Sandbox Escape in Aspnet with Dynamodb

A sandbox escape in the context of an ASP.NET application that uses Amazon DynamoDB typically involves an attacker who has achieved limited code execution or unsafe deserialization within the application leveraging DynamoDB as a backend store. Because DynamoDB is often used to store configuration, user metadata, or application state, an attacker can manipulate stored items to influence how the application behaves, potentially bypassing intended access controls or runtime constraints.

In ASP.NET, common vectors leading to a sandbox escape include unsafe deserialization of objects retrieved from DynamoDB, insecure permission boundaries around IAM credentials used by the application, and over-permissive trust levels in dynamically compiled or reflection-based code that processes DynamoDB responses. For example, if an application stores serialized user data or policy objects in DynamoDB and later reconstructs them without strict type validation or with full trust, an attacker may embed malicious assemblies or crafted payloads that escape the application domain or execute unintended operations when deserialized.

DynamoDB-specific risks arise from how data is modeled and retrieved. Because DynamoDB is a NoSQL store, developers may store complex nested structures or use sparse attributes. If the application uses these attributes to drive dynamic behavior—such as selecting which modules to load or which handlers to invoke—manipulated or injected attribute values can redirect execution flow. Additionally, if the application uses the AWS SDK for .NET with default credentials chaining in environments where instance metadata is reachable, a compromised application container might leverage the DynamoDB API permissions to escalate access beyond the intended sandbox, for instance by calling sensitive AWS APIs.

Consider an ASP.NET Core service that retrieves an item from a DynamoDB table and uses a Type from the item to perform deserialization or dynamic method invocation:

var client = new AmazonDynamoDBClient();
var request = new GetItemRequest
{
    TableName = "AppConfig",
    Key = new Dictionary<string, AttributeValue> { { "Key", new ScalarAttributeValue("Policies") } }
};
var response = await client.GetItemAsync(request);
if (response.Item.TryGetValue("Type", out var typeAttr))
{
    var policyType = Type.GetType(typeAttr.S);
    var policyData = JsonConvert.DeserializeObject(response.Item["Data"].S, policyType);
    // Use policyData to make runtime decisions
}

If an attacker can write an item where Type points to a malicious type available in the application’s load context, or where the deserialization logic does not restrict target types, they may execute arbitrary code within the application’s process. This becomes a sandbox escape when the application runs with higher privileges than expected, for example by leveraging IAM credentials assigned to the instance that grant broader AWS access than intended.

Because middleBrick tests unauthenticated attack surfaces and includes checks for SSRF and unsafe consumption patterns, it can surface risks where DynamoDB integrations expose endpoints or behaviors that allow manipulation of stored types or attributes. Findings often highlight missing input validation on data retrieved from DynamoDB and overly permissive runtime trust levels, which are precursors to sandbox escape in ASP.NET environments.

Remediation focuses on strict type handling, input validation, and least-privilege execution boundaries. Avoid deserializing arbitrary types from DynamoDB, and do not use data from DynamoDB to decide which types to load or invoke.

1. Avoid dynamic type resolution

Do not use Type.GetType with strings from DynamoDB. Instead, use a closed set of known types and map via a whitelist:

private static readonly Dictionary<string, Type> KnownPolicyTypes = new()
{
    { "AccessPolicy", typeof(AccessPolicy) },
    { "RateLimitPolicy", typeof(RateLimitPolicy) }
};

if (response.Item.TryGetValue("PolicyType", out var typeAttr) && KnownPolicyTypes.TryGetValue(typeAttr.S, out var policyType))
{
    var policyData = JsonConvert.DeserializeObject<object>(response.Item["Data"].S);
    // Use a common interface instead of policyType-specific logic
    if (policyData is IAccessControlPolicy policy)
    {
        policy.Apply();
    }
}

2. Validate and constrain data from DynamoDB

Treat all DynamoDB responses as untrusted. Validate schemas and lengths before use:

if (response.Item.TryGetValue("Data", out var dataAttr) && dataAttr.S != null)
{
    var json = dataAttr.S;
    if (json.Length > 10_000) throw new SecurityException("Payload too large");
    // Validate expected shape before processing
    using var doc = JsonDocument.Parse(json);
    if (!doc.RootElement.TryGetProperty("requiredField", out _))
        throw new SecurityException("Invalid payload structure");
}

3. Use least-privilege IAM for the SDK client

Configure the AmazonDynamoDBClient with credentials and permissions scoped strictly to the required table and actions. In ASP.NET, avoid using instance profile permissions that allow broad AWS actions:

var config = new AmazonDynamoDBConfig { RegionEndpoint = RegionEndpoint.USEast1 };
// Use explicit credentials with limited scope in production
var client = new AmazonDynamoDBClient(awsAccessKeyId, awsSecretAccessKey, config);

4. Disable risky runtime features

If your ASP.NET application uses runtime compilation or reflection to load handlers based on DynamoDB values, replace with static registration and configuration-driven routing that does not invoke arbitrary types:

// Instead of dynamic loading, use a factory with registered handlers
public interface IPolicyHandler { void Handle(JsonElement data); }
public static class PolicyHandlers { public static Dictionary<string, IPolicyHandler> Registry = new(); }

// Registration at startup
PolicyHandlers.Registry["access"] = new AccessPolicyHandler();

// Usage
if (PolicyHandlers.Registry.TryGetValue(typeKey, out var handler))
{
    handler.Handle(dataElement);
}

5. Mitigate SSRF and network exposure

If the ASP.NET service calls DynamoDB via a backend endpoint or uses user-supplied URLs to configure SDK behavior, validate and restrict endpoints to prevent SSRF that could lead to further escape via DynamoDB metadata:

// Reject non-expected hostnames for any constructed service URLs
var allowedHosts = new HashSet<string> { "dynamodb.us-east-1.amazonaws.com" };
if (!allowedHosts.Contains(uri.Host))
    throw new SecurityException("Blocked endpoint");