Dns Rebinding in Koa with Cockroachdb

DNS Rebinding is a client-side attack that manipulates DNS responses to make a browser believe a remote host is reachable at an attacker-controlled IP after the initial page load. In a Koa application that connects to CockroachDB from server-side code, the typical risk chain involves a compromised or malicious dependency, an injected route, or a server-side request that is inadvertently exposed to an attacker-controlled hostname. If Koa dynamically constructs a CockroachDB connection string using values derived from DNS or HTTP request hostnames without strict validation, an attacker may be able to pivot the intended destination.

Consider a Koa route that reads a database host from a request header or subdomain and uses it to form a CockroachDB connection string. Because DNS Rebinding can cause a hostname to resolve to different IPs over time, an attacker may first cause the client or server to resolve to a benign host, then rebind it to a sensitive internal host such as a CockroachDB node that would normally be unreachable from the public internet. If the Koa server does not enforce strict allowlists on hostnames and does not validate that the resolved address belongs to an expected network, the server may open a connection to the attacker-controlled endpoint, effectively bypassing network-level segregation between public-facing services and internal databases.

The vulnerability in this combination is not about CockroachDB itself being DNS-rebindable, but about the Koa application logic that trusts dynamic host inputs when forming CockroachDB connection URIs. For example, if the server uses a hostname supplied by the client to build a connection string like postgresql://user:pass@${host}:26257/dbname, and does not verify that the host resolves to an allowed IP range, an attacker may cause the Koa server to connect to a malicious database proxy or an internal CockroachDB node. This can lead to unauthorized data access, credential exposure if the attacker can capture authentication material, or abuse of trust relationships in the cluster.

In practice, this scenario requires several conditions: the Koa app must accept or derive database hostnames from untrusted inputs, the CockroachDB node must accept connections from the Koa server’s network path, and the server must not enforce network-level restrictions or certificate pinning where applicable. MiddleBrick scans detect such risks by correlating API surface characteristics, input validation checks, and runtime behavior to highlight places where hostname-based redirection or SSRF-like paths may exist.

To mitigate DNS Rebinding risks when your Koa application interacts with CockroachDB, enforce strict hostname allowlisting, avoid dynamic hostnames in connection strings, and validate all inputs that influence database connectivity. Below are concrete patterns you can adopt.

1. Use a fixed connection string with static configuration

Do not construct CockroachDB URIs from request-derived values. Instead, store the connection string in environment variables and reference it directly.

import Koa from 'koa';
import { Pool } from 'pg';

const app = new Koa();

// Fixed connection string from environment, never from request
const pool = new Pool({
  connectionString: process.env.COCKROACHDB_URI,
});

app.use(async (ctx) => {
  const client = await pool.connect();
  try {
    const res = await client.query('SELECT now()');
    ctx.body = res.rows;
  } finally {
    client.release();
  }
});

app.listen(3000);

2. Validate and restrict hostnames if dynamic routing is required

If your application must support multi-tenant routing, validate the hostname against a strict allowlist before using it to influence database selection. Do not rely on DNS alone; also perform IP resolution checks and compare against expected CIDRs.

import Koa from 'koa';
import net from 'net';

const app = new Koa();
const ALLOWED_HOSTS = new Set(['db.example.com', 'internal.example.com']);

function isValidHost(host) {
  if (!ALLOWED_HOSTS.has(host)) return false;
  // Additional check: resolve and ensure IP is in expected range
  try {
    const addresses = net.resolve(host, '4');
    // Implement further checks as needed
    return addresses.length > 0;
  } catch {
    return false;
  }
}

app.use(async (ctx) => {
  const host = ctx.request.hostname;
  if (!isValidHost(host)) {
    ctx.status = 400;
    ctx.body = { error: 'invalid host' };
    return;
  }
  const pool = new Pool({
    connectionString: `postgresql://user:pass@${host}:26257/db`,
  });
  const client = await pool.connect();
  try {
    const result = await client.query('SELECT $1::text as message', ['ok']);
    ctx.body = result.rows;
  } finally {
    client.release();
  }
});

3. Enforce network policies and avoid exposing CockroachDB directly

Ensure CockroachDB is bound to private interfaces and that the Koa server runs in a network zone that does not permit arbitrary inbound connections to the database port. Use service meshes or firewall rules to restrict source IPs. In code, prefer using Unix domain sockets or internal VPC endpoints where available, and disable external listeners on the CockroachDB node.

4. Use TLS and strong authentication

Always enable TLS for CockroachDB connections and avoid embedding credentials in code. Rotate credentials regularly and use IAM or certificate-based authentication to reduce the impact of leaked connection strings.

const pool = new Pool({
  connectionString: process.env.COCKROACHDB_URI,
  ssl: {
    rejectUnauthorized: true,
  },
});