Api Key Exposure in Gorilla Mux with Firestore

Gorilla Mux is a widely used HTTP router for Go that supports route variables, regex patterns, and nested subrouters. When developers use Gorilla Mux to expose endpoints that interact with Google Cloud Firestore, a common misconfiguration can lead to API key exposure. This typically occurs when API keys or service account credentials are passed in HTTP headers, query parameters, or request bodies, and are inadvertently logged, reflected in error messages, or exposed through debug endpoints.

Firestore, Google’s native NoSQL database, requires authentication via an API key or a service account token for REST and gRPC calls. In a Gorilla Mux service, if routes such as /api/v1/firestore/{collection}/{document} pass keys in headers (e.g., Authorization: Bearer {key}) or as query strings (e.g., ?api_key=xxx), those keys can leak through server logs, browser history, or intermediary proxies when responses include verbose errors or stack traces.

The risk is compounded when Firestore security rules are misconfigured or when the Gorilla Mux handler echoes request parameters back to the client. For example, returning the document ID alongside a Firestore read operation without validating the caller’s permissions can reveal that a specific API key was used to access a given resource. An attacker can then correlate repeated requests, inspect response times, and infer whether a valid key exists, leading to enumeration or privilege escalation.

Additionally, if the Gorilla Mux application uses middleware that logs incoming headers for debugging, API keys can be captured in plaintext logs. This is especially dangerous when combined with Firestore’s predictable document paths, as an attacker can use leaked keys to probe collections and infer data structures. The combination of Gorilla Mux’s flexible routing and Firestore’s token-based authentication increases the attack surface if keys are not strictly isolated from logs, error responses, or client-side code.

To illustrate, consider a handler that forwards an API key to Firestore without sanitizing outputs:

package main

import (
	"context"
	"fmt"
	"log"
	"net/http"

	"github.com/gorilla/mux"
	"cloud.google.com/go/firestore"
	google "google.golang.org/api/option"
)

func firestoreHandler(w http.ResponseWriter, r *http.Request) {
	vars := mux.Vars(r)
	apiKey := r.Header.Get("X-API-Key")
	log.Printf("Incoming request with key: %s", apiKey) // Risk: key logged

	ctx := context.Background()
	client, err := firestore.NewClient(ctx, "my-project-id", option.WithAPIKey(apiKey))
	if err != nil {
		fmt.Fprintf(w, "Error: %v", err)
		return
	}
	defer client.Close()

	collection := vars["collection"]
	docID := vars["document"]
	doc, err := client.Collection(collection).Doc(docID).Get(ctx)
	if err != nil {
		fmt.Fprintf(w, "Failed to fetch document: %v", err)
		return
	}
	fmt.Fprintf(w, "Document data: %v", doc.Data())
}

In this example, the API key is extracted from a custom header, logged, and then used to initialize a Firestore client. If the log is exposed or an error message reflects the key, the credential is compromised. Moreover, if the route is accessible without authentication at the application layer, an unauthenticated scan (as performed by middleBrick) could detect the presence of the key in responses or error strings, leading to further exploitation.

middleBrick’s LLM/AI Security checks and unauthenticated endpoint testing can surface such exposures by probing endpoints without credentials and analyzing responses for sensitive patterns. This helps identify whether API keys are being reflected or leaked through error handling or logging practices.

To prevent API key exposure when using Gorilla Mux with Firestore, follow these concrete remediation steps with corrected code examples.

1. Avoid passing API keys via headers or query parameters. Instead, use Application Default Credentials (ADC) and rely on environment-based authentication. This removes the key from the request path entirely.

package main

import (
	"context"
	"log"
	"net/http"

	"github.com/gorilla/mux"
	"cloud.google.com/go/firestore"
)

func firestoreHandler(w http.ResponseWriter, r *http.Request) {
	vars := mux.Vars(r)
	ctx := context.Background()

	// Uses ADC; no API key in request
	client, err := firestore.NewClient(ctx, "my-project-id")
	if err != nil {
		log.Printf("Firestore client init failed: %v", err)
		http.Error(w, "Internal server error", 500)
		return
	}
	defer client.Close()

	collection := vars["collection"]
	docID := vars["document"]
	doc, err := client.Collection(collection).Doc(docID).Get(ctx)
	if err != nil {
		http.Error(w, "Document not found", 404)
		return
	}
	w.Write([]byte("OK"))
}

2. Never log or echo API keys or sensitive request data. Ensure middleware and error handlers do not include credentials in logs or responses.

// BAD: log.Printf("Key: %s", apiKey)

// GOOD: structured logging without sensitive data
log.Printf("Request for collection=%s document=%s", collection, docID)

3. Enforce Firestore security rules and validate inputs. Ensure that the Gorilla Mux handler does not rely solely on client-supplied identifiers without server-side authorization checks.

// Example: validate document ownership before access
doc, err := client.Collection(collection).Doc(docID).Get(ctx)
if err != nil {
    // handle error without revealing key usage
}
// Apply business logic checks here (e.g., user permissions)

4. Use environment variables or secret managers for credentials, and rotate keys regularly. If you must pass keys (e.g., for third-party services), use encrypted headers and remove them from logs immediately.

middleBrick’s dashboard and CLI can scan your endpoints to verify that no API keys appear in responses or logs, and its GitHub Action can enforce these rules in CI/CD to prevent regressions.