Injection Flaws in Aspnet with Dynamodb

Injection flaws in an ASP.NET application that uses Amazon DynamoDB typically arise when user-controlled input is concatenated into DynamoDB API calls, especially within low-level condition expressions or key attribute values. Because DynamoDB uses a JSON-like data model and condition expressions are string-based, unsanitized input can alter query intent, leading to unintended reads, writes, or privilege escalation. This risk is pronounced when developers map HTTP query parameters or JSON payload fields directly into DynamoDB KeyConditionExpression, FilterExpression, or ExpressionAttributeNames without validation.

ASP.NET endpoints often expose data access logic via route or query parameters (e.g., GET /orders/{orderId}). If the orderId is used to build a DynamoDB key without type or format checks, an attacker can inject logical operators or wildcards. For example, an input like orderId = "12345 OR #status = :open", when placed into a KeyConditionExpression or FilterExpression, can change access patterns or bypass intended partition key restrictions. DynamoDB does not support SQL-style injection, but expression-language injection is possible through malformed attribute names or values used in condition expressions.

The combination of ASP.NET’s model binding and DynamoDB’s expression syntax increases the attack surface in two practical ways. First, if an endpoint uses ExpressionAttributeNames to map reserved words (e.g., status or order), attacker-controlled input can overwrite these names to point to different item attributes. Second, if numeric or string inputs are weakly validated, an attacker may supply operators such as begins_with, contains, or comparison operators to manipulate result sets. This can lead to Insecure Direct Object Reference (IDOR) or Horizontal Privilege Escalation when users access items outside their scope by injecting alternate key conditions.

Real-world patterns include concatenating strings to build KeyConditionExpression or using raw user input in Select projections. For instance, constructing an expression like "PK = " + partitionKey + " AND SK = " + sortKey from request data enables injection if the keys are not strictly validated. Another scenario involves using untrusted input in FilterExpression to bypass intended filtering, effectively exposing more data than intended. These issues map to OWASP API Top 10 categories such as Broken Object Level Authorization (BOLA) and Injection, and can be discovered during unauthenticated scans when endpoints expose DynamoDB-derived responses without proper authorization checks.

middleBrick detects these risks through black-box testing of the unauthenticated attack surface, checking for missing input validation, improper use of expression attributes, and overly broad data exposure in DynamoDB-driven endpoints. Findings include severity ratings and remediation guidance to help developers tighten expression construction and enforce strict schema-aware validation before constructing DynamoDB API calls.

To remediate injection flaws when using DynamoDB in ASP.NET, treat all user input as untrusted and never directly interpolate request data into DynamoDB expressions. Use strongly typed models, input validation, and the AWS SDK’s built-in expression builders to ensure attribute names and values are properly escaped. Below are concrete patterns that reduce risk.

1. Use ExpressionBuilder for KeyConditionExpression

Avoid string concatenation for key expressions. Instead, use the AWS SDK’s ExpressionBuilder (or construct expressions with explicit attribute value placeholders). This ensures partition key and sort key values are passed separately and safely.

// Good: Explicit key construction using SDK patterns
var request = new QueryRequest
{
    TableName = "Orders",
    KeyConditionExpression = "PK = :pk AND SK = :sk",
    ExpressionAttributeValues = new Dictionary<string, AttributeValue>
    {
        { ":pk", new AttributeValue { S = $"ORDER#{orderId}" } },
        { ":sk", new AttributeValue { S = sortKey } }
    }
};
var response = await dynamoClient.QueryAsync(request);

2. Validate and constrain input formats

Enforce strict format rules for IDs and keys in ASP.NET model binders or action filters. Reject unexpected characters (e.g., logical operators, semicolons) and ensure length and charset constraints match your domain model.

// Example validation in ASP.NET Core MVC
public class OrderController : ControllerBase
{
    [HttpGet("/orders/{orderId}")]
    public IActionResult GetOrder(string orderId)
    {
        if (!Regex.IsMatch(orderId, @"^ORDER#[A-Za-z0-9_-]{1,36}$"))
        {
            return BadRequest("Invalid order identifier");
        }

        var request = new GetItemRequest
        {
            TableName = "Orders",
            Key = new Dictionary<string, AttributeValue>
            {
                { "PK", new AttributeValue { S = $"ORDER#{orderId}" } },
                { "SK", new AttributeValue { S = $"METADATA" } }
            }
        };
        // proceed with safe DynamoDB call
    }
}

3. Use ExpressionAttributeNames safely with allowlisting

If you must use ExpressionAttributeNames, maintain an allowlist of known attribute names and map only those. Do not pass user input directly into attribute name placeholders.

// Safe allowlist-based attribute name mapping
var attributeNames = new Dictionary<string, string>
{
    { "#dt", "DeliveryTime" },
    { "#st", "Status" }
};

var filter = new FilterExpression()
    .WithCondition(new Condition("#dt", ConditionOperator.GreaterThan, DateTime.UtcNow)
    .WithCondition(new Condition("#st", ConditionOperator.Equal, "Open"));

var request = new ScanRequest("Orders")
    .WithFilterExpression(filter)
    .WithExpressionAttributeNames(attributeNames);

4. Prefer strongly-typed DocumentModel over low-level JSON patches

When possible, use the DocumentModel pattern to avoid manual expression assembly. This reduces the surface for injection because attribute names are defined in code, not built from strings.

public class Order
{
    public string PK { get; set; }
    public string SK { get; set; }
    public string Status { get; set; }
}

// Safe retrieval using DocumentModel
var doc = await context.LoadAsync<Order>(partitionKey, sortKey);

5. Apply defense-in-depth: validation, allowlists, and least privilege

Combine input validation, tight IAM policies, and runtime monitoring. Ensure the DynamoDB credentials used by ASP.NET have the minimum required permissions (e.g., dynamodb:GetItem for read-only endpoints). Log and alert on anomalous query patterns that may indicate injection attempts.

6. MiddleBrick integration for continuous detection

Use the middleBrick CLI to scan endpoints regularly: middlebrick scan <url>. The GitHub Action can enforce a security score threshold in CI/CD, while the MCP Server lets you scan APIs directly from your IDE. The dashboard tracks findings over time, helping you detect regressions in expression handling.