Cache Poisoning in Sinatra with Dynamodb

Cache poisoning in a Sinatra application that uses DynamoDB typically occurs when responses from DynamoDB are cached in a shared or downstream cache (for example, an HTTP cache or a CDN) and subsequently served to other users or requests. Because DynamoDB stores data keyed by user or tenant identifiers, improper handling of request inputs before caching can cause one user’s data to be cached under a key that other users can later retrieve.

Consider a Sinatra endpoint that queries DynamoDB using an identifier taken directly from user-controlled parameters without normalization or strict validation. If the response for a given identifier is cached, an attacker may manipulate the parameter to substitute another user’s identifier and observe cached content that should be private. This is a form of Insecure Cache Handling and can be discovered by security scans such as those performed by middleBrick, which tests input validation and authorization checks in parallel with cache-aware probes.

DynamoDB itself does not introduce caching at the service level for query responses; caching behavior comes from how your application stores and retrieves results. If you store the result of a DynamoDB query keyed by something like a raw path or query parameter, and that cache is shared across requests, you risk returning data belonging to one user to another. This maps to common OWASP API Top 10 categories such as Broken Object Level Authorization (BOLA) and Improper Cache Handling, and can be reflected in scan findings related to BOLA/IDOR and Property Authorization.

Because middleBrick scans the unauthenticated attack surface, it can identify endpoints where identifiers flow directly into DynamoDB queries and where outputs may be cached or reflected in a way that enables cross-user data exposure. The scan also checks input validation and authorization logic, highlighting cases where user-supplied values are used to form DynamoDB key expressions without proper access checks or canonicalization.

Remediation guidance centers on ensuring that cache keys are deterministic, tenant-aware, and never directly reflect raw user input. You should enforce authorization checks before returning or caching any data, and design your caching layer so that cached entries are scoped to the correct principal. MiddleBrick’s per-category breakdowns and prioritized findings can help you locate these patterns and apply targeted fixes.

To remediate cache poisoning when using DynamoDB in Sinatra, enforce strict input validation, scope cache keys to the authenticated principal, and avoid storing or caching raw query results keyed by user-controlled values. Below are concrete code examples using the official AWS SDK for Ruby.

First, validate and normalize the user identifier before using it in a DynamoDB request. Use a UUID format check or a known user ID pattern to ensure the value is safe and canonical:

require 'aws-sdk-dynamodb'
require 'sinatra'
require 'securerandom'

# Validate a user ID to ensure it matches expected format
def safe_user_id(input)
  return nil unless input&.match?(\A[a-f0-9]{8}-[a-f0-9]{4}-4[a-f0-9]{3}-[89ab][a-f0-9]{3}-[a-f0-9]{12}\i)
  input
end

# Build a scoped cache key that includes the tenant or user context
def cache_key(user_id, path)
  "user:#{user_id}:path:#{Digest::SHA256.hexdigest(path)}"
end

# Configure DynamoDB client
client = Aws::DynamoDB::Client.new(region: 'us-east-1')

get '/profile/:user_id' do
  user_id = safe_user_id(params['user_id'])
  halt 400, { error: 'Invalid user identifier' }.to_json unless user_id

  # Ensure the requesting user is allowed to view this profile
  # (authorization check should use your application's identity model)
  unless authorized?(current_user, user_id)
    halt 403, { error: 'Forbidden' }.to_json
  end

  key = cache_key(user_id, request.path_info)
  cached = Cache.get(key)
  return cached if cached

  resp = client.get_item({
    table_name: 'Profiles',
    key: {
      'user_id' => { s: user_id }
    }
  })

  profile = resp.item
  if profile
    Cache.set(key, profile.to_json, expires_in: 300) # short TTL to limit stale data
    profile.to_json
  else
    status 404
    { error: 'Not found' }.to_json
  end
end

# Example authorization helper (implementation-specific)
def authorized?(requester, target_user_id)
  requester && requester['user_id'] == target_user_id
end

In this example, the cache key incorporates both the validated user ID and a hash of the request path to ensure uniqueness and prevent collisions across users or endpoints. Authorization is checked before any data is retrieved or cached, reducing the risk of BOLA/IDOR. The short TTL on cached entries further limits the impact of any accidental cross-user exposure.

For DynamoDB operations, always use parameterized key expressions and avoid string interpolation that could lead to unexpected query behavior. When multiple callers share infrastructure, ensure that cache entries are namespaced with tenant or user context, as shown above. MiddleBrick can help verify that these controls are present and that findings related to input validation, authorization, and cache handling are addressed.