Injection Flaws in Rails with Dynamodb

DynamoDB is a NoSQL database, and Rails applications often interact with it through the AWS SDK rather than an ORM that enforces strict query parameterization. Injection flaws occur when application input is directly interpolated into DynamoDB API calls, particularly in expression parameter values, condition expressions, and key schema usage. Unlike SQL, DynamoDB does not support prepared statements in the same way, so developers must manually avoid concatenating user input into request structures.

In a Rails context, this risk is amplified when developers map RESTful routes and params directly into DynamoDB request hashes. For example, using params[:id] in a KeyConditionExpression without validation or escaping can lead to NoSQL injection. Attackers may supply values like 1 OR #attr = :val where #attr is a reserved word placeholder and :val is an injected condition, altering query logic.

Another common pattern is using Scan or Query with a FilterExpression that includes unchecked user input. If a developer writes FilterExpression: "user_id = #{user_input}", the input is not parameterized, enabling injection through specially crafted values that change the filter semantics or cause unexpected item access.

Reserved words in DynamoDB can also be abused. If user input is used to specify attribute names without proper placeholder syntax (e.g., {"id" => params[:id]} for key names), attackers may inject expressions that reference reserved keywords, causing logical bypass or data exposure.

These patterns violate the principle of least privilege and can lead to unauthorized data access or manipulation, mapping to OWASP API Top 10 A03:2021 Injection. Real-world examples include scenarios where an attacker enumerates users by injecting condition expressions or extracts sensitive data via crafted scan filters.

Remediation centers on strict input validation, use of expression placeholders, and avoiding dynamic construction of request parameters. Always treat user input as untrusted and never directly interpolate values into DynamoDB request structures.

1. Use Expression Attribute Names and Values

For key conditions and filter expressions, use placeholder tokens to separate structure from data. This prevents attackers from altering query logic.

require 'aws-sdk-dynamodb'

# Unsafe: direct interpolation (vulnerable)
# client.query(
#   table_name: 'Users',
#   key_condition_expression: "user_id = #{params[:id]}"
# )

# Safe: using placeholders
client = Aws::DynamoDB::Client.new(region: 'us-east-1')
response = client.query({
  table_name: 'Users',
  key_condition_expression: 'user_id = :uid',
  expression_attribute_names: { '#u' => 'user_id' }, # optional, for reserved words
  expression_attribute_values: { ':uid' => params[:id] }
})

2. Validate and Whitelist Input for Key Operations

Ensure that inputs used in key conditions conform to expected formats. For numeric IDs, enforce integer constraints; for string identifiers, use allowlists or strict regex patterns.

def safe_query(user_id)
  # Validate input: must be a positive integer
  unless user_id =~ /^\d+$
    raise ArgumentError, 'Invalid user identifier'
  end

  client.query({
    table_name: 'Orders',
    key_condition_expression: 'user_id = :uid',
    expression_attribute_values: { ':uid' => user_id }
  })
end

3. Avoid Dynamic FilterExpression Construction

Never build FilterExpression strings via concatenation. Use expression attribute values for all data inputs.

# Unsafe
# filter = "status = '#{params[:status]}' AND category = '#{params[:category]}'"

# Safe
response = client.scan({
  table_name: 'Products',
  filter_expression: 'status = :s AND category = :c',
  expression_attribute_values: {
    ':s' => params[:status],
    ':c' => params[:category]
  }
})

4. Handle Reserved Words Explicitly

When attribute names match DynamoDB reserved words (e.g., order, status), always use expression_attribute_names to map placeholders.

response = client.scan({
  table_name: 'Items',
  filter_expression: '#s = :status_val',
  expression_attribute_names: { '#s' => 'status' },
  expression_attribute_values: { ':status_val' => 'active' }
})

5. Use Strong Parameter Patterns in Rails Controllers

Leverage Rails parameter sanitization before passing values to DynamoDB calls. This adds a layer of validation consistent with Rails conventions.

def item_params
  params.require(:item).permit(:id, :status, :category)
end

# In action
def show
  item = safe_query(item_params[:id])
  render json: item
end

6. Prefer Query over Scan When Possible

Scan operations examine every item and are more prone to injection if filter expressions are dynamic. Use Query with indexed key attributes to reduce risk and improve performance.

# Prefer this
client.query({
  table_name: 'Sessions',
  key_condition_expression: 'session_token = :token',
  expression_attribute_values: { ':token' => SecureRandom.hex }
})

# Over this (if scan is unavoidable)
# client.scan(table_name: 'Sessions', filter_expression: "session_token = '#{token}'")

These practices align with middleBrick’s checks for Input Validation and help reduce the risk of injection. The scanner can detect unsafe patterns such as string interpolation in expression fields and flag them for review.