Do I need to validate JWT tokens in asp.net core 2 REST API?

Question

I have the following code in my asp.net core REST API configuration:

services
    .AddAuthentication(options => { options.DefaultScheme = JwtBearerDefaults.AuthenticationScheme; })
    .AddJwtBearer(options =>
        {
            options.Authority = "https://login.microsoftonline.com/XXXTenantIDXXX";
            options.Audience = "XXXX clientId XXXX";
        });

services.AddMvc(o =>
    {
        var policy = new AuthorizationPolicyBuilder()
            .RequireAuthenticatedUser()
            .Build();
        o.Filters.Add(new AuthorizeFilter(policy));

It authenticates requests. It is working fine.

I am concerned and worried about jwt token forgery or jwt tokens that come from other AAD applications in the tenant.

I expect above code provides all the information to the asp.net core authentication to verify the jwt is valid and its audience is the right AAD application.

I wanted to confirm my expectation here and ask if I need to have additional logic (code) to verify the JWT token?

Answer 1

Yes, Asp.Net Core Middleware validates JWT Token. Make sure you are configuring JWT Bearer Options and token validation parameters in order for Asp.Net Core Middleware to validate it.

For example:

      services.AddAuthentication(auth =>
        {
            auth.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
            auth.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
        }).AddJwtBearer(options =>
        {               
            options.ClaimsIssuer = jwtAuthSettings.ValidIssuer;//Your issuer
            options.IncludeErrorDetails = true;
            options.RequireHttpsMetadata = true;
            options.SaveToken = true;
            options.Validate(JwtBearerDefaults.AuthenticationScheme);
            options.TokenValidationParameters = new TokenValidationParameters()
            {
                ClockSkew = TimeSpan.FromMinutes(30),
                ValidateIssuer = true,
                ValidateAudience = true,
                ValidateLifetime = true,
                ValidateIssuerSigningKey = true,
                ValidIssuer = jwtAuthSettings.ValidIssuer, //Your issuer
                ValidAudience = jwtAuthSettings.ValidAudience,//Your Audience
                IssuerSigningKey = jwtAuthSettings.SymmetricSecurityKey, //Your Key
                NameClaimType = ClaimTypes.NameIdentifier,
                RequireSignedTokens = true,
                RequireExpirationTime = true

            };
        });