Dns Rebinding in Dynamodb

Dns rebinding attacks exploit the gap between how browsers resolve DNS names and how backend services validate origins. When a malicious site controls DNS resolution timing, it can trick a browser into connecting to internal services that trust requests from localhost or trusted domains.

In DynamoDB contexts, this manifests through several specific attack vectors:

• Local DynamoDB endpoint abuse: Developers often use http://localhost:8000 for DynamoDB Local during development. A rebinding attack can make a browser connect to this endpoint when the user visits a malicious site, potentially exposing development credentials stored in environment variables.
• VPC endpoint enumeration: If an application uses VPC endpoints for DynamoDB (like vpce-123456789abcdef0.us-east-1.vpce.amazonaws.com), a rebinding attack might attempt to resolve and connect to internal VPC resources that trust traffic from within the same network.
• Cross-account metadata service access: Some DynamoDB configurations integrate with EC2 instance metadata services. A successful rebinding could potentially expose metadata that contains temporary credentials.

The attack typically follows this pattern: A malicious site serves a page that loads a script from a domain it controls. The DNS record for this domain has a very short TTL (like 1 second). The script makes requests to what appears to be an external API, but the DNS resolution switches mid-connection to an internal IP address. If the DynamoDB client or SDK doesn't properly validate the endpoint's origin, it may accept these connections.

Here's a vulnerable pattern you might see in Node.js applications:

const AWS = require('aws-sdk');
const ddb = new AWS.DynamoDB({
  endpoint: process.env.DYNAMODB_ENDPOINT || 'http://localhost:8000',
  region: process.env.AWS_REGION || 'us-east-1'
});

// Vulnerable: No validation of endpoint origin
async function scanDynamo(tableName) {
  const params = { TableName: tableName };
  return await ddb.scan(params).promise();
}

This code trusts whatever endpoint is configured without validating that it's the intended service. A rebinding attack could redirect requests to a malicious local service that mimics DynamoDB's API, capturing all requests and credentials.

Detecting Dns rebinding vulnerabilities in DynamoDB implementations requires examining both configuration and runtime behavior. Here are specific detection methods:

1. Endpoint validation scanning: Check if DynamoDB clients validate that endpoints match expected patterns. Legitimate DynamoDB endpoints should match patterns like *.amazonaws.com or specific VPC endpoint formats.
2. Credential exposure analysis: Scan for hardcoded credentials or environment variables that might be exposed if a malicious endpoint is reached.
3. Network boundary verification: Verify that DynamoDB clients don't accept connections to private IP ranges (10.x.x.x, 172.16-31.x.x, 192.168.x.x) unless explicitly configured for VPC endpoints.
4. Time-based DNS resolution checks: Test if applications handle DNS changes appropriately during long-lived connections.

middleBrick's DynamoDB-specific scanning includes these checks:

Using middleBrick's CLI for DynamoDB security scanning:

npm install -g middlebrick
middlebrick scan https://yourapi.com/dynamodb-endpoint \
  --category=dynamodb \
  --output=json > dynamodb-report.json

The scan tests your API's DynamoDB integration by attempting to identify misconfigurations that could enable Dns rebinding. It checks endpoint validation, credential handling, and network boundary configurations specific to DynamoDB's authentication and connection patterns.

For GitHub Actions integration:

- name: DynamoDB Security Scan
  uses: middlebrick/middlebrick-action@v1
  with:
    url: https://yourapi.com/dynamodb-endpoint
    category: dynamodb
    fail-threshold: C
  env:
    MIDDLEBRICK_API_KEY: ${{ secrets.MIDDLEBRICK_API_KEY }}

This automatically scans your DynamoDB endpoints in CI/CD, failing builds if vulnerabilities like Dns rebinding are detected.

Securing DynamoDB against Dns rebinding requires specific code patterns and configuration practices. Here are DynamoDB-specific remediations:

1. Strict Endpoint Validation

Always validate that DynamoDB endpoints match expected patterns:

const AWS = require('aws-sdk');

function validateDynamoEndpoint(endpoint) {
  const validPatterns = [
    /.*\.amazonaws\.com$/,           // Standard AWS endpoints
    /vpce-[a-f0-9]{17}\..*\.vpce\.amazonaws\.com$/ // VPC endpoints
  ];
  
  if (!validPatterns.some(pattern => pattern.test(endpoint))) {
    throw new Error(`Invalid DynamoDB endpoint: ${endpoint}`);
  }
}

const ddbEndpoint = process.env.DYNAMODB_ENDPOINT || 'https://dynamodb.us-east-1.amazonaws.com';
validateDynamoEndpoint(ddbEndpoint);

const ddb = new AWS.DynamoDB({
  endpoint: ddbEndpoint,
  region: process.env.AWS_REGION || 'us-east-1'
});

2. Credential Security

Use IAM roles instead of hardcoded credentials, and implement credential validation:

const AWS = require('aws-sdk');
const { validateIAMRole } = require('./iam-utils');

async function createSecureDynamoClient() {
  // Verify we have valid credentials
  const sts = new AWS.STS();
  try {
    const identity = await sts.getCallerIdentity().promise();
    if (!validateIAMRole(identity.Arn)) {
      throw new Error('DynamoDB access denied for this IAM role');
    }
  } catch (error) {
    throw new Error('Invalid or missing AWS credentials');
  }

  return new AWS.DynamoDB({
    region: process.env.AWS_REGION || 'us-east-1'
  });
}

function validateIAMRole(arn) {
  const dynamodbAllowedRoles = [
    'arn:aws:iam::*:role/DynamoDBFullAccess',
    'arn:aws:iam::*:role/DynamoDBReadOnly'
  ];
  return dynamodbAllowedRoles.some(pattern => arn.startsWith(pattern.replace('*', '')));
}

3. Network Boundary Enforcement

Prevent connections to private networks unless explicitly configured:

const { URL } = require('url');

function isPrivateIP(host) {
  const privateRegex = /^(
    10\.\d{1,3}\.\d{1,3}\.\d{1,3}
    |172\.(?:1[6-9]|2\d|3[01])\.\d{1,3}\.\d{1,3}
    |192\.168\.\d{1,3}\.\d{1,3}
    |localhost
    |127\.\d{1,3}\.\d{1,3}\.\d{1,3}
  )$/x;
  
  return privateRegex.test(host);
}

function createSecureDynamoClient() {
  const endpoint = process.env.DYNAMODB_ENDPOINT || 'https://dynamodb.us-east-1.amazonaws.com';
  const url = new URL(endpoint);
  
  if (isPrivateIP(url.hostname) && !process.env.ALLOW_PRIVATE_DYNAMODB) {
    throw new Error('Private DynamoDB endpoints are not allowed');
  }
  
  return new AWS.DynamoDB({
    endpoint: endpoint,
    region: process.env.AWS_REGION || 'us-east-1'
  });
}

4. VPC Endpoint Whitelisting

When using VPC endpoints, validate the specific VPC ID:

function validateVPCFormat(endpoint) {
  const vpcPattern = /^vpce-[a-f0-9]{17}\..*\.vpce\.amazonaws\.com$/;
  if (!vpcPattern.test(endpoint)) {
    throw new Error('Invalid VPC endpoint format');
  }
  
  // Extract VPC ID from endpoint
  const vpcId = endpoint.split('.')[1];
  const allowedVpcs = ['vpc-12345678', 'vpc-87654321']; // From config
  
  if (!allowedVpcs.includes(vpcId)) {
    throw new Error(`VPC endpoint not in allowed list: ${vpcId}`);
  }
}

These remediations work together to create a defense-in-depth approach against Dns rebinding attacks targeting DynamoDB implementations. The key principles are: validate all endpoints, never trust localhost or private IPs without explicit configuration, use IAM roles with principle of least privilege, and implement network boundary checks.