Dns Cache Poisoning in Dynamodb

DNS Cache Poisoning (also called DNS spoofing) targets the integrity of domain name resolution. When an application interacts with Amazon DynamoDB, it typically connects to a regional endpoint like dynamodb.us-east-1.amazonaws.com. If an attacker can poison a recursive DNS resolver or a local host's cache, they can redirect this legitimate hostname to a malicious IP address under their control. In a DynamoDB context, this enables a Man-in-the-Middle (MitM) attack against the database connection.

The attack manifests through specific DynamoDB code paths and configurations:

• Custom Domain Names: Many organizations use CNAME records to point a custom domain (e.g., api.mycompany.com) to the official DynamoDB endpoint. If the DNS for this custom domain is poisoned, all SDK/API calls intended for DynamoDB are routed to the attacker's server.
• VPC Endpoint DNS Override: When using DynamoDB VPC endpoints (interface endpoints), the DNS name dynamodb.us-east-1.amazonaws.com resolves to private IP addresses within the VPC. An attacker with network access who poisons the VPC's DNS resolver (e.g., a compromised EC2 instance running a local resolver) could redirect this to an external IP, exfiltrating traffic.
• SDK Configuration with Hardcoded/Overridden Endpoints: Developer errors, such as manually setting the endpoint parameter in the AWS SDK to a hostname that later gets poisoned, create a direct path for exploitation. This is common in local development or testing environments where a mock server's hostname is used.
• Regional Endpoint Exploitation: The standard DynamoDB endpoint format is predictable. An attacker poisoning DNS for dynamodb.<region>.amazonaws.com can target any application using that region without needing to compromise the custom domain first.

The impact is severe: an attacker can intercept, read, and modify all data in transit between the application and DynamoDB. They can steal AWS credentials if they are sent in headers (though Signature V4 signs the request, the body and headers are visible), inject malicious responses to trigger application logic flaws, or perform actions on the table as the compromised application's IAM role, leading to data destruction or theft. This directly violates the OWASP API Security Top 10's API7:2023 – Server-Side Request Forgery (SSRF) category, as the poisoned DNS is a form of SSRF that forces the server (the application) to make a request to an attacker-controlled destination.

Detecting DNS Cache Poisoning vulnerabilities in a DynamoDB integration requires analyzing both the application's runtime behavior and its configuration. middleBrick performs black-box scanning of the API endpoint, testing for indicators that the underlying DynamoDB connection is susceptible to DNS manipulation.

The scanner focuses on:

• Endpoint Validation: middleBrick checks if the API's responses or error messages leak information about the resolved IP address of the DynamoDB endpoint it uses. For example, detailed error messages might include network diagnostics or connection strings.
• TLS Certificate Validation: The scanner initiates TLS handshakes with the hostname the application uses to reach DynamoDB (whether a standard AWS endpoint or a custom CNAME). It verifies that the presented certificate is issued to *.amazonaws.com or the specific custom domain. A certificate for an unexpected domain (e.g., attacker.com) is a strong indicator of a successful DNS poisoning attack or a misconfigured proxy.
• HTTP Header Analysis: middleBrick inspects response headers for security best practices. The absence of Strict-Transport-Security (HSTS) on responses from the custom domain (if used) weakens the defense against subsequent MitM attacks after an initial poisoning.
• OpenAPI/Swagger Spec Analysis: If an OpenAPI spec is provided or discovered, middleBrick resolves all $ref and examines any servers blocks. It flags configurations where the url for a DynamoDB-backed operation uses a non-Amazon hostname without clear justification, as this expands the DNS attack surface.

A typical middleBrick scan of an API that uses DynamoDB might produce a finding like:

Finding: Potential DNS Spoofing Risk (BOLA/IDOR Category)
Severity: High
Description: The API endpoint 'https://api.example.com/v1/items' appears to proxy requests to DynamoDB. The TLS certificate presented for the upstream host 'dynamodb.custom.corp' is not issued by Amazon. This suggests a possible DNS cache poisoning or a malicious proxy. An attacker could intercept and modify database traffic.
Evidence: TLS handshake to 'dynamodb.custom.corp' returned certificate for '*.malicious-domain.net'.
Remediation: Verify the DNS resolution for all DynamoDB endpoints. Enforce TLS certificate pinning for AWS service endpoints. Use VPC Interface Endpoints to keep traffic within the AWS network and avoid public DNS resolution.

Developers and security teams can use the middleBrick CLI to scan their API endpoints directly from their terminal: middlebrick scan https://api.example.com. The output in JSON format will include any DNS-related findings under the relevant categories. Integrating the middleBrick GitHub Action into a CI/CD pipeline allows for automatic detection of such misconfigurations before deployment, failing the build if a high-severity DNS spoofing risk is introduced.

Remediation focuses on eliminating the single point of failure—untrusted DNS resolution—when connecting to DynamoDB. The fixes leverage AWS-native features and secure SDK configurations.

1. Use VPC Interface Endpoints for Private Connectivity
The most robust defense is to avoid public DNS resolution entirely for DynamoDB traffic. Create a DynamoDB Interface VPC Endpoint (powered by AWS PrivateLink). This assigns private IP addresses within your VPC to the DynamoDB service. Your application, running in the same VPC, connects to these private IPs. The DNS name dynamodb.us-east-1.amazonaws.com now resolves to these private IPs via the VPC's DNS resolver, which cannot be poisoned from outside the VPC.

// AWS Infrastructure (Terraform example)
resource "aws_vpc_endpoint" "dynamodb" {
  vpc_id       = aws_vpc.main.id
  service_name = "com.amazonaws.us-east-1.dynamodb"
  vpc_endpoint_type = "Interface"
  private_dns_enabled = true // Critical for standard SDK compatibility
}

2. Enforce TLS Certificate Validation and Pinning
Ensure the AWS SDK is configured to strictly validate the server's TLS certificate against Amazon's trusted CA. Do not disable certificate validation (NODE_TLS_REJECT_UNAUTHORIZED=0 in Node.js is a catastrophic misconfiguration). For high-security applications, implement certificate pinning for the Amazon root CA or specific DynamoDB endpoint certificates within the application code.

// Node.js AWS SDK v3 - Correct Configuration
import { DynamoDBClient } from "@aws-sdk/client-dynamodb";

const client = new DynamoDBClient({
  region: "us-east-1",
  tls: true, // Default is true, but be explicit
  // The SDK inherently validates the certificate chain for *.amazonaws.com
});

// For custom domains, ensure the certificate matches the domain exactly.
// Do NOT use a self-signed certificate without adding it to the trust store.

// Python (boto3) - Secure Configuration
import boto3

session = boto3.Session()
dynamodb = session.client(
    'dynamodb',
    region_name='us-east-1',
    # boto3 uses the system's CA bundle by default. Ensure it's up-to-date.
    # To pin a certificate, you would need a custom HTTP adapter.
)

3. Avoid Custom Domain Names for DynamoDB Endpoints
Do not create CNAME records that point to DynamoDB endpoints unless absolutely necessary. If you must use a custom domain (e.g., for legacy reasons), you must:

• Implement DNSSEC on your domain to protect against DNS spoofing at the resolver level.
• Use a DNS provider that offers DNS over HTTPS (DoH) or DNS over TLS (DoT) for your recursive resolvers.
• Combine with strict TLS certificate pinning in the application as described above.

4. Implement Network-Level Controls
Use Security Groups and Network ACLs to restrict outbound traffic from your application servers to only the known IP ranges of AWS DynamoDB service endpoints (published in the AWS IP address ranges JSON file). This acts as a failsafe if DNS is poisoned and the attacker's IP is not in the allowed set.

5. Regular Auditing
Use tools like dig or nslookup to periodically verify the IP addresses your custom domains resolve to, comparing them against the expected AWS service IPs. Automate this check in your monitoring.

By using VPC endpoints, you fundamentally remove the dependency on public DNS for DynamoDB, which is the core of the poisoning attack. The other measures provide defense-in-depth.

DNS Cache Poisoning against DynamoDB is a high-impact, low-complexity attack that bypasses traditional application-layer security controls. It exploits the fundamental trust applications place in the DNS system to locate their database service. The vulnerability is not in DynamoDB itself, but in the network path and configuration surrounding it. Mitigation requires a shift from trusting public DNS to using private, AWS-controlled network paths via VPC Interface Endpoints, combined with rigorous TLS practices. Regular scanning of your API's external behavior with a tool like middleBrick can help identify if your application inadvertently leaks signals of a compromised DNS chain or if your custom domain configurations introduce unnecessary risk. Remember, the scanner detects the symptom—the potential for traffic redirection—and the remediation lies in securing the underlying network and DNS architecture.