Nosql Injection in Aspnet with Dynamodb

NoSQL injection in an ASP.NET application that uses Amazon DynamoDB typically occurs when application code builds query parameters, filter expressions, or key condition expressions by concatenating user input without validation or parameterization. In a black-box scan, middleBrick tests this unauthenticated attack surface by submitting crafted payloads that attempt to change the semantics of DynamoDB requests, bypass authorization checks, or force broader data retrieval.

DynamoDB’s query and scan APIs accept expression strings (e.g., KeyConditionExpression, FilterExpression, ExpressionAttributeNames, ExpressionAttributeValues). If ASP.NET code interpolates raw input into these strings, an attacker can inject logical operators such as OR and AND to always-true conditions, or exploit type confusion between numeric and string values. For example, a user ID taken directly from a query string could turn PK = :uid into PK = :uid OR begins_with(PK, 'admin'), exposing other partitions. Because DynamoDB does not support parameterized queries in the same way as SQL, developers must explicitly separate expression names and values; failing to do so results in injection.

In an ASP.NET Core controller, a vulnerable endpoint might construct a query by concatenating a partition key value provided by the client. middleBrick’s authentication and BOLA/IDOR checks will probe whether an attacker can manipulate identifiers to access other users’ resources. Input validation checks will also attempt payloads such as {"$ne": ""} or {"username": {"$gt": ""}} to see if they are interpreted as logical operators rather than literal attribute values. When DynamoDB’s response reflects unexpected items or reveals additional attributes, the scan reports a finding with severity high and references the broader OWASP API Top 10 and Injection category.

Because DynamoDB is a managed NoSQL service, there is no server-side SQL-like parser to provide safety nets; the responsibility is entirely on the client-side code. ASP.NET applications that use the AWS SDK for .NET must treat expression attribute names and values as untrusted, apply strict allow-lists on identifiers, and avoid building expressions through string concatenation. middleBrick’s LLM/AI Security checks are designed to surface API endpoints that are susceptible to prompt or injection-like techniques, which in this context maps to injection-style manipulation of expression semantics.

Remediation focuses on strict separation of expression structure from data, allow-listing identifiers, and using the SDK’s built-in parameterization for attribute values. The AWS SDK for .NET provides ExpressionAttributeName and ExpressionAttributeValue dictionaries specifically to avoid string interpolation. Always prefer parameterized condition expressions over concatenated strings.

In an ASP.NET Core controller, a safe pattern for retrieving an item by partition key and sort key looks like this:

var request = new GetItemRequest
{
    TableName = "MyTable",
    Key = new Dictionary<string, AttributeValue>
    {
        ["PK"] = new AttributeValue { S = partitionKeyValue },
        ["SK"] = new AttributeValue { S = sortKeyValue }
    }
};
var response = await _client.GetItemAsync(request);

For queries with filter expressions, use expression names and values instead of string concatenation:

var queryRequest = new QueryRequest
{
    TableName = "MyTable",
    KeyConditionExpression = "PK = :pkey",
    ExpressionAttributeNames = new Dictionary<string, string>
    {
        ["#pk"] = "PK"
    },
    ExpressionAttributeValues = new Dictionary<string, AttributeValue>
    {
        [":pkey"] = new AttributeValue { S = partitionKeyValue }
    },
    FilterExpression = "AttrType = :t AND begins_with(#pk, :prefix)",
    ExpressionAttributeValues = new Dictionary<string, AttributeValue>
    {
        [":t"] = new AttributeValue { S = "User" },
        [":prefix"] = new AttributeValue { S = partitionKeyValue.Substring(0, Math.Min(3, partitionKeyValue.Length)) }
    }
};
var queryResponse = await _client.QueryAsync(queryRequest);

Important notes: avoid reusing the same key for both ExpressionAttributeNames and ExpressionAttributeValues; keep them distinct to prevent accidental collisions. Validate and allow-list attribute names (e.g., map known safe names from client-supplied identifiers) and reject any input that contains characters not permitted in DynamoDB identifiers. For FilterExpression, prefer server-side validation on attribute value formats instead of dynamic expression building. middleBrick’s CLI tool can be run as middlebrick scan <url> to verify that endpoints no longer reflect injected expressions in responses, and the GitHub Action can enforce a maximum risk score threshold in CI/CD pipelines.

Additionally, apply defensive coding in ASP.NET model binders and middleware to reject unexpected operators such as $ne, $gt, $lt, and logical tokens like OR and AND when they appear in string fields intended to be simple identifiers or values. These practices reduce the likelihood of NoSQL injection and align with findings that map to OWASP API Top 10 and compliance frameworks.