Container Escape in Sinatra with Dynamodb

A container escape in a Sinatra application that uses DynamoDB typically arises from a chain of weaknesses: the web framework exposes an API surface, DynamoDB is used as a backend data store, and container isolation is relied upon to limit blast radius. If the Sinatra app processes untrusted input and forwards it to DynamoDB without strict validation and least-privilege controls, an attacker may leverage malformed requests to probe behavior and eventually break out of the container.

Consider a Sinatra endpoint that accepts a user-supplied key and fetches an item from a DynamoDB table using the AWS SDK for Ruby:

require 'sinatra' require 'aws-sdk-dynamodb'  # BAD: no input validation, permissive CORS, and broad IAM  get '/user' do  table = Aws::DynamoDB::Table.new(client: Aws::DynamoDB::Client.new(region: 'us-east-1'), table_name: 'users')  user_id = params['id']  item = table.get_item(key: { 'id' => user_id })  item.item.to_json end

In this setup, the container might run with network access to DynamoDB and broad IAM permissions. If the container is compromised (for example, via a separate vulnerability such as input validation bypass or insecure deserialization), the attacker can use the Sinatra app as a pivot: craft requests that cause excessive DynamoDB operations, probe metadata service access from within the container, or exploit misconfigured IAM to escalate privileges. The risk is heightened when the same container runs multiple services or when environment variables (such as AWS credentials or instance metadata access) are reachable from within the app process.

An attacker might also leverage SSRF via the Sinatra app to force the backend SDK to make AWS service calls to attacker-controlled endpoints, potentially intercepting or manipulating metadata queries. Because DynamoDB SDK calls originate from the container, any exposed metadata endpoint reachable from within the container becomes a potential target. The combination of Sinatra’s flexible routing, DynamoDB’s API surface, and a container’s shared kernel or network namespace can amplify findings flagged by security scans, such as excessive agency or unsafe consumption patterns.

Remediation focuses on strict input validation, least-privilege IAM, and reducing the container’s attack surface. Always validate and sanitize user input before using it in DynamoDB calls, and avoid exposing raw SDK errors to clients.

Here is a hardened Sinatra example with whitelist validation, parameterized table access, and structured error handling:

require 'sinatra' require 'aws-sdk-dynamodb' require 'json'  VALID_TABLES = ['users', 'profiles']  def dynamodb_client  Aws::DynamoDB::Client.new(region: 'us-east-1') end  get '/user' do  content_type :json  table_name = params['table']  user_id = params['id']   # Validate table against a strict allowlist  halt 400, { error: 'invalid table' }.to_json unless VALID_TABLES.include?(table_name)  # Validate user_id format (alphanumeric, 1..64 chars)  halt 400, { error: 'invalid id' }.to_json unless user_id&.match?(/\\[A-Za-z0-9_-]{1,64}\\)   begin   table = Aws::DynamoDB::Table.new(client: dynamodb_client, table_name: table_name)   item = table.get_item(key: { 'id' => user_id })   halt 404, { error: 'not found' }.to_json unless item.respond_to?(:item) && item.item    item.item.to_json  rescue Aws::DynamoDB::Errors::ServiceError => e   # Log full details server-side; return generic message to client   halt 502, { error: 'upstream service error' }.to_json  end end

On the IAM side, assign the container’s execution role only the permissions required for the specific DynamoDB actions on the specific resources. For example, allow dynamodb:GetItem and dynamodb:Query on the designated tables, and deny access to other AWS services unless explicitly needed. Avoid using wildcard actions or resource ARNs in production.

Additionally, ensure the container does not have access to the EC2 instance metadata service if it’s not required, and enforce network policies that limit egress to known DynamoDB endpoints only. Scan the API with middleBrick to validate these controls; the scanner checks Authentication, BOLA/IDOR, Input Validation, and other relevant checks for Sinatra endpoints that interact with DynamoDB. If you automate scans in pipelines, the middleBrick GitHub Action can fail builds when risk scores drop below your chosen threshold.