Api Key Exposure in Grape with Dynamodb

Grape is a REST-like API micro-framework for Ruby, commonly used to build endpoints that return sensitive data or authentication tokens. When a Grape-based service integrates with Amazon DynamoDB to store or retrieve API keys, improper handling can lead to API key exposure. This typically occurs when keys are stored in plaintext, when responses inadvertently include key material, or when authorization checks are bypassed due to misconfigured resource policies.

DynamoDB stores items as JSON-like documents. If an item contains an attribute such as api_key or secret, and the application returns the entire item or a subset without filtering, the key can be exposed. For example, a GET endpoint that retrieves user configuration may return the full item including the key, especially if the developer uses DynamoDB.get_item without projection expressions.

Additionally, IAM policies attached to the role used by the Grape service may grant broader dynamodb:GetItem or dynamodb:Scan permissions than necessary. Overly permissive policies can allow an attacker who compromises the service or tricks the endpoint into scanning to extract items containing keys. Insufficient validation of input parameters (e.g., the partition key) may lead to unauthorized access to other users’ items, inadvertently returning keys belonging to other accounts.

The combination of Grape endpoints that directly proxy to DynamoDB increases risk because errors in parameter handling, response serialization, or error handling can surface raw key values in logs, HTTP responses, or client-side storage. For instance, if an exception handler dumps the item for debugging, the key may appear in application logs or error messages returned to the caller.

Consider a vulnerable Grape endpoint that retrieves a user’s API key from DynamoDB:

require 'aws-sdk-dynamodb'
class KeyResource & Grape::API
  resource :keys do
    get ':user_id' do
      client = Aws::DynamoDB::Client.new(region: 'us-east-1')
      resp = client.get_item(
        table_name: 'AppSecrets',
        key: { 'user_id' => { s: params[:user_id] } }
      )
      item = resp.item
      { api_key: item['api_key'], status: 'ok' } if item
    end
  end
end

This example exposes the API key in the JSON response because the code returns item['api_key'] directly. An attacker who compromises the client or intercepts traffic can obtain the key. Moreover, if the AppSecrets table contains multiple items and the endpoint does not enforce strict ownership checks, a user could modify user_id to access another user’s key (Insecure Direct Object Reference/IDOR).

Middleware security scans, such as those provided by services like middleBrick, detect this class of issue by examining endpoints that return sensitive attributes and by testing for missing authorization on object references. These scans can identify whether responses include keys, whether input validation is insufficient, and whether IAM configurations expose excessive read permissions.

To remediate API key exposure when using DynamoDB with Grape, apply least privilege, filter responses, and enforce strict ownership checks. Avoid returning raw key attributes in API responses; instead, return opaque references or tokens when possible. Use projection expressions in DynamoDB requests to retrieve only non-sensitive metadata, and ensure IAM policies restrict actions to the minimum required attributes.

Below is a secure version of the earlier endpoint that excludes the key from responses and validates ownership:

require 'aws-sdk-dynamodb'
class KeyResource & Grape::API
  resource :keys do
    helpers do
      def current_user
        # Assume authentication sets current_user with an id
        @current_user ||= User.find_by(id: params[:user_id])
      end
    end

    get ':user_id' do
      user_id = params[:user_id]
      raise Grape::Errors::Forbidden unless current_user&.id == user_id

      client = Aws::DynamoDB::Client.new(region: 'us-east-1')
      resp = client.get_item(
        table_name: 'AppSecrets',
        key: { 'user_id' => { s: user_id } },
        projection_expression: 'created_at,status' # Exclude api_key
      )
      item = resp.item
      { status: item ? 'active' : 'not_found' } if item
    end
  end
end

If you must return the key (for example, to the authenticated user during initial creation), ensure it is transmitted over TLS and never logged. Use environment variables or secret management integrations for keys at rest, and avoid storing them in DynamoDB when alternatives exist. When storage is necessary, enable DynamoDB encryption at rest and use IAM conditions to restrict access by requester attributes.

Define an IAM policy that limits the action to specific attributes and enforces ownership conditions:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "dynamodb:GetItem"
      ],
      "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/AppSecrets",
      "Condition": {
        "ForAllValues:StringEquals": {
          "dynamodb:LeadingKeys": ["${cognito-identity.amazonaws.com:sub}"]
        }
      }
    }
  ]
}

This policy ensures that a request can only access items where the partition key matches the authenticated user’s identity, reducing the risk of horizontal privilege escalation. Combine this with input validation to ensure the user_id parameter conforms to expected formats and reject requests that attempt to traverse paths or include special characters.

Finally, rotate keys if exposure is suspected, and audit access patterns using DynamoDB Streams or AWS CloudTrail to detect anomalous scans or unauthorized get operations. Tools like middleBrick can be used to verify that endpoints no longer expose sensitive attributes and that authorization logic correctly restricts access.