Graphql Introspection in Buffalo with Bearer Tokens

GraphQL introspection is a feature that allows an attacker to query the schema of a GraphQL endpoint to understand available types, queries, and mutations. In the Buffalo framework, GraphQL endpoints are often exposed via standard HTTP routes, and if the introspection operation is not explicitly disabled, any authenticated or unauthenticated request can retrieve the full schema. When Bearer tokens are used for authorization, developers may assume that simply requiring a token is sufficient to protect introspection. However, if the token is accepted but introspection is still enabled, an attacker who obtains or guesses a valid Bearer token can still probe the API to map its capabilities, revealing sensitive data models and potential attack surfaces. This combination creates a scenario where authentication exists, but authorization controls are incomplete because introspection is not restricted based on token scopes or roles.

During a black-box scan, middleBrick tests GraphQL endpoints by sending introspection queries over HTTP, including scenarios where Bearer tokens are provided. If the endpoint responds with schema data even when the token lacks appropriate permissions, the scan flags this as BOLA/IDOR and Property Authorization risks. The presence of a Bearer token does not automatically imply that introspection should be allowed; tokens should be validated against a policy that explicitly denies introspection unless the caller has the required administrative privileges. Without such controls, attackers can leverage introspection results to craft more targeted injection, privilege escalation, or data exposure attempts, aligning with OWASP API Top 10:2023 A1 (Broken Object Level Authorization) and A7 (Identification and Authentication Failures).

Real-world examples show that GraphQL servers in Buffalo applications often expose introspection by default when using libraries such as graphql-ruby without additional configuration. If a Bearer token is passed in the Authorization header but the server does not differentiate between read-only and administrative tokens, the introspection query succeeds and returns fields like internal IDs, relationships, and business logic structures. This information can be combined with other findings, such as Input Validation weaknesses, to build exploit chains. middleBrick’s checks include sending introspection queries with valid Bearer tokens and analyzing whether the response contains schema details, which would indicate a misconfiguration in token-based access controls.

To secure GraphQL introspection in Buffalo when using Bearer tokens, you must explicitly disable introspection for non-administrative contexts and enforce scope-based checks within your route handlers. Below are concrete code examples demonstrating how to implement these controls.

Disabling Introspection in Production

When initializing your GraphQL schema, configure the introspection option to be environment-aware. In development, introspection may be useful, but it should be disabled in production unless explicitly required and protected.

# config/initializers/graphql.rb
if ENV["APP_ENV"] == "production"
  GraphQL::Schema.define do
    use GraphQL::Schema::Authorization
    introspection: false
  end
else
  GraphQL::Schema.define do
    use GraphQL::Schema::Authorization
    introspection: true
  end
end

Token-Based Scope Validation for Introspection

Implement a custom authorization check in your GraphQL context to verify that the Bearer token includes a specific scope (e.g., schema:read) before allowing introspection queries.

# app/graphql/my_schema.rb
class MySchema < GraphQL::Schema
  field :introspection_queries, [String], null: false, authorize: :admin_or_schema_reader

  def self.introspection_resolver(obj, args, ctx)
    unless ctx[:token_scopes]&.include?("schema:read")
      raise GraphQL::ExecutionError, "Not authorized to view schema"
    end
    # Return filtered schema data if needed
    { __schema: obj.__schema }.to_json
  end
end

Securing the HTTP Endpoint in Buffalo

In your Buffalo route definition, add middleware or a before action to validate the Bearer token and set scopes in the context before the GraphQL request is processed.

# bootstrap/api/app/controllers/api_controller.go
package controllers

import (
  "github.com/gobuffalo/buffalo"
  "net/http"
  "strings"
)

func RequireBearerToken(next buffalo.Handler) buffalo.Handler {
  return func(c buffalo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    if auth == "" || !strings.HasPrefix(auth, "Bearer ") {
      return c.Render(http.StatusUnauthorized, r.JSON(map[string]string{"error": "unauthorized"}))
    }
    token := strings.TrimPrefix(auth, "Bearer ")
    scopes, err := validateTokenScopes(token)
    if err != nil || !hasScope(scopes, "api:access") {
      return c.Render(http.StatusForbidden, r.JSON(map[string]string{"error": "forbidden"}))
    }
    c.Set("token_scopes", scopes)
    return next(c)
  }
}

func validateTokenScopes(token string) ([]string, error) {
  // Implement JWT validation and scope extraction
  return []string{"api:access", "schema:read"}, nil
}

func hasScope(scopes []string, required string) bool {
  for _, s := range scopes {
    if s == required {
      return true
    }
  }
  return false
}

Testing with middleBrick

After applying these fixes, use the middleBrick CLI to verify that introspection is properly restricted. Run middlebrick scan <your-api-url> and confirm that no findings related to GraphQL introspection or Bearer token misconfiguration appear. The CLI integrates cleanly into your workflow and provides JSON output for automation.