Dns Rebinding in Koa with Dynamodb

DNS rebinding occurs when an attacker forces a victim’s browser to resolve a domain to an IP address under the attacker’s control, then switches the resolved IP to a different target on the network, typically an internal service. In a Koa application that interacts with DynamoDB, this can expose internal endpoints or administrative interfaces that are otherwise not directly reachable from the public internet.

Consider a Koa service that accepts user-supplied table names or keys and forwards them to DynamoDB using AWS SDK calls. If the Koa app runs inside a VPC or a restricted network and relies on DNS names (e.g., internal service names or custom endpoints) to reach DynamoDB, an attacker may exploit DNS rebinding to trick the client into sending requests that resolve to an internal DynamoDB endpoint or a proxy that reveals metadata or misconfigurations.

For example, an attacker might host a page that causes the browser to resolve api.example.com first to a public IP (so the initial request succeeds) and then to 169.254.169.254 (the AWS instance metadata service). If the Koa backend uses IAM roles assigned to the host and does not validate or restrict outbound endpoints, the attacker could potentially probe internal services or intercept requests intended for DynamoDB. This risk is higher when the Koa application uses SDK clients that resolve endpoints dynamically or when custom HTTP agents are configured with permissive DNS settings.

In the context of middleBrick scanning, such a setup would be flagged under SSRF and DNS rebinding checks, highlighting that the application’s network path to DynamoDB relies on names that can be manipulated. The scan does not alter runtime behavior but surfaces the exposure so operators can apply network-level hardening, such as endpoint pinning and strict egress controls.

To mitigate DNS rebinding risks when a Koa application communicates with DynamoDB, enforce strict endpoint configuration, validate and constrain outbound connections, and avoid relying on dynamic or internal DNS resolution for sensitive operations. Below are concrete remediation steps with working Koa and AWS SDK code examples.

1. Use explicit, static DynamoDB endpoints

Instead of allowing the SDK to resolve endpoints via DNS, configure the DynamoDB client with a fixed endpoint URL that points to a known, public AWS endpoint. This reduces the chance that DNS rebinding will redirect requests to internal services.

import Koa from 'koa';
import { DynamoDB } from 'aws-sdk';

const app = new Koa();

// Explicit endpoint prevents DNS-based redirection to internal hosts
const ddb = new DynamoDB({
  region: 'us-east-1',
  endpoint: 'https://dynamodb.us-east-1.amazonaws.com',
  sslEnabled: true,
  httpOptions: {
    agent: new (require('https').Agent)({
      rejectUnauthorized: true,
    }),
  },
});

app.use(async (ctx) => {
  const params = {
    TableName: process.env.DYNAMODB_TABLE || 'public-read-table',
    Key: { id: { S: ctx.query.id } },
  };
  try {
    const data = await ddb.getItem(params).promise();
    ctx.body = data.Item || {};
  } catch (err) {
    ctx.status = 500;
    ctx.body = { error: err.message };
  }
});

app.listen(3000);

2. Validate and constrain table names and keys

Prevent attackers from specifying arbitrary table identifiers by validating input against an allowlist or strict pattern. This ensures that even if DNS rebinding influences name resolution, the application will not inadvertently access unintended resources.

import Koa from 'koa';
import { DynamoDB } from 'aws-sdk';

const app = new Koa();
const ddb = new DynamoDB({ region: 'us-east-1' });

const ALLOWED_TABLES = new Set(['public-items', 'audit-logs']);

function isValidTable(name) {
  return /^[a-z][a-z0-9\-]{1,64}$/.test(name) && ALLOWED_TABLES.has(name);
}

app.use(async (ctx) => {
  const tableName = ctx.query.table;
  if (!isValidTable(tableName)) {
    ctx.status = 400;
    ctx.body = { error: 'Invalid table name' };
    return;
  }
  const params = {
    TableName: tableName,
    Key: { id: { S: ctx.query.id } },
  };
  try {
    const data = await ddb.getItem(params).promise();
    ctx.body = data.Item || {};
  } catch (err) {
    ctx.status = 500;
    ctx.body = { error: err.message };
  }
});

app.listen(3000);

3. Enforce network-level egress controls

While not code, it is critical to pair application-level fixes with infrastructure controls. Configure security groups, VPC endpoint policies, and egress rules to allow connections only to approved DynamoDB endpoints. This prevents outbound requests from being redirected by an attacker even if the application logic is compromised.

4. Use AWS SDK’s built-in retry and timeout settings

Configure reasonable timeouts and disable automatic retries for sensitive operations to reduce the window of exposure during rebinding attempts. This complements endpoint pinning and ensures that slow or malicious redirects do not cause hanging requests.

const ddb = new DynamoDB({
  region: 'us-east-1',
  endpoint: 'https://dynamodb.us-east-1.amazonaws.com',
  httpOptions: {
    timeout: 5000,
    retries: 0,
  },
});