Crlf Injection in Rails with Dynamodb

Crlf Injection occurs when an attacker can inject carriage return (CR, \r) and line feed (\n) characters into an HTTP header or a log entry, causing header splitting or log forging. In a Rails application that uses Amazon DynamoDB as a persistence layer, this risk exists at the intersection of Rails request handling, the application’s use of DynamoDB operations, and the data stored in DynamoDB items.

DynamoDB itself does not interpret HTTP headers or newlines in the same way a web server does; however, Rails constructs HTTP responses and logs that include data read from or written to DynamoDB. If user-controlled input (such as a parameter used in a DynamoDB PutItem or UpdateItem) is reflected in HTTP headers, Set-Cookie values, or log messages without sanitization, an attacker can inject CRLF sequences. For example, a user-supplied name or identifier stored in a DynamoDB table and later rendered in a response header can enable response splitting, session fixation, or log injection.

Consider a Rails controller that retrieves a user profile from DynamoDB and sets a custom header with the user’s display name:

user = dynamodb.get_item(table_name: 'Users', key: { user_id: user_id })
username = user['item']['username']['S']
response.headers['X-User-Display'] = username

If the username contains \r\nSet-Cookie: auth=attacker, the injected CRLF can split the header and set an arbitrary cookie. DynamoDB stores the raw string, so if the application does not validate or encode data on read, stored attacker-controlled values become a vector when used in headers or logs. Logging is another area: Rails logs DynamoDB responses or errors; if a stored item contains CRLF, the log entry can be forged to inject additional log lines or hide actions.

The DynamoDB scan or query results may also include user-controlled attributes used in pagination tokens or URLs. If these values are placed into HTTP Location headers or JSON responses without sanitization, CRLF injection can occur. Because DynamoDB is often used for structured storage, nested attributes may still contain newline characters when originally input via forms or APIs. Rails’ default parameter parsing and log formatting will treat bare CR/LF as line breaks, enabling the injection when the data is later rendered in headers or logs.

In summary, the vulnerability arises not from DynamoDB itself but from the way Rails uses data retrieved from DynamoDB in contexts that interpret CRLF as control characters: HTTP headers, Set-Cookie, and log lines. The risk is compounded when the application trusts data stored in DynamoDB as safe simply because it came from a database.

Remediation focuses on preventing CRLF characters from being interpreted as control characters when data from DynamoDB is used in HTTP headers, cookies, or logs. Input validation at the boundary and safe output encoding are essential.

1. Validate and sanitize DynamoDB inputs

Ensure that any user-supplied data stored in DynamoDB does not contain CR or LF characters. Normalize and reject such values at the model or service layer.

class UserService
  CRLF = /[\r\n]/

  def self.store_user(params)
    raise ArgumentError, 'CRLF not allowed' if params[:username] =~ CRLF

    dynamodb.put_item(
      table_name: 'Users',
      item: {
        user_id: { s: params[:user_id] },
        username: { s: params[:username] }
      }
    )
  end
end

2. Encode data when reflecting DynamoDB values into headers

Use strict header assignment and avoid directly inserting raw DynamoDB strings into headers. Replace or encode CR/LF and ensure only one header line is produced.

user = dynamodb.get_item(table_name: 'Users', key: { user_id: user_id })
username = user['item']['username']['S']
# Remove or replace CRLF before using in headers
safe_username = username.gsub(/[\r\n]/, '_')
response.headers['X-User-Display'] = safe_username

3. Avoid CRLF in logging and error messages

Sanitize any DynamoDB data that may be included in logs. Replace newlines to prevent log injection and ensure log lines remain singular.

def log_user_activity(user_id, action)
  item = dynamodb.get_item(table_name: 'Users', key: { user_id: user_id })
  username = item['item']['username']['S']
  # Replace CR/LF to prevent log forging
  clean_username = username.gsub(/[\r\n]/, ' ')
  Rails.logger.info("user=#{clean_username} action=#{action}")
end

4. Use strong parameter filtering and allowlists

Treat DynamoDB data as untrusted. When building responses that include DynamoDB fields, apply Rails’ built-in helpers and avoid inserting raw strings into headers or cookies. For JSON APIs, ensure serialization escapes control characters appropriately.

def show
  user = dynamodb.get_item(table_name: 'Users', key: { user_id: params[:id] })
  render json: {
    user_id: user['item']['user_id']['S'],
    username: user['item']['username']['S'].gsub(/[\r\n]/, '')
  }
end

5. MiddleBrick scans for CRLF in API responses

Use the middlebrick scan <url> CLI or Web Dashboard to detect CRLF injection risks across endpoints that consume DynamoDB data. The scanner checks for missing input validation and unsafe reflection of stored data into headers and logs, helping you find problematic patterns without manual code review.