Dns Rebinding in Echo Go with Firestore

DNS Rebinding is a client-side network attack that bypasses same-origin policies by having a victim load a page that forces their browser to resolve a domain to an internal IP address after the initial page load. In an Echo Go service that integrates with Google Cloud Firestore, this can expose sensitive Firestore operations to a remote attacker when the application exposes administrative or data endpoints without proper authorization checks and relies on IP-based trust.

Consider an Echo Go handler that uses a service account to create a Firestore client and returns documents from a user-supplied collection name. If the handler trusts internal network origin (e.g., localhost or a corporate IP) and does not enforce authentication or validate the Firestore resource being accessed, an attacker can craft a malicious page that causes the victim’s browser to make requests to the Echo Go service on an internal address. The Firestore client, initialized with the service account credentials on the server side, may perform actions the attacker can indirectly influence via request parameters such as collection or document IDs.

For example, an attacker might use JavaScript to repeatedly change the browser’s perceived IP address and send requests to the Echo Go endpoint, attempting to enumerate or read Firestore documents. Because the Echo Go server uses the same Firestore client for each request and does not validate whether the requesting user is authorized to access the specific document, the attacker may read or trigger writes to sensitive collections. This becomes especially risky when the Firestore security rules rely only on request IP allowlisting, which DNS Rebinding can circumvent by making the malicious client appear to originate from an allowed IP.

The interaction with Firestore amplifies the impact: if the Echo Go handler uses a privileged service account, the attacker may be able to perform actions beyond read access, such as updating or deleting documents, depending on how the Firestore client is configured and whether additional authorization checks are applied at the application layer. This highlights the need to treat all incoming requests as untrusted, validate inputs such as collection and document names, and enforce user-level authentication and authorization rather than relying on network boundaries or IP-based checks alone.

Using middleBrick to scan your Echo Go endpoint with Firestore integrations can surface these risks by identifying missing authentication, weak input validation, and overly permissive behavior in Firestore-related handlers. The scanner tests unauthenticated attack surfaces and, with its LLM/AI Security checks, can detect whether exposed endpoints might be leveraged in prompt injection or data exfiltration scenarios, providing prioritized findings and remediation guidance tailored to your API surface.

To mitigate DNS Rebinding risks in an Echo Go service using Firestore, apply defense-in-depth: strict input validation, per-request authorization, and avoiding reliance on network-level trust. Below are concrete code examples that demonstrate a secure approach.

1. Validate and sanitize user input before using it with Firestore

Never directly use user-controlled values such as collection or document IDs in Firestore operations. Validate against a whitelist or strict pattern to prevent traversal or access to unintended resources.

import (
    "net/http"
    "regexp"

    "github.com/labstack/echo/v4"
    "cloud.google.com/go/firestore"
    "google.golang.org/api/iterator"
)

// isValidCollectionName ensures the collection name contains only safe characters.
func isValidCollectionName(name string) bool {
    matched, _ := regexp.MatchString(`^[a-z][a-z0-9_]{0,99}$`, name)
    return matched
}

func GetDocument(c echo.Context) error {
    db := c.Get("firestore").(*firestore.Client)
    collection := c.Param("collection")
    docID := c.Param("docID")

    if !isValidCollectionName(collection) || !isValidDocumentID(docID) {
        return echo.NewHTTPError(http.StatusBadRequest, "invalid resource identifier")
    }

    ctx := c.Request().Context()
    docRef := db.Collection(collection).Doc(docID)
    snap, err := docRef.Get(ctx)
    if err != nil {
        return echo.NewHTTPError(http.StatusInternalServerError, "failed to fetch document")
    }
    if snap.Exists {
        return c.JSON(http.StatusOK, snap.Data())
    }
    return echo.NewHTTPError(http.StatusNotFound, "document not found")
}

func isValidDocumentID(id string) bool {
    // Allow alphanumeric, underscore, dash, and must not be empty.
    matched, _ := regexp.MatchString(`^[a-zA-Z0-9_-]{1,100}$`, id)
    return matched
}

2. Enforce per-request authorization and avoid relying on service account privileges

Ensure that each Firestore operation checks that the authenticated subject has permission to access the specific document or collection. Do not use a highly privileged service account for all operations; instead, use Firestore security rules and application-level checks.

import (
    "context"
    "fmt"

    "cloud.google.com/go/firestore"
)

// userCanAccessDocument returns true if the user identified by userID is allowed to access the document.
// Implement your own authorization logic, e.g., checking Firestore roles or a permissions collection.
func userCanAccessDocument(ctx context.Context, client *firestore.Client, userID, collection, docID string) (bool, error) {
    // Example: check a user_permissions collection
    permRef := client.Collection("user_permissions").Doc(userID)
    snap, err := permRef.Get(ctx)
    if err != nil || !snap.Exists {
        return false, nil
    }
    allowedCollections, ok := snap.Data()["collections"].([]interface{})
    if !ok {
        return false, nil
    }
    for _, c := range allowedCollections {
        if c == collection {
            return true, nil
        }
    }
    return false, nil
}

func SecureGetDocument(c echo.Context) error {
    db := c.Get("firestore").(*firestore.Client)
    collection := c.Param("collection")
    docID := c.Param("docID")
    userID := c.Get("user_id").(string) // set by prior authentication middleware

    allowed, err := userCanAccessDocument(c.Request().Context(), db, userID, collection, docID)
    if err != nil {
        return echo.NewHTTPError(http.StatusInternalServerError, "authorization check failed")
    }
    if !allowed {
        return echo.NewHTTPError(http.StatusForbidden, "access denied")
    }

    docRef := db.Collection(collection).Doc(docID)
    snap, err := docRef.Get(c.Request().Context())
    if err != nil {
        return echo.NewHTTPError(http.StatusInternalServerError, "failed to fetch document")
    }
    if snap.Exists {
        return c.JSON(http.StatusOK, snap.Data())
    }
    return echo.NewHTTPError(http.StatusNotFound, "document not found")
}

3. Use Firestore security rules and avoid IP-based trust

While Echo Go handles application logic, Firestore security rules provide an essential layer. Ensure rules validate user identity and scope access to the user’s own data where applicable, rather than relying on request origin IPs.

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read, write: if request.auth != null && request.auth.uid == userId;
    }
    match /shared/{docId} {
      allow read: if request.auth != null && resource.data.visibility == 'public';
      allow write: if request.auth != null && request.auth.token.role == 'editor';
    }
  }
}

4. Apply middleware to enforce authentication and restrict network exposure

Use Echo middleware to ensure all Firestore-related endpoints require authenticated sessions, reducing the window for DNS Rebinding attacks that exploit unauthenticated or IP-trusted handlers.

func AuthMiddleware(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        user := c.Get("user")
        if user == nil {
            return echo.NewHTTPError(http.StatusUnauthorized, "unauthorized")
        }
        return next(c)
    }
}

// Usage:
e.GET("/data/:collection/:docID", AuthMiddleware(SecureGetDocument))

By combining strict input validation, per-request authorization, secure Firestore rules, and authenticated middleware, you reduce the risk that DNS Rebinding can exploit Firestore endpoints in an Echo Go service. Regular scans with tools like middleBrick can further surface missing controls and help maintain a strong security posture for your API.