The Fundamental Misconception

Many people mistakenly compare OAuth 2.0 and JWT as if they were alternatives. This is incorrect. OAuth 2.0 is an authorization framework, while JWT (JSON Web Token) is a token format. They solve different problems and are frequently used together.

What OAuth 2.0 Is

OAuth 2.0 is a protocol that defines how a client application can obtain permission to access resources on behalf of a user. It specifies the flow of authorization, including:

• How a client requests authorization
• How an authorization server grants access tokens
• How a resource server validates tokens
• How tokens can be refreshed or revoked

What JWT Is

JWT is a compact, URL-safe token format defined in RFC 7519. It's a way to encode information (claims) in a structured format that can be digitally signed or encrypted. A JWT consists of three parts separated by dots:

• Header — Contains metadata about the token (algorithm, type)
• Payload — Contains the claims (user ID, permissions, expiration)
• Signature — Ensures the token hasn't been tampered with

How They Work Together

In most modern API implementations, OAuth 2.0 uses JWT as the format for access tokens. Here's how the flow typically works:

1. A client application requests authorization from the user
2. The user grants permission through an authorization server
3. The authorization server issues an access token (often in JWT format)
4. The client includes this token in API requests using the Bearer authentication scheme
5. The resource server validates the JWT and grants access to protected resources

Bearer Token Authentication

The term "Bearer JWT" refers to using a JWT token with the Bearer authentication scheme in HTTP requests. The Bearer scheme is defined in RFC 6750 and is part of the OAuth 2.0 specification.

When you see a request like this:

GET /api/users/123 HTTP/1.1
Host: api.example.com
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...

This means:

• The client is using the Bearer authentication scheme
• The token format is JWT (indicated by the token structure)
• The token was likely obtained through an OAuth 2.0 flow

Security Implications

From an API security perspective, understanding the difference between OAuth 2.0 and JWT is critical for several reasons:

Token Validation

When testing APIs, you need to understand what you're validating:

• OAuth 2.0 validation — Checks if the token was issued by a trusted authorization server, if it's still valid, and if it hasn't been revoked
• JWT validation — Verifies the signature, checks expiration claims, and validates the token structure

Many security vulnerabilities arise from incomplete validation. For example:

• Accepting unsigned JWTs (none algorithm)
• Not verifying token expiration
• Failing to check token revocation status
• Trusting client-provided claims without server-side verification

Token Storage and Transmission

JWTs are self-contained, meaning all necessary information is in the token itself. This has security implications:

• Advantage — No need to query a database for each request (stateless)
• Risk — If a JWT is stolen, it remains valid until expiration unless revocation is implemented
• Risk — JWTs can be large if they contain many claims
• Risk — Sensitive data in JWTs can be decoded (though not modified without the secret)

OAuth 2.0 Flow Security

The OAuth 2.0 framework itself has security considerations:

• Authorization Code Flow — Most secure for server-side applications
• Implicit Flow — Deprecated due to security concerns (tokens exposed in URLs)
• Client Credentials Flow — For machine-to-machine communication
• PKCE (Proof Key for Code Exchange) — Essential for public clients to prevent authorization code interception

Common Security Vulnerabilities

Understanding both OAuth 2.0 and JWT helps identify common security issues:

JWT-Specific Vulnerabilities

• Algorithm confusion attacks — Changing the algorithm from RS256 to HS256 to forge tokens
• Weak secrets — Using predictable or weak signing keys
• Missing expiration — Tokens that never expire
• Insecure token storage — Storing tokens in localStorage or cookies without proper flags
• Token leakage — Exposing tokens in logs, error messages, or URLs

OAuth 2.0-Specific Vulnerabilities

• Authorization code interception — Attacker intercepts the authorization code
• Token reuse — Using the same token across multiple sessions
• Insufficient scope validation — Not properly checking if a token has required permissions
• Redirect URI manipulation — Redirecting tokens to attacker-controlled endpoints
• Missing state parameter — Vulnerable to CSRF attacks

Testing APIs: What to Look For

When performing API security testing, examine both OAuth 2.0 implementation and JWT handling:

JWT Testing Checklist

• Verify token signature validation
• Test algorithm confusion attacks
• Check token expiration enforcement
• Attempt to modify claims and see if validation fails
• Look for sensitive data in token payloads
• Test token revocation mechanisms
• Verify that expired tokens are rejected

OAuth 2.0 Testing Checklist

• Verify proper authorization flow implementation
• Test for authorization code interception
• Check redirect URI validation
• Verify state parameter usage
• Test scope validation on protected endpoints
• Check token refresh mechanisms
• Verify token revocation works correctly
• Test for token reuse vulnerabilities

Best Practices

For secure API implementation, follow these practices:

JWT Best Practices

• Always use strong, unpredictable signing keys
• Set appropriate token expiration times
• Use RS256 (asymmetric) instead of HS256 (symmetric) when possible
• Never include sensitive data in JWT payloads
• Validate all claims server-side
• Implement token revocation mechanisms
• Use HTTPS for all token transmission

OAuth 2.0 Best Practices

• Use Authorization Code Flow with PKCE for all clients
• Validate redirect URIs strictly
• Always use the state parameter
• Implement proper scope validation
• Use short-lived access tokens with refresh tokens
• Implement token revocation
• Log all authorization attempts for security monitoring

Conclusion

OAuth 2.0 and Bearer JWT are not competitors—they are complementary technologies. OAuth 2.0 defines the authorization framework and flow, while JWT provides a secure, standardized token format. Understanding both is essential for:

• Building secure APIs
• Implementing proper authentication and authorization
• Identifying security vulnerabilities during testing
• Understanding how modern API security works

When testing APIs, don't just check if tokens are present—verify that both the OAuth 2.0 flow and JWT validation are implemented correctly. Many security breaches occur not because of flaws in OAuth 2.0 or JWT themselves, but because of improper implementation or missing security controls.

Remember: OAuth 2.0 is the "what" and "how" of authorization, while JWT is the "format" of the token. Both must be implemented securely for your API to be protected.