JWT Verification

Skill: jwt-verification

Purpose

Verify JWT tokens in backend services and enforce authenticated user context. This skill establishes patterns for token validation, claim extraction, and user context propagation throughout the backend request lifecycle.

When to Use

• When implementing JWT token validation in FastAPI
• When extracting user identity from JWT claims
• When designing authenticated endpoint middleware
• When establishing user context for request processing
• When handling token expiration and refresh scenarios
• When debugging authentication failures

When NOT to Use

• When working on frontend authentication (use better-auth-integration)
• When designing the overall auth flow (use jwt-authentication)
• When tokens haven't been issued yet
• When working on public/unauthenticated endpoints
• When designing authorization rules (use auth-boundary-design)

Responsibilities

• Validate JWT signature using appropriate secret/public key
• Verify token expiration (exp claim)
• Extract user identity from token claims (sub, email, etc.)
• Propagate user context through request lifecycle
• Handle invalid/expired token errors appropriately
• Configure JWT validation middleware for FastAPI
• Support token refresh when needed
• Log authentication events for security auditing

Inputs

• JWT tokens from Authorization header
• JWT secret or public key configuration
• Expected token claims and structure
• Token issuer configuration (Better Auth)
• Authentication requirements from specifications

Outputs

• Validated user context object
• Authentication middleware for FastAPI
• Error responses for invalid tokens (401 Unauthorized)
• User identity extraction patterns
• Token validation configuration

Constraints

• Never trust token claims without signature verification
• Never log full token contents (security risk)
• Never store tokens in backend (stateless verification)
• Never bypass verification for any authenticated endpoint
• Always verify token expiration
• Always use secure key management (environment variables)
• Always return 401 for invalid/expired tokens

Interaction With Other Skills

• jwt-authentication: Operates within broader authentication flow design
• better-auth-integration: Validates tokens issued by Better Auth
• fastapi-architecture: Integrates as middleware in FastAPI application
• auth-boundary-design: Provides verified user context for authorization
• python-backend-structure: Fits within backend code organization

Anti-Patterns

• Signature skip: Accepting tokens without signature verification
• Expiration ignore: Not checking token expiration claims
• Claim trust: Blindly trusting token claims without verification
• Token logging: Logging full tokens which exposes credentials
• Inconsistent handling: Different verification logic across endpoints
• Secret exposure: Hardcoding JWT secrets in source code
• Error leakage: Exposing verification failure details to attackers

Phase Applicability

Phase II only. Phase I has no authentication requirements.