Api Key Exposure in Fiber with Oauth2

When an API built with Fiber uses OAuth 2.0 but still exposes API keys in requests, it introduces a hybrid risk where two authentication mechanisms collide. OAuth 2.0 is designed to delegate access using tokens, while API keys are simple shared secrets. If both are accepted by the same endpoint without clear scoping or rejection rules, an API key can unintentionally authorize actions or bypass intended OAuth 2.0 protections.

Consider a Fiber route that accepts either an OAuth 2.0 Bearer token or an API key in a header. If the route does not strictly enforce a single authentication scheme, an attacker who discovers or guesses a static API key can call protected endpoints without needing to obtain an OAuth token. This can happen when middleware is configured to check for multiple credentials and defaults to the weaker one instead of rejecting the request.

Key exposure often occurs through logs, error messages, or client-side code. For example, if a Fiber application logs incoming headers for debugging and an API key is present, the key may be persisted in log stores and exposed to unauthorized parties. Similarly, if API keys are embedded in JavaScript or mobile clients that interact with the Fiber backend, they can be extracted via browser dev tools or reverse engineering. In an OAuth 2.0 context, this is particularly risky because the API key may grant broad access, effectively undermining token-based scopes and lifetimes.

Another scenario involves misconfigured proxy servers or load balancers that forward API keys to downstream Fiber services. If these intermediaries are compromised or overly permissive, they can inject legacy API keys into requests that appear to carry valid OAuth 2.0 tokens. The Fiber application may then incorrectly associate the request with the key’s permissions, leading to privilege escalation or data exposure across OAuth 2.0-protected resources.

During a scan, middleBrick checks whether API keys are accepted in routes that also support OAuth 2.0, whether authentication middleware properly prioritizes and isolates schemes, and whether keys appear in logs or error traces. Findings include references to the OAuth 2.0 specification and common implementation pitfalls observed in real-world services, helping you understand how the combination of Fiber and OAuth 2.0 can unintentionally surface static credentials.

To secure a Fiber API that uses OAuth 2.0, enforce a single, explicit authentication scheme per route and avoid accepting API keys where OAuth 2.0 is intended. Define middleware that validates Bearer tokens according to the OAuth 2.0 specification and reject requests that include legacy keys on protected endpoints.

Below is a concrete example of OAuth 2.0 Bearer token validation in Fiber using JWT verification. This middleware extracts the token, validates its signature and claims, and ensures the request is authorized before reaching the handler.

// OAuth 2.0 Bearer token validation in Fiber
package main

import (
    "github.com/gofiber/fiber/v2"
    "github.com/golang-jwt/jwt/v5"
)

func JWTMiddleware() fiber.Handler {
    return func(c *fiber.Ctx) error {
        auth := c.Get("Authorization")
        if auth == "" {
            return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "authorization header required"})
        }

        const prefix = "Bearer "
        if len(auth) < len(prefix) || auth[:len(prefix)] != prefix {
            return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid authorization header format"})
        }

        tokenString := auth[len(prefix):]
        token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
            if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
                return nil, fiber.ErrUnauthorized
            }
            return []byte("your-secret-key"), nil
        })

        if err != nil || !token.Valid {
            return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid or expired token"})
        }

        c.Locals("user", token)
        return c.Next()
    }
}

func main() {
    app := fiber.New()
    app.Get("/protected", JWTMiddleware(), func(c *fiber.Ctx) error {
        return c.JSON(fiber.Map{"message": "access granted with OAuth 2.0"})
    })
    app.Listen(":3000")
}

If you must support API keys for legacy clients, isolate them to non-OAuth routes and explicitly disable them for endpoints that validate Bearer tokens. Do not allow both credentials to be accepted simultaneously unless you have a strict precedence rule enforced in middleware.

Additionally, ensure tokens are issued with appropriate scopes and short lifetimes, and use HTTPS to prevent interception. middleBrick can validate that your Fiber endpoints do not accept API keys where OAuth 2.0 is expected and highlight routes where headers or logs may expose sensitive credentials.