Broken Access Control in Sinatra with Api Keys

Broken Access Control occurs when API endpoints do not properly enforce authorization between subjects. In Sinatra applications that rely on API keys, the vulnerability arises when key validation is incomplete, keys are accepted via multiple interchangeable headers, or keys are validated but not checked against the required scope or tenant context. Because middleBrick tests unauthenticated attack surfaces, missing or misapplied authorization checks are detectable even without authentication, making weak API key implementations a high‑risk finding.

Common patterns that lead to Broken Access Control in Sinatra include:

• Accepting API keys via query parameters, which can leak in logs, browser history, and referrer headers.
• Using a single before filter that validates the presence of a key but does not verify whether the key grants access to the requested resource or user.
• Failing to enforce per‑endpoint or per‑action authorization once the key is considered valid.
• Reusing a global key for multiple users or services without scoping, enabling horizontal privilege escalation (BOLA/IDOR) where one user can access another’s data.

For example, consider a Sinatra route that lists user profiles. If the route only checks for a key and does not ensure the key maps to the requested user ID, an attacker can enumerate user IDs and access other users’ data. Because middleBrick’s BOLA/IDOR and Property Authorization checks correlate spec definitions with runtime behavior, such missing ownership checks are surfaced as findings. Similarly, if the OpenAPI spec describes key-based security requirements but the implementation ignores scopes or roles, the mismatch is detectable during unauthenticated scans.

The LLM/AI Security checks do not apply here, as this section focuses on traditional access control weaknesses introduced by improper handling of API keys in Sinatra. However, the same principles of precise authorization and scoping are relevant when API keys are used to gate access to AI endpoints or administrative interfaces.

Remediation centers on strict validation, scoping, and avoiding unsafe exposure of keys. Use middleware or before filters to centralize authorization logic, and ensure keys are transmitted only via secure headers.

Secure API key validation with scope and ownership checks

The following example demonstrates a robust approach in Sinatra. It validates the key, maps it to a user and allowed scopes, and enforces ownership before accessing a user-specific resource.

require 'sinatra'
require 'json'

# In production, store hashed keys and metadata in a database or vault
API_KEYS = {
  'abc123' => { user_id: 1, scopes: ['read:profile', 'read:activity'] },
  'def456' => { user_id: 2, scopes: ['read:profile'] }
}

helpers do
  def find_api_key
    key = request.env['HTTP_X_API_KEY'] || params['api_key']
    return nil unless key
    API_KEYS[key]
  end

  def require_key!
    halt 401, { error: 'unauthorized' }.to_json unless find_api_key
  end

  def require_scope!(required_scope)
    key_meta = find_api_key
    halt 403, { error: 'forbidden' }.to_json unless key_meta && key_meta[:scopes].include?(required_scope)
  end
end

before do
  content_type :json
  require_key!
end

# Example: User profile endpoint with ownership check
get '/users/:user_id/profile' do
  key_meta = find_api_key
  # BOLA protection: ensure the key maps to the requested user_id
  halt 403, { error: 'forbidden' }.to_json unless key_meta[:user_id].to_s == params['user_id']
  require_scope!('read:profile')
  { user_id: params['user_id'], profile: 'public data' }.to_json
end

# Example: Scoped activity endpoint
get '/users/:user_id/activity' do
  key_meta = find_api_key
  halt 403, { error: 'forbidden' }.to_json unless key_meta[:user_id].to_s == params['user_id']
  require_scope!('read:activity')
  { user_id: params['user_id'], activity: [] }.to_json
end

# Avoid query-parameter keys in production; if necessary, treat them as less trusted
configure :production do
  before do
    if request.query_string.include?('api_key=')
      halt 400, { error: 'query parameters not allowed for API keys' }.to_json
    end
  end
end

Transport and operational safeguards

• Accept API keys only via the X-API-Key header to reduce leakage risk.
• Use HTTPS to prevent interception; middleBrick’s Encryption check can verify this in scans.
• Rotate keys regularly and implement revocation logic; consider adding a key-introspection endpoint for centralized control.
• Apply rate limiting to reduce brute-force risk; combine with logging for anomalous key usage.

For teams seeking managed detection and tracking, the middleBrick CLI allows scanning from the terminal with middlebrick scan <url>, while the GitHub Action can add API security checks to your CI/CD pipeline and fail builds if risk scores drop below your chosen threshold. The Pro plan supports continuous monitoring and configurable schedules so that regressions are caught early.