Brute Force Attack in Sinatra with Firestore

A brute force attack against a Sinatra application backed by Google Cloud Firestore typically targets authentication or token-verification endpoints. Sinatra’s lightweight routing and lack of built-in throttling can allow rapid, unthrottled requests when developers rely solely on Firestore for user state without adding rate controls. Attackers may attempt credential guessing or token enumeration against endpoints such as /login or /verify. Because Firestore rules are evaluated at runtime per request, an attacker can probe many user identifiers or API keys without triggering server-side denial by default, especially when rules are permissive for read operations on user documents.

The combination exposes risk patterns such as:

• Unbounded login attempts against a known user field (e.g., email) without exponential backoff or account lockout logic in Sinatra middleware.
• Overly broad Firestore security rules that allow read access to user documents based only on UID, enabling enumeration via timing differences or existence checks.
• Lack of request metadata validation (e.g., missing origin checks or API key binding), which permits automated scripts to iterate over identifiers or API keys stored (or referenced) in Firestore.
• Use of predictable identifiers (e.g., sequential UIDs or email-based tokens) that make enumeration feasible across many documents in Firestore.

Because middleBrick scans test Authentication, Rate Limiting, and BOLA/IDOR in parallel, it can identify whether a Sinatra endpoint allows unchecked attempts and whether Firestore rules expose user data that facilitates brute force enumeration. Findings often highlight missing rate constraints, weak token entropy, or rules that do not adequately scope reads to the requesting user.

Remediation focuses on reducing the attack surface for brute force by combining Sinatra-side controls with secure Firestore usage. Apply rate limiting at the Sinatra layer, avoid leaking user existence, and enforce least-privilege rules in Firestore.

1. Rate limiting in Sinatra

Use a middleware-based rate limiter to bound requests per source. Below is a concise example using rack-attack to limit login attempts per IP and per user identifier, integrated with a Firestore lookup only after basic throttling passes.

# Gemfile
# gem 'rack-attack'
# gem 'google-cloud-firestore'

require 'sinatra'
require 'rack/attack'
require 'google/cloud/firestore'

# Rate limit configuration
Rack::Attack.throttle('logins/ip', limit: 5, period: 60) do |req|
  req.ip if req.path == '/login' && req.post?
end

Rack::Attack.throttle('logins/email', limit: 5, period: 60) do |req|
  req.params['email'] if req.path == '/login' && req.post?
end

Rack::Attack.blocklist('block suspicious agents') do |req|
  req.user_agent =~ /(sqlmap|nikto|nmap)/i
end

# Sinatra route with throttling check before Firestore access
post '/login' do
  email = params['email']
  password = params['password']

  halt 429, { error: 'Too many attempts, try later' }.to_json unless Rack::Attack.allow_request?(env)

  firestore = Google::Cloud::Firestore.new
  users_ref = firestore.col ref: 'users'
  user_doc = users_ref.where(eq: email).limit(1).get

  if user_doc.empty?
    # Return generic message to avoid user enumeration
    halt 401, { error: 'Invalid credentials' }.to_json
  end

  # Assume user_data is retrieved and password verification is done securely
  halt 401, { error: 'Invalid credentials' }.to_json
end

2. Firestore security rules to prevent enumeration and over-permissiveness

Rules should scope access to a user’s own document and avoid broad list/read permissions. Use request.auth and derived claims to ensure users can only interact with their own data.

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read, write: if request.auth != null && request.auth.uid == userId;
      // Deny enumeration by not allowing list operations on the collection
      allow list: if false;
    }

    // Avoid rules that permit read access based only on predictable fields
    match /tokens/{tokenId} {
      allow read: if request.auth != null && request.auth.token.can_access == true;
    }
  }
}

3. Secure token and identifier handling

Avoid exposing predictable identifiers in URLs or responses. When generating tokens or references, use high-entropy values and bind them to Firestore documents with restricted rules. Validate the presence and correctness of tokens server-side before performing any sensitive operation.

By combining Sinatra-side throttling, generic error messages, and tightly scoped Firestore rules, you reduce the feasibility and impact of brute force attempts while maintaining a functional authentication flow.