Shellshock in Grape with Dynamodb

Shellshock is a family of command injection vulnerabilities that arise when untrusted data is passed into bash environment variables and then used to construct shell commands. In a Grape API, this typically occurs when user-controlled input flows into system or backtick calls that invoke a shell. When that API uses Amazon DynamoDB as a backend data store, the risk expands because sensitive data such as AWS credentials and table metadata may be accessible to the application process environment. An attacker who can inject environment variables or command sequences may be able to read or manipulate DynamoDB operations, or exfiltrate data that the application later stores in DynamoDB.

Consider a Grape endpoint that queries DynamoDB based on a user-supplied table name. If the implementation builds a shell command using string interpolation and passes user input into the command environment, the classic Shellshock pattern emerges:

get '/dynamodb/:table' do
  table = params[:table]
  result = `aws dynamodb scan --table-name #{table}`
  result
end

If the request includes table crafted as foo; echo $AWS_SECRET_ACCESS_KEY or uses environment variable assignment such as FOO=bar aws dynamodb scan ..., the injected code executes in the shell with the same permissions as the API process. Because the application process often inherits AWS credentials from its environment (e.g., instance profile or exported keys), the injected command can read or modify any DynamoDB resource accessible to those credentials. This turns a command injection flaw into a data exposure vector against DynamoDB.

Additionally, if the API parses environment variables to configure its DynamoDB client (e.g., ENV['AWS_REGION']), an attacker who can control or influence environment construction may indirectly affect which DynamoDB endpoint is used, enabling SSRF-like patterns or data exfiltration through misconfigured endpoints. The combination of Grape’s flexible routing, shell usage, and DynamoDB’s environment-dependent client configuration creates a scenario where improper input handling can lead to unauthorized data access or manipulation.

Remediation focuses on removing shell involvement and validating all inputs that affect DynamoDB behavior. Avoid constructing shell commands with user input; instead use the AWS SDK directly. If shell invocation is unavoidable, rigorously sanitize and whitelist inputs, and avoid passing untrusted data through environment variables.

Use the AWS SDK without shell commands

Replace backticks or system calls with the official AWS SDK for Ruby. This eliminates shell injection and gives you fine-grained control over requests.

require 'aws-sdk-dynamodb'

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

get '/dynamodb/items' do
  table = params[:table]
  # Validate table name against a whitelist or strict pattern
  unless table =~ /^\w+$/ && known_tables.include?(table)
    halt 400, { error: 'invalid_table' }.to_json
  end
  resp = client.scan(table_name: table)
  resp.items.to_json
end

Input validation and safe configuration

Validate and constrain all inputs that influence DynamoDB operations. Use strict allowlists for table names and parameter values. Do not rely on environment variables constructed from user input.

# Safe environment defaults, not derived from user input
ENV['AWS_REGION'] ||= 'us-east-1'
ENV['AWS_MAX_ATTEMPTS'] ||= '3'

# Explicit client configuration
client = Aws::DynamoDB::Client.new(
  region: ENV['AWS_REGION'],
  max_attempts: ENV['AWS_MAX_ATTEMPTS'].to_i
)

Avoid environment variable injection

If you must invoke external commands, construct the command without injecting environment variables derived from users. Use the SDK whenever possible; if shell use is required, avoid backticks with interpolated input and prefer Open3.capture3 with explicit argument arrays.

require 'open3'

# Safe: arguments are passed directly, not interpolated into a shell string
def safe_scan(table)
  return unless table =~ /^\w+$/ && known_tables.include?(table)
  stdout, status = Open3.capture3('aws', 'dynamodb', 'scan', '--table-name', table)
  status.success? ? stdout : nil
end

By keeping user input out of the shell and out of environment variables used by the DynamoDB client, you mitigate the risk that an attacker could leverage Shellshock-style injection to access or modify DynamoDB data.