feed refreshes seamlessly with new posts?

The answer lies in a language understood by machines: API calls.

These digital messengers are the backbone of how different applications talk to each other, exchanging information and functionality behind the scenes.

But what exactly is an API call, and how does it work?

This blog post guides you through the fascinating world of APIs and their calls. We’ll break down the essential components, explore how they interact, and some real-time examples.

What is an API call?

An API call, short for Application Programming Interface call, is like a structured message sent by one software program to another. It’s like sending a request saying, “Hey, can you share that information you have, or do that special thing you can do?”

Simply put – think of it as ordering food at a restaurant. You (the program) tell the waiter (the API) what you want (data or functionality), and the waiter delivers it to you (the program) from the kitchen (the server).

Here’s the simple breakdown:

1. You (the program): You need information or want to act on another program.

2. API (the messenger): You send your request through the API, which acts like a messenger translating your request into a language the other program understands.

3. Other program (the friend): It receives your request, processes it, and sends back the information or performs the action you asked for.

4. API (the deliverer): It translates the response back into your program’s language and delivers it.

Components of an API Call

Here are the key components of an API call:

1. Endpoint:

  • This is the specific URL that the client sends the request to. It identifies the resource or functionality that the client is requesting.
  • For example, an API endpoint for getting a list of users might be /users.

2. Method:

  • The method specifies the type of operation that the client wants to perform on the resource.
  • Common HTTP methods used in REST APIs include:
    1. GET: To retrieve data
    2. POST: To create new data
    3. PUT/PATCH: To update existing data
    4. DELETE: To delete data
  • The method is specified in the first line of the request message

3. Headers:

  • Headers are key-value pairs that provide additional information about the request.
  • Some common headers include:
    1. Content-Type: Specifies the format of the data in the request body (e.g., JSON, XML)
    2. Authorization: Contains authentication credentials if the API requires them
  • Headers are also specified in the first line of the request message.

4. Request Body:

  • For some API operations, data needs to be sent in the request body. This is common for POST and PUT requests.
  • The data format (e.g., JSON or XML) is specified in the “Content-Type” header.

5. Response:

  • The server sends back a response message to the client.
  • The response contains the following components:
    1. Status code: A three-digit number that indicates the outcome of the request (e.g., 200 for success, 404 for not found
    2. Headers: Key-value pairs that provide additional information about the response.
    3. Body: The requested data or the result of the requested operation
  • Headers are also specified in the first line of the request message.

Here is an example of an API call to get a list of users:


					GET /users HTTP/1.1
Content-Type: application/json

  "limit": 10,
  "offset": 0


HTTP/1.1 200 OK
Content-Type: application/json

    "id": 1,
    "name": "John Doe",
    "email": ""
    "id": 2,
    "name": "Jane Doe",
    "email": ""


How do API Calls Work?

How do API Calls Work


1. User Initiates Request: The process starts with a user initiating an action in the client application. This action could be anything like searching for a product or booking a flight.

2. Client App Makes API Request: The client application then formulates an API request based on the user’s action. This request is sent to the API.

3. API receives and validates the request: The API receives the request and performs the following actions:

  • Checks the request’s validity: The API ensures the request is formatted correctly and includes all the necessary information.
  • Authenticates the request: The API verifies the identity of the client application making the request. This is often done using an API key or another authentication mechanism.
  • Authorizes the request: The API checks if the authenticated client application has permission to perform the requested action.

4. API sends a request to the database: If the request is valid and authorized, the API formulates a new request to the database. This request is specific to the database system and translates the original user action into a language the database understands.

5. Database processes the request and retrieves data: The database receives the request, processes it, and retrieves the relevant data. This data could be anything from product information to user details, depending on the original user action.

6. Database sends data back to the API: The database sends the retrieved data back to the API.

7. API processes and prepares the response: The API receives the data from the database and may perform some additional processing, such as formatting the data or applying any necessary transformations.

8. API sends the response to the client application: The API sends the processed data back to the client application in the form of an API response. The arrow labeled “Response with data” indicates this flow in the image.

9. Client application receives and processes the response: The client application receives the API response and extracts the relevant information from the data.

10. Client application displays the information to the user: Finally, the client application uses the extracted information to update its user interface and display the requested information to the user.

How API Calls Can Be Used for Attacks?

How API Calls Can Be Used for Attacks

While API calls fuel the smooth operation of our digital world, unfortunately, they can also be exploited for malicious purposes. Hackers and attackers can leverage the structure and functionality of API calls to launch various attacks, causing harm and disruption. Here’s how API calls can be weaponized:

Unauthorized Access (API Authentication Bypass):

If an API doesn’t have proper authentication mechanisms or if there are vulnerabilities in the authentication process, attackers may attempt to bypass authentication and gain unauthorized access to sensitive data or functionality.

Injection Attacks:

Similar to SQL injection in web applications, injection attacks in API calls involve inserting malicious code or payloads into API requests to manipulate data, execute arbitrary commands, or compromise the integrity of the system.

Man-in-the-Middle (MitM) Attacks:

Attackers may intercept and manipulate API calls between the client and server. This can lead to unauthorized access, data tampering, or eavesdropping on sensitive information being transmitted over the network.

Cross-Site Scripting (XSS):

Attackers inject malicious scripts into API responses, tricking users into executing them, potentially stealing their data or hijacking their sessions.

DoS and DDoS Attacks:

Attackers may flood an API with a large number of requests (either directly or through a botnet) to overwhelm the server, causing it to become unresponsive or unavailable. This can lead to a denial of service for legitimate users.

Security Misconfigurations:

Improperly configured API permissions, inadequate logging, or exposed sensitive information in error responses can be exploited by attackers to gather information or launch more sophisticated attacks.

How to secure APIs from invalid API calls?

How to secure APIs from invalid API calls

Securing APIs from invalid calls is crucial for protecting data, preventing attacks, and ensuring seamless functionality. Here are some key strategies to achieve this:

1. Input Validation:

  • Validate all data: Implement rigorous validation checks on every parameter received in an API call. Ensure data types, formats, and lengths adhere to defined standards. Reject invalid calls immediately with informative error messages.
  • Sanitize inputs: Sanitize all data to remove potentially harmful characters or code that could exploit vulnerabilities like SQL injection or cross-site scripting (XSS).

2. Authentication and Authorization:

  • Strong authentication: Implement robust authentication mechanisms like multi-factor authentication (MFA) to ensure only authorized users access the API.

  • Granular authorization: Define clear access control lists (ACLs) specifying which users or applications can access specific resources and actions within the API. Prevent unauthorized access to sensitive data or functionalities.

3. Rate Limiting:

  • Limit requests: Set limits on the number of API calls a user or application can make within a specific timeframe. This prevents brute-force attacks and DoS attempts.

  • Adaptive rate limiting: Implement adaptive rate limiting algorithms that adjust limits based on user behavior and risk profiles. This helps identify and limit suspicious activity while allowing legitimate users smooth access.

4. Input Data Sanitization:

  • Escape special characters: Escape special characters like quotes, apostrophes, and backslashes that could be used for injection attacks.
  • Validate data structure: Ensure data adheres to predefined formats and structures.

5. Error Handling and Logging:

  • Return informative error messages: Instead of generic error messages, provide specific details about the invalid call, helping developers debug and identify potential issues.
  • Log all API calls: Log all API calls, including details like timestamps, source IP addresses, and requested resources. This helps identify suspicious activity and troubleshoot issues.

6. Security Best Practices:

  • Keep APIs updated: Regularly update APIs with the latest security patches to address known vulnerabilities.
  • Use secure protocols: Utilize HTTPS for encrypted communication and prevent data sniffing or tampering.
  • Perform security audits: Conduct regular security audits and penetration testing to identify and address potential weaknesses in your API implementation.

7. API Gateway Protection:

  • Utilize API gateways: Implement API gateways as a single entry point for API calls. They can enforce security policies, perform additional validation, and route requests to appropriate services.
  • Implement API throttling: Implement throttling mechanisms within the API gateway to limit the rate of incoming requests and prevent DoS attacks.


APIs, the silent messengers behind our connected world, can be potent tools for good. They fuel innovation, streamline processes, and connect us seamlessly. But like any powerful tool, they demand responsible handling. Understanding how API calls work is crucial, but so is safeguarding them from malicious intent.

Ready to take your API security to the next level? CloudDefense.AI’s intuitive scanning solution empowers you to proactively identify and address vulnerabilities before they become threats. Book a free demo today and see how we can help you achieve bulletproof API security. Let’s keep the invisible hand working for good, together.

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Anshu Bansal
Anshu Bansal
Anshu Bansal, a Silicon Valley entrepreneur and venture capitalist, currently co-founds CloudDefense.AI, a cybersecurity solution with a mission to secure your business by rapidly identifying and removing critical risks in Applications and Infrastructure as Code. With a background in Amazon, Microsoft, and VMWare, they contributed to various software and security roles.
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