Security Misconfiguration in Chi with Firestore

Security misconfiguration in a Chi application that uses Google Cloud Firestore often arises from overly permissive Firestore security rules and from runtime assumptions that differ between development and production. When rules are not explicitly scoped, an unauthenticated or low-privilege caller may be able to read or write collections intended for specific users or system processes. In Chi, this can occur when route handlers construct Firestore paths from user-supplied identifiers without validating ownership or access rights.

For example, a common pattern is to store user data under a path like users/{userId}. If the rule for that path relies on request.auth != null but does not also enforce that the requesting user ID matches the document ID, an attacker who can manipulate the userId parameter may access or modify other users’ documents. Chi endpoints that parse parameters from the request path or query string can inadvertently expose this mismatch when developers skip authorization checks or assume route safety alone is sufficient.

Another misconfiguration vector is the use of Firestore emulator references in production code or relaxed rules during initial development that are never tightened before deployment. Rules that allow read or write access to broad collections, or that use conditions based on easily spoofed request properties, can permit enumeration of documents or unauthorized bulk reads. In Chi, this becomes a risk when handlers pass unvalidated query parameters into Firestore calls, enabling query manipulation or injection-style behavior through crafted inputs.

The interaction between Chi’s routing and Firestore’s permission model also highlights the importance of principle of least privilege. Service accounts used by backend code may have broader access than necessary, and if credentials are inadvertently exposed or over-scoped, an attacker who compromises the environment can leverage Firestore rules weaknesses to escalate access. Real-world rule patterns such as allowing writes if request.auth != null without checking resource.data fields or request.time-based constraints can lead to mass assignment or deletion scenarios that violate intended boundaries.

middleBrick detects Security Misconfiguration findings by scanning the unauthenticated attack surface of your Chi endpoints and correlating runtime behavior with Firestore security rules patterns where observable. It identifies issues such as missing ownership checks, overly permissive rule conditions, and unsafe data exposure paths, then provides prioritized findings with severity ratings and remediation guidance mapped to frameworks like OWASP API Top 10 and common cloud misconfiguration patterns.

Remediation focuses on tightening Firestore security rules and ensuring Chi route handlers enforce authorization consistently. Begin by writing rules that scope access to the authenticated user’s own data and validate all input used to construct document paths.

Rule examples

Instead of a broad read rule, scope reads to documents where the document ID matches the authenticated user ID:

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read, write: if request.auth != null && request.auth.uid == userId;
    }
  }
}

For collections that require shared access, use allow-listed groups or explicitly grant roles rather than open reads:

match /publicData/{docId} {
  allow read: if get(/databases/$(database)/documents/roles/$(request.auth.uid)).data.role in ['viewer', 'editor'];
  allow write: if false; // or stricter conditions
}

Chi handler examples

In Chi, derive the user identifier from the request context rather than trusting URL parameters. Use sessions or JWT claims to obtain the UID, and ensure Firestore paths are constructed from this trusted source:

// Chi endpoint with verified user identity
import "github.com/go-chi/chi/v5"
import "context"
import "firebase.google.com/go/v4"
import "google.golang.org/api/option"

func getUserProfile(w http.ResponseWriter, r *http.Request) {
  // Assume appCtx.Auth contains verified claims with UID
  uid := appCtx.Auth.UID
  client := appCtx.FirestoreClient
  docRef := client.Collection("users").Doc(uid)
  var profile Profile
  if err := docRef.Get(r.Context()).DataTo(&profile); err != nil {
    http.Error(w, "not found", http.StatusNotFound)
    return
  }
  // respond with profile
}

When accepting identifiers for admin operations, validate against an allow-list or a roles document instead of using the parameter directly as a Firestore path segment:

// Safe lookup by validating against allowed targets
func updateSettings(w http.ResponseWriter, r *http.Request) {
  requestedId := chi.URLParam(r, "targetId")
  // Ensure requestedId is permitted for the caller, e.g., via a mapping or role check
  if !isAllowedTarget(r.Context(), requestedId) {
    http.Error(w, "forbidden", http.StatusForbidden)
    return
  }
  client := getClient(r)
  settingsRef := client.Collection("settings").Doc(requestedId)
  // perform update
}

Avoid constructing Firestore queries by concatenating user input into collection or field names. Use parameterized field accesses and validate query constraints server-side. For continuous protection, middleBrick can be added to your workflow via the CLI (middlebrick scan <url>), GitHub Action to fail builds on regressions, or MCP Server inside your AI coding assistant to surface misconfiguration risks during development.

Finally, rotate service account credentials, apply the principle of least privilege to IAM roles, and periodically review rules with simulated requests. These steps reduce the likelihood that misconfiguration leads to unintended data exposure or modification.