Command Injection in Sinatra with Dynamodb

Command Injection occurs when untrusted input is concatenated into system or shell commands. In a Sinatra application that interacts with Amazon DynamoDB, this typically arises when developers build shell commands or subprocess invocations using data that originates from HTTP requests, path segments, query parameters, or headers. Even though DynamoDB itself is a managed NoSQL database and does not execute shell commands, the surrounding Ruby runtime can be leveraged to run arbitrary executables via methods like system, ` (backticks), or Open3. If user-controlled values such as table_name, partition_key, or custom metadata are passed into these shell constructs without proper sanitization, an attacker can inject additional shell commands.

For example, suppose a Sinatra endpoint accepts a name parameter and uses it to tag a DynamoDB put item while also invoking a shell command for logging or file operations:

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

get '/log_item' do
  name = params['name']
  # Unsafe: name is concatenated into a shell command
  system("echo Writing item for #{name}")
  dynamo = Aws::DynamoDB::Client.new(region: 'us-east-1')
  dynamo.put_item(
    table_name: 'Items',
    item: { 'name' => { s: name }, 'timestamp' => { n: Time.now.to_i.to_s } }
  )
  'ok'
end

Here, name reaches the system call unsanitized. An attacker could provide foo; cat /etc/passwd as the name, causing the shell to execute arbitrary commands. The DynamoDB operation may still succeed, but the injected command runs with the privileges of the Sinatra process. The risk is compounded if the application runs in an environment with access to sensitive metadata (IMDSv1) or secrets, enabling broader compromise. This pattern violates secure handling of external input and demonstrates why input validation and strict command construction are essential even when the primary interaction is with DynamoDB.

Another scenario involves generating shell commands to inspect or manage DynamoDB streams or backups using CLI tools, where user input influences command arguments. Without rigorous escaping or use of non-shell APIs (e.g., the AWS SDK directly), the application remains vulnerable to injection despite using DynamoDB as the data store.

To mitigate Command Injection while interacting with DynamoDB in Sinatra, avoid constructing shell commands with user input. Prefer the AWS SDK for all DynamoDB operations and use Ruby’s built-in mechanisms safely. If shell execution is unavoidable, use strong input validation, allowlisting, and process invocation without a shell.

1. Use the AWS SDK directly and avoid shell construction

Always use the official aws-sdk-dynamodb client for database operations. Do not embed request parameters into shell commands. The following example shows a safe Sinatra handler:

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

get '/item/:name' do
  name = params['name']
  # Validate: allow only alphanumeric and limited punctuation
  unless name =~ /^\w{1,64}$/
    halt 400, 'Invalid name'
  end

  dynamo = Aws::DynamoDB::Client.new(region: 'us-east-1')
  resp = dynamo.get_item(
    table_name: 'Items',
    key: { 'name' => { s: name } }
  )
  if resp.item
    resp.item.to_h
  else
    status 404
    { error: 'not found' }.to_json
  end
end

This approach eliminates shell involvement entirely. Input validation ensures name conforms to an expected pattern, reducing risk if validation is bypassed elsewhere.

2. If shell commands are required, use strict allowlisting and the 3-argument form of system

When invoking external utilities is necessary, avoid string interpolation. Use the array form of system to bypass shell interpretation:

# Safe: array form prevents shell injection
system('echo', 'Writing item for', name)

Even better, avoid external utilities for DynamoDB-related tasks. If you must interact with the AWS CLI (e.g., for administrative operations), validate and allowlist command and argument values rigorously:

allowed_commands = %w[describe-table list-tables]
cmd = params['cmd']
table = params['table']

if allowed_commands.include?(cmd) && table =~ /^\w-[a-z0-9_-]+$/
  # Safe: no user input in the shell command beyond allowlisted args
  system(cmd, table)
else
  halt 400, 'Invalid request'
end

3. Secure runtime environment and least privilege

Ensure the Sinatra process runs with minimal IAM permissions. For DynamoDB, grant only the necessary actions on specific resources. Disable IMDSv1 to prevent SSRF-related credential theft, which could amplify an injection impact. Combine these measures with continuous scanning to detect regressions; the middleBrick CLI can be integrated into development workflows to surface such issues early:

# Example: scanning a Sinatra endpoint with middlebrick CLI
# middlebrick scan http://localhost:4567/items

By adhering to these practices—direct SDK usage, input validation, and avoiding shell construction—you significantly reduce the likelihood of Command Injection in Sinatra applications that leverage DynamoDB.