NestJS: Adding verification options to AuthGuard with JWT

When you look at the code of the AuthGuard, it seems like the options.callback function is the only possible customization.

I think instead of writing your own AuthGuard that supports scope checks, it is cleaner to have a ScopesGuard (or RolesGuard) with its own decorater like @Scopes('manage_server') instead. For this, you can just follow the RolesGuard example in the docs, which also just checks an attribute of the JWT payload under the user property in the request.

---

Essential steps

Create a @Scopes() decorator:

export const Scopes = (...scopes: string[]) => SetMetadata('scopes', scopes);

Create a ScopesGuard:

@Injectable()
export class ScopesGuard implements CanActivate {
  constructor(private readonly reflector: Reflector) {}

  canActivate(context: ExecutionContext): boolean {
    const scopes = this.reflector.get<string[]>('scopes', context.getHandler());
    if (!scopes) {
      return true;
    }
    const request = context.switchToHttp().getRequest();
    const user = request.user;
    const hasScope = () => user.scopes.some((scope) => scopes.includes(scope));
    return user && user.scopes && hasScope();
  }
}

Use the ScopesGuard as a global guard for all routes (returns true when no scopes are given):

@Module({
  providers: [
    {
      provide: APP_GUARD,
      useClass: ScopesGuard,
    },
  ],
})
export class ApplicationModule {}

And then use it on an endpoint:

@Get('protected')
@UseGuards(AuthGuard('jwt'))
@Scopes('manage_server')
async protected(): Promise<string> {
    return 'Hello Protected World';
}

I tried a slightly different approach, by extending the AuthGuard guard. I wanted to maintain the ability to use different Passport Strategies, so I included a mixin. Feedback is appreciated.

In your Jwt strategy you could simply return the JwtPaylozd so that the user has a scopes attribute. Then the custom AuthGuard looks like this:

import { UnauthorizedException, mixin } from "@nestjs/common";
import { AuthGuard } from "@nestjs/passport";

export function AuthScopes(scopes: string[], type?: string | string[]) {
    return mixin(class ScopesAuth extends AuthGuard(type) {
        protected readonly scopes = scopes;
        handleRequest(err, user, info, context) {
        if (err || !user) {
            throw err || new UnauthorizedException();
        }

        if(!this.scopes.some(s => user.scopes.split(' ').includes(s)))
        {
            throw new UnauthorizedException(`JWT does not possess one of the required scopes (${this.scopes.join(',')})`);
        }
        return user;
        }
    });
  }

You can then use this guard like so:

@Get('protected')
@UseGuards(AuthScopes(['secret:read'], 'jwt'))
async protected(): Promise<string> {
    return 'Hello Protected World';
}

'jwt' represents the strategy.