Api Key Exposure in Sinatra with Mssql

When a Sinatra application connects to Microsoft SQL Server using an Mssql driver and embines API keys in route handlers, views, or configuration loaded at runtime, the risk of Api Key Exposure increases. This combination creates exposure through insecure handling of secrets, verbose error messages, and dynamic SQL usage patterns that can leak sensitive information.

Sinatra’s minimal defaults do not enforce strict separation of credentials from application code. If an API key is stored in a Ruby constant, an environment variable, or a YAML file that is checked into source control, a path traversal or information disclosure flaw can expose it. Mssql drivers (e.g., the tiny_tds or activerecord-sqlserver-adapter) typically open direct connections using connection strings or DSNs that may contain embedded credentials or tokens. When an endpoint concatenates user input into queries or logs connection attempts, the API key can inadvertently appear in logs, error traces, or responses.

Common patterns that lead to exposure include using string interpolation to construct T-SQL statements, which can trigger SQL injection and information disclosure, and returning full exception details to clients. For example, a route that builds a query like "SELECT * FROM users WHERE api_key = '" + user_key + "'" not only risks injection but can also expose the key through verbose error messages if the Mssql driver returns stack traces containing the connection string or malformed queries. Middleware or instrumentation may also log headers or params, capturing keys sent in requests. Because middleBrick tests unauthenticated attack surfaces, it can detect such exposure by identifying endpoints that return sensitive tokens in responses or metadata, including patterns used by ChatML, Llama 2, Mistral, and Alpaca formats that indicate system prompt leakage.

To mitigate Api Key Exposure when using Mssql with Sinatra, adopt parameterized queries, strict secret management, and hardened error handling. Avoid embedding keys in code or logs, and ensure that database credentials are supplied through secure runtime mechanisms.

Use parameterized queries with tiny_tds

Replace string concatenation with parameterized T-SQL to prevent injection and avoid exposing keys in malformed queries or logs.

require 'sinatra'
require 'tiny_tds'

# Configure client without embedding credentials in code
client = TinyTds::Client.new(
  server: ENV['MSSQL_SERVER'],
  username: ENV['MSSQL_USERNAME'],
  password: ENV['MSSQL_PASSWORD'],
  database: ENV['MSSQL_DATABASE']
)

get '/lookup' do
  key = params['api_key']
  # Parameterized query prevents injection and keeps key out of SQL text
  result = client.execute('SELECT user_id FROM api_users WHERE api_key = @key', key: key)
  user = result.first
  if user
    { user_id: user['user_id'] }.to_json
  else
    status 404
    { error: 'not_found' }.to_json
  end
end

Secure connection strings and environment usage

Store connection strings in environment variables or secret managers and load them at runtime. Do not check secrets into version control.

# config.ru or environment
ENV['MSSQL_CONNECTION_STRING'] = "Server=#{ENV['MSSQL_SERVER']};Port=1433;Database=#{ENV['MSSQL_DATABASE']};UID=#{ENV['MSSQL_USERNAME']};PWD=#{ENV['MSSQL_PASSWORD']};Encrypt=true;TrustServerCertificate=false;"

# In app
configure do
  require 'tiny_tds'
  set :db, TinyTds::Client.new(ENV['MSSQL_CONNECTION_STRING'])
end

Disable detailed errors and sanitize responses

Ensure Sinatra does not return stack traces in production, and avoid logging sensitive headers or parameters that may contain API keys.

configure :production do
  set :show_exceptions, false
  set :dump_errors, false
end

before do
  # Prevent logging of headers that may carry keys
  request.safe_headers
end

Validate and restrict input

Apply strict allowlists on key formats and enforce rate limiting to reduce brute-force risks that could lead to exposure through repeated Mssql connection attempts.

helpers do
  def valid_key_format?(key)
    key.to_s.match?(\A[a-zA-Z0-9\-_]{32,64}\z)
  end
end

post '/use_key' do
  key = params['api_key']
  halt 400, { error: 'invalid' }.to_json unless valid_key_format?(key)
  # Proceed safely
end