Dns Cache Poisoning in Express with Dynamodb

Dns Cache Poisoning in an Express application that uses DynamoDB can occur when the Express server resolves hostnames for DynamoDB service endpoints or for downstream services referenced in DynamoDB records. If the DNS resolution within the runtime environment is not secured, an attacker may inject a malicious IP address into the DNS cache, redirecting DynamoDB traffic to a rogue endpoint. This can lead to data interception or manipulation when Express queries DynamoDB using the poisoned DNS entry, effectively bypassing intended network segmentation.

The vulnerability is not inherent to DynamoDB itself but is introduced through the interaction between Express runtime networking and the AWS SDK or custom HTTP clients used to communicate with DynamoDB. For instance, if Express uses a hostname such as dynamodb.us-east-1.amazonaws.com and the DNS cache is poisoned, the SDK may unwittingly send requests to an attacker-controlled server that mimics the DynamoDB API surface. This is particularly relevant when DynamoDB streams or DynamoDB Accelerator (DAX) endpoints are used, as they may involve additional hostnames subject to resolution.

Express applications that store or retrieve sensitive data from DynamoDB can inadvertently process maliciously redirected responses. An attacker might exploit this to cause data leakage, inject falsified records into the application’s logic, or trigger unauthorized operations that appear to originate from legitimate DynamoDB responses. The risk is compounded when Express does not validate the identity of the endpoint beyond DNS, such as by enforcing AWS Signature Version 4 or strict certificate pinning for DynamoDB hosts.

In practice, a typical Express route that queries DynamoDB might look like the following, where the AWS SDK resolves the endpoint internally:

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

AWS.config.update({
  region: 'us-east-1'
});

const docClient = new AWS.DynamoDB.DocumentClient();

app.get('/user/:id', async (req, res) => {
  const params = {
    TableName: 'Users',
    Key: {
      id: req.params.id
    }
  };
  try {
    const data = await docClient.get(params).promise();
    res.json(data.Item);
  } catch (err) {
    res.status(500).json({ error: 'Failed to fetch user' });
  }
});

app.listen(3000, () => console.log('Server running on port 3000'));

In this setup, if the DNS entry for dynamodb.us-east-1.amazonaws.com is poisoned, the docClient.get call may reach a malicious host. The Express application has no inherent mechanism to detect such DNS manipulation, and the AWS SDK will trust the resolved IP, treating the malicious host as a valid DynamoDB endpoint.

middleBrick can help detect whether an API’s DNS resolution behavior exposes unexpected hostnames or whether responses from DynamoDB endpoints lack proper validation. By scanning the unauthenticated attack surface, middleBrick checks for insecure runtime dependencies and provides findings mapped to frameworks such as OWASP API Top 10, including indicators related to data exposure and insecure network design.

To mitigate Dns Cache Poisoning risks in Express applications that interact with DynamoDB, implement host validation and secure communication practices. Avoid relying solely on DNS for endpoint integrity by enforcing strict AWS service endpoint resolution and validating server identity.

• Use explicit regional endpoints with HTTPS and avoid dynamic hostname construction. Hardcode or securely configure the DynamoDB endpoint to reduce reliance on external DNS resolution at runtime.
• Enable AWS Signature Version 4 for all requests and ensure the AWS SDK is configured to validate TLS certificates. Do not disable certificate validation or use insecure protocols.
• Pin the public key hash (certificate pinning) for known DynamoDB endpoints if your runtime environment supports it, particularly when using custom HTTP agents or proxy setups.
• Monitor DNS resolution behavior within your deployment environment and use trusted DNS resolvers with DNSSEC enabled to reduce poisoning probability.
• Apply the principle of least privilege to IAM roles used by the Express application so that even if a request is redirected, the attacker cannot perform destructive actions.

Here is a revised example that configures the AWS SDK with a fixed endpoint and enforces secure defaults:

const AWS = require('aws-sdk');
const https = require('https');
const express = require('express');
const app = express();

// Use a fixed, known-good endpoint to reduce DNS dependency
const fixedEndpoint = 'https://dynamodb.us-east-1.amazonaws.com';

const agent = new https.Agent({
  rejectUnauthorized: true
});

AWS.config.update({
  region: 'us-east-1',
  endpoint: fixedEndpoint,
  httpOptions: {
    agent: agent
  },
  sslEnabled: true
});

const docClient = new AWS.DynamoDB.DocumentClient();

app.get('/user/:id', async (req, res) => {
  const params = {
    TableName: 'Users',
    Key: {
      id: req.params.id
    }
  };
  try {
    const data = await docClient.get(params).promise();
    if (!data.Item) {
      return res.status(404).json({ error: 'User not found' });
    }
    res.json(data.Item);
  } catch (err) {
    res.status(500).json({ error: 'Failed to fetch user' });
  }
});

app.listen(3000, () => console.log('Server running on port 3000'));

For broader protection, integrate middleBrick into your workflow using the CLI to scan endpoints for insecure configurations or use the GitHub Action to fail builds when risky patterns are detected in API definitions. The MCP Server allows you to run these checks directly from development environments, ensuring DynamoDB-related endpoints are validated early.