OAuth 2.0 and JWT Tokens: One-Stop Interview Guide

Security is a critical aspect of modern web applications. OAuth 2.0 and JWT (JSON Web Tokens) are widely used authentication and authorization mechanisms. This guide provides a detailed, interview-ready explanation of these concepts. 1. What is OAuth 2.0? OAuth 2.0 is an authorization framework that allows third-party applications to securely access user resources without exposing credentials. It enables secure delegated access through access tokens. Key Terminologies: Resource Owner: The user who owns the data. Client: The application requesting access. Authorization Server: Issues access tokens. Resource Server: The API or service that requires authentication. OAuth 2.0 Grant Types OAuth 2.0 provides different flows (grant types) for various use cases: 1️⃣ Authorization Code Flow (Most Secure) Use case: Web applications with a backend server. ✅ Steps: User logs in and authorizes the client. Client receives an authorization code. Client exchanges the code for an access token. Client uses the token to access resources. 2️⃣ Client Credentials Flow Use case: Machine-to-machine (M2M) communication. ✅ Steps: Client authenticates itself with the authorization server. Client receives an access token. Client uses the token to access resources. 3️⃣ Implicit Flow (Deprecated) Use case: Legacy single-page applications (SPA). ✅ Steps: Access token is directly returned in the URL. ⚠️ Not recommended due to security vulnerabilities (e.g., token leakage). 4️⃣ Resource Owner Password Credentials Flow Use case: Trusted applications (not recommended for third parties). ✅ Steps: User provides username and password directly to the client. Client exchanges credentials for an access token. ⚠️ Not recommended due to security risks (password exposure). 2. What is JWT (JSON Web Token)? JWT is a compact, self-contained token used for authentication and information exchange. It is digitally signed to ensure integrity and authenticity. Structure of JWT JWT consists of three parts: Header: Contains metadata (e.g., signing algorithm) { "alg": "HS256", "typ": "JWT" } Payload: Contains claims (user data, roles, expiration) { "sub": "1234567890", "name": "John Doe", "exp": 1710000000 } Signature: Ensures the token hasn’t been tampered with HMACSHA256( base64UrlEncode(header) + "." + base64UrlEncode(payload), secretKey ) JWT example: HEADER.PAYLOAD.SIGNATURE Types of JWTs: Access Tokens: Short-lived tokens for accessing resources. Refresh Tokens: Long-lived tokens used to obtain new access tokens. 3. OAuth 2.0 vs JWT Feature OAuth 2.0 JWT Purpose Authorization framework Token format Usage API authentication Stateless authentication Storage Can use JWT or opaque tokens Self-contained token Expiration Short-lived tokens Expiration embedded Signature Not mandatory Mandatory for integrity 4. Securing OAuth 2.0 and JWT

Mar 7, 2025 - 03:51

OAuth 2.0 and JWT Tokens: One-Stop Interview Guide

Security is a critical aspect of modern web applications. OAuth 2.0 and JWT (JSON Web Tokens) are widely used authentication and authorization mechanisms. This guide provides a detailed, interview-ready explanation of these concepts.

1. What is OAuth 2.0?

OAuth 2.0 is an authorization framework that allows third-party applications to securely access user resources without exposing credentials. It enables secure delegated access through access tokens.

Key Terminologies:

Resource Owner: The user who owns the data.
Client: The application requesting access.
Authorization Server: Issues access tokens.
Resource Server: The API or service that requires authentication.

OAuth 2.0 Grant Types

OAuth 2.0 provides different flows (grant types) for various use cases:

1️⃣ Authorization Code Flow (Most Secure)

Use case: Web applications with a backend server.

✅ Steps:

User logs in and authorizes the client.
Client receives an authorization code.
Client exchanges the code for an access token.
Client uses the token to access resources.

2️⃣ Client Credentials Flow

Use case: Machine-to-machine (M2M) communication.

✅ Steps:

Client authenticates itself with the authorization server.
Client receives an access token.
Client uses the token to access resources.

3️⃣ Implicit Flow (Deprecated)

Use case: Legacy single-page applications (SPA).

✅ Steps:

Access token is directly returned in the URL.

⚠️ Not recommended due to security vulnerabilities (e.g., token leakage).

4️⃣ Resource Owner Password Credentials Flow

Use case: Trusted applications (not recommended for third parties).

✅ Steps:

User provides username and password directly to the client.
Client exchanges credentials for an access token.

⚠️ Not recommended due to security risks (password exposure).

2. What is JWT (JSON Web Token)?

JWT is a compact, self-contained token used for authentication and information exchange. It is digitally signed to ensure integrity and authenticity.

Structure of JWT

JWT consists of three parts:

Header: Contains metadata (e.g., signing algorithm)

   {
     "alg": "HS256",
     "typ": "JWT"
   }

Payload: Contains claims (user data, roles, expiration)

   {
     "sub": "1234567890",
     "name": "John Doe",
     "exp": 1710000000
   }

Signature: Ensures the token hasn’t been tampered with

   HMACSHA256(
     base64UrlEncode(header) + "." + base64UrlEncode(payload),
     secretKey
   )

JWT example:

HEADER.PAYLOAD.SIGNATURE

Types of JWTs:

Access Tokens: Short-lived tokens for accessing resources.
Refresh Tokens: Long-lived tokens used to obtain new access tokens.

3. OAuth 2.0 vs JWT

Feature	OAuth 2.0	JWT
Purpose	Authorization framework	Token format
Usage	API authentication	Stateless authentication
Storage	Can use JWT or opaque tokens	Self-contained token
Expiration	Short-lived tokens	Expiration embedded
Signature	Not mandatory	Mandatory for integrity