Api Key Exposure in Fiber with Mongodb

When a Fiber application uses MongoDB as a backend data store, mishandling of sensitive credentials—particularly API keys used to authenticate to MongoDB—can lead to direct API key exposure. This typically occurs when keys are embedded in route handlers, configuration objects, or response payloads that are inadvertently returned to the client. Because Fiber is a fast, expressive web framework built on Fasthttp, developers may inadvertently expose secrets through logging, error messages, or unguarded endpoint behavior.

Consider a route that initializes a MongoDB client using a connection string containing an API key and then passes database handles or configuration objects to downstream handlers. If these objects or their properties are serialized into JSON responses—such as when returning a database metadata object or debug info—the API key can be exposed to unauthenticated attackers. For example, returning the full MongoDB URI or a client configuration object from an endpoint like /debug/db can disclose the key in cleartext to anyone who can trigger that route.

Another common pattern in Fiber apps is storing the MongoDB connection string in environment variables and then constructing the client with options that include the URI directly. If the application also logs configuration details for troubleshooting, the API key can appear in application logs. An attacker who gains access to logs or can trigger verbose error output may harvest the key and use it to pivot to the MongoDB deployment, bypassing network-level protections.

Additionally, improper error handling in Fiber middleware can amplify exposure. When errors from the MongoDB driver are returned to the client—such as validation failures or connection issues—stack traces or message strings may include connection parameters, including the API key. Without careful sanitization, these messages become a leakage channel for credentials that should remain confined to the server environment.

These risks align with broader API security concerns such as BFLA (Business Logic Flaws and Abuse), where logic intended for internal debugging or configuration is exposed to external callers. middleBrick scans for such issues by correlating OpenAPI specifications with runtime behavior, detecting when sensitive data like API keys appear in responses or when endpoints expose configuration objects that should remain internal.

To prevent API key exposure when using MongoDB with Fiber, ensure that sensitive credentials are never serialized, logged, or returned to clients. Use environment variables for configuration, restrict response objects to safe data, and sanitize errors before sending them upstream.

First, store the MongoDB connection string in environment variables and load it securely at startup:

// config.go
package config

import "os"

var MongoURI = os.Getenv("MONGO_URI")

Initialize the MongoDB client without exposing the URI in route handlers or responses:

// main.go
package main

import (
	"context"
	"log"

	"github.com/gofiber/fiber/v2"
	"go.mongodb.org/mongo-driver/mongo"
	"go.mongodb.org/mongo-driver/mongo/options"
)

var client *mongo.Client

func initClient() {
	var err error
	client, err = mongo.Connect(context.TODO(), options.Client().ApplyURI(config.MongoURI))
	if err != nil {
		log.Fatalf("failed to connect to MongoDB: %v", err)
	}
}

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

	app.Get("/health", func(c *fiber.Ctx) error {
		return c.JSON(fiber.Map{"status": "ok"})
	})

	app.Get("/debug/connection", func(c *fiber.Ctx) error {
		// Unsafe: exposing internal client configuration
		// return c.JSON(client.Database("test").Client())
		// Safe: return only non-sensitive health info
		return c.JSON(fiber.Map{"connected": true})
	})

	app.Listen(":3000")
}

Second, avoid returning the MongoDB client or its configuration in HTTP responses. Never serialize objects that contain connection strings or keys. If you must expose metadata, provide a sanitized subset:

// handlers.go
package handlers

import (
	"context"
	"github.com/gofiber/fiber/v2"
)

func GetCollectionInfo(c *fiber.Ctx) error {
	// Safe: returning only collection names, no credentials
	names, err := database.CollectionNames(context.TODO())
	if err != nil {
		return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": "unable to fetch collections"})
	}
	return c.JSON(fiber.Map{"collections": names})
}

Third, sanitize errors to prevent credential leakage. Use middleware to intercept and redact sensitive information from error messages before they reach the client:

// middleware.go
package middleware

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

func ErrorHandler(c *fiber.Ctx, err error) error {
	// Avoid exposing internal error details that may contain connection strings
	message := "internal server error"
	if c.App().Get("env") == "development" {
		message = err.Error()
	}
	return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": message})
}

Finally, enforce strict access controls and audit logs on the MongoDB side. Use role-based access control (RBAC) to limit what the application-level API key can do, and ensure that logs do not capture full connection strings. middleBrick can help identify when endpoints expose sensitive configuration or when error messages risk leaking credentials.