Cache Poisoning in Rails with Dynamodb

Cache poisoning occurs when an attacker causes cached data to store malicious or incorrect values that are subsequently served to other users. In a Ruby on Rails application that uses DynamoDB as a persistent store or cache backend, the risk arises when cache keys are derived from attacker-controlled input without proper validation or normalization. If user input directly influences the cache key or the cached response, an attacker can force the application to write values under predictable keys and poison the cache for subsequent requests.

DynamoDB does not inherently introduce cache poisoning, but patterns in Rails can create the conditions. For example, using raw user parameters to construct cache keys without sanitization can allow an attacker to overwrite keys used by other users or critical application flows. Consider a scenario where Rails caches an API response keyed by a user-supplied product_id and an unvalidated currency parameter:

# Dangerous: user input directly shapes the cache key
cache_key = "product_#{params[:product_id]}_currency_#{params[:currency]}"
Rails.cache.write(cache_key, expensive_query_result, expires_in: 15.minutes)

If an attacker manipulates currency to a shared key, they can cause the application to overwrite a commonly used cache entry with malicious content. When other users request the same product with the default currency, they receive the attacker’s poisoned response. This becomes more impactful when the cached data is used for authorization or pricing decisions.

Another vector involves template fragment caching where unsafe input is included in partial paths or cache digests. If a partial is rendered with a path derived from user data, an attacker can inject path traversal or specially crafted segments that map to the same cache key across different contexts. In DynamoDB-backed caching strategies (e.g., using a custom store that writes to DynamoDB), these poisoned entries persist and are served across sessions, amplifying the impact.

Rails’ default cache stores abstract storage details, but when integrating DynamoDB explicitly—such as via a custom store or through an API-layer cache—developers must ensure cache keys are deterministic, scoped, and isolated per tenant or user context. Without scoping, an attacker can force collisions across unrelated users or environments, effectively turning DynamoDB into a shared cache where poisoned entries propagate.

Additionally, if responses cached in DynamoDB include sensitive data derived from unvalidated input, an attacker may probe key patterns to enumerate valid keys and retrieve or overwrite sensitive entries. This is especially relevant when using DynamoDB for storing rendered fragments or API responses that should remain isolated between users or roles. The lack of built-in isolation in key design leads to privilege escalation or data exposure when combined with cache poisoning.

To mitigate cache poisoning in Rails applications using DynamoDB, adopt strict cache key construction and isolation practices. Always treat user input as untrusted and avoid using raw parameters directly in cache keys. Use strong namespacing, tenant or user context, and cryptographic hashing to ensure keys are predictable only to the intended scope.

Below are concrete code examples for safe DynamoDB-backed caching in Rails.

1. Key scoping with user and tenant context

Scope cache keys by tenant and user identifiers to prevent cross-user collisions. Use a deterministic hash for variable inputs like currency or locale rather than inserting them raw into the key.

# Safe: scoped and hashed user input
cache_key = "tenant/#{current_tenant.id}/user/#{current_user.id}/product/#{params[:product_id]}/currency/#{Digest::SHA256.hexdigest(params[:currency])}"
data = Rails.cache.fetch(cache_key, expires_in: 15.minutes) do
  # Expensive DynamoDB query or computation
  ProductData.fetch_from_dynamodb(params[:product_id], params[:currency])
end

2. Using DynamoDB attributes for cache metadata

If you store cache entries as items in DynamoDB, structure items with explicit metadata fields such as cache_key, tenant_id, and user_id to support safe lookups and prevent accidental cross-tenant reads. Always validate tenant and user IDs server-side before querying.

# Example using AWS SDK for DynamoDB in a Rails service object
class DynamoDbCacheStore
  TABLE_NAME = 'app_cache'

  def initialize
    @dynamodb = Aws::DynamoDB::Client.new(region: 'us-east-1')
  end

  def fetch(key:, tenant_id:, user_id:, &block)
    item = @dynamodb.get_item(
      table_name: TABLE_NAME,
      key: { pk: { s: "tenant##{tenant_id}" }, sk: { s: "user##{user_id}" } }
    )
    if item.item && item.item[:cache_key].s == key
      JSON.parse(item.item[:value].s)
    else
      value = block.call
      @dynamodb.put_item(
        table_name: TABLE_NAME,
        item: {
          pk: { s: "tenant##{tenant_id}" },
          sk: { s: "user##{user_id}" },
          cache_key: { s: key },
          value: { s: value.to_json },
          expires_at: { n: (Time.now.to_f + 900).to_s } // 15 minutes
        }
      )
      value
    end
  end
end

# Usage in controller
cache_store = DynamoDbCacheStore.new
cache_store.fetch(
  key: "product_data_v2",
  tenant_id: current_tenant.id,
  user_id: current_user.id
) do
  Product.fetch_details(params[:product_id])
end

3. Validation and normalization of inputs

Validate and normalize inputs before using them in cache logic. Whitelist acceptable values for parameters like currency or locale to reduce the attack surface for key manipulation.

# Input normalization and validation
class CacheParams
  CURRENCIES = %w[usd eur gbp].freeze

  def self.currency(param)
    CURRENCIES.include?(param) ? param : 'usd'
  end
end

normalized_currency = CacheParams.currency(params[:currency])
cache_key = "product/#{params[:product_id]}/currency/#{normalized_currency}"
data = Rails.cache.fetch(cache_key) { ProductData.fetch(param[:product_id], normalized_currency) }

4. Avoiding path-based fragment caching with user input

When using fragment caching, avoid including raw user input in partial paths or cache digests. Instead, pass safe objects or hashes that Rails can serialize deterministically.

# Unsafe: partial path includes raw user input
# render partial: "items/#{params[:section]}"

# Safer: use a hash with normalized values
render partial: 'items/item', locals: { section: { name: params[:section], version: 1 } }

By combining strict scoping, input normalization, and explicit storage semantics, Rails applications using DynamoDB can reduce the risk of cache poisoning while maintaining performance.