Dns Rebinding in Chi with Firestore

Chi is a minimal, functional router for Scala that encourages explicit route definitions and type-safe parameter handling. When a Chi service interacts with Google Cloud Firestore, Dns Rebinding can bypass network-based trust boundaries if the application resolves hostnames at runtime and uses the results to form Firestore client configurations or document paths. Dns Rebinding is an attack where a malicious domain returns A records that switch between attacker-controlled IPs and internal targets, enabling an external client to reach a service that is not directly exposed on the public internet.

In a Chi application, this typically occurs when a Firestore client is initialized using a hostname resolved from user input or environment variables without strict validation. For example, if a Chi route accepts a project ID or instance hostname and passes it to Firestore libraries that perform DNS resolution at connection time, an attacker can use a rebinding domain to point initial probes to a benign IP and later redirect traffic to internal Firestore endpoints or metadata services. This can lead to unauthorized access to Firestore resources if the application also relies on implicit trust based on network location, such as allowing connections from internal IP ranges.

Consider a Chi route that forwards a project identifier to initialize a Firestore client:

import cats.effect.IO
import com.google.auth.oauth2.GoogleCredentials
import com.google.cloud.firestore.FirestoreOptions

val firestoreClient = (projectId: String) =>
  FirestoreOptions.newBuilder()
    .setProjectId(projectId)
    .setCredentials(GoogleCredentials.getApplicationDefault())
    .build()
    .getService

If the projectId or underlying host used by FirestoreOptions is derived from a Chi route parameter without strict allowlisting, an attacker can manipulate DNS responses to redirect the client to a malicious or internal host. Because Chi does not validate or sanitize hostnames used by downstream clients, the rebinding can expose Firestore operations that would otherwise be protected by firewall rules or VPC Service Controls.

The risk is compounded when the Chi service runs in environments where metadata service access is permitted, as rebinding can redirect traffic to cloud metadata endpoints to obtain temporary credentials. This does not imply Chi or Firestore are flawed; it highlights the importance of validating and restricting hostnames and project identifiers before they are used in client construction. middleBrick can detect such configurations during scans by analyzing unauthenticated attack surfaces and identifying endpoints that accept hostnames or project identifiers without proper input validation.

To mitigate Dns Rebinding in Chi applications using Firestore, enforce strict validation and static configuration for all hostnames and project identifiers. Avoid constructing Firestore client options from user-controlled input. If dynamic configuration is necessary, use allowlisted values and resolve hostnames at build time rather than runtime.

Here is a secure pattern for initializing Firestore in Chi routes:

import cats.effect.IO
import com.google.auth.oauth2.GoogleCredentials
import com.google.cloud.firestore.FirestoreOptions

// Static project ID defined via environment or configuration, not from user input
val safeProjectId: IO[String] = IO(System.getenv("FIRESTORE_PROJECT_ID"))

// Validate project ID against an allowlist
val allowedProjects = Set("my-secure-project-123", "backup-project-456")

val validatedProject: String => IO[String] = (projectId: String) =
  if (allowedProjects.contains(projectId)) IO.pure(projectId)
  else IO.raiseError(new IllegalArgumentException("Invalid project ID"))

// Build Firestore client once and reuse
val firestoreClient: IO[com.google.cloud.firestore.Firestore] = for {
  project <- safeProjectId
  validated <- validatedProject(project)
  opts = FirestoreOptions.newBuilder()
    .setProjectId(validated)
    .setCredentials(GoogleCredentials.getApplicationDefault())
    .build()
  client = opts.getService
} yield client

In Chi route definitions, ensure that any path or query parameters used to influence Firestore behavior are strictly validated:

import cats.data.NonEmptyList
import com.twitter.util.Future
import io.chisquare.routes.dto.ProjectRequest
import io.chisquare.firestore.firestoreClient

val routes = Router(
  post {
    path("Firestore" / Segment) { projectId =>
      get {
        // Reject non-allowlisted project IDs early
        validatedProject(projectId).flatMap { validId =>
          firestoreClient.use { client =>
            // Safe Firestore operations using validated client
            val docRef = client.collection("users").document(validId)
            // Perform Firestore read
            Future.value(ResponseOk(s"Read document for $validId"))
          }
        }.getOrElseF(IO.pure(ResponseBadRequest("Invalid project")))
      }
    }
  }
)

Additionally, configure your Chi server to reject requests with suspicious Host headers and enforce HTTPS to reduce the impact of rebinding attempts. middleBrick can support this remediation process by scanning your Chi endpoints and identifying places where hostnames or project identifiers are used without validation, providing prioritized findings and remediation guidance.