Zone Transfer in Fiber with Api Keys

A DNS zone transfer is a mechanism that replicates DNS records between servers. When misconfigured, it can expose internal hostnames, IP addresses, and network topology. In a Fiber-based application, if zone transfer functionality is exposed via an HTTP endpoint and protected only by API keys, the combination creates a significant information disclosure risk.

API keys in Fiber are typically passed in headers (e.g., x-api-key) and used for simple authorization. However, API keys alone do not enforce scope or context. If a zone transfer endpoint (such as /dns/zone/:origin) relies solely on key presence for access, an attacker who obtains the key—through source code leakage, logs, or insecure storage—can request transfers of any configured zone. Unlike more structured access controls, API keys do not restrict operations to specific actions or data subsets, so a key valid for one purpose can inadvertently permit zone transfers.

Consider a Fiber route that forwards zone transfer requests to an internal DNS server:

const { Router } = require('express'); // Fiber uses Express under the hood
const router = Router();

router.get('/dns/zone/:origin', (req, res) => {
  const { origin } = req.params;
  const apiKey = req.headers['x-api-key'];
  if (!apiKey) return res.status(401).send('Missing key');
  // Forward to internal DNS server (e.g., dns.js or similar)
  dnsClient.query({ qname: origin, qtype: 'ANY', axfr: true }, (err, data) => {
    if (err) return res.status(500).send('DNS error');
    res.json(data);
  });
});

If the API key is leaked, this route enables an attacker to perform a DNS zone transfer against any configured origin, revealing internal hostnames and IPs. This maps to the BOLA/IDOR category in middleBrick’s checks, where lack of ownership validation allows one authenticated context to access another’s data. The presence of an API key does not mitigate this; it only adds a single factor that can be compromised.

Moreover, if the same API key is used across multiple services—including those that expose zone transfer logic—the blast radius increases. middleBrick’s Data Exposure and Inventory Management checks would flag this as a finding, noting that DNS infrastructure data should never be accessible via a public endpoint without additional constraints like IP whitelisting or scope-bound tokens.

In practice, zone transfer over HTTP should be disabled entirely. If administrative DNS operations are required, they should be restricted to private networks and protected by mechanisms that enforce context, such as role-based access or mutual TLS, rather than static API keys alone.

To remediate zone transfer risks when using API keys in Fiber, move beyond simple key presence checks and add strict scoping, validation, and network controls.

• Require key and validate scope: Ensure the key maps to a specific, limited permission set. Do not use a single key for administrative and read-only operations.
• Restrict endpoints and methods: Disable zone transfer routes in production or guard them behind private networks.
• Add request validation: Validate and sanitize all inputs to prevent SSRF or parameter manipulation when interacting with DNS libraries.

Example of improved routing in Fiber (using the standard fiber package):

const { Fiber } = require('fiber');
const app = new Fiber();

// Middleware to validate API key and scope
app.use('/dns/*', (req, res, next) => {
  const apiKey = req.get('x-api-key');
  const validKeys = {
    'read-only-key': { zones: ['public.example.com'], operations: ['query'] },
    'admin-key': { zones: ['internal.example.com'], operations: ['query', 'transfer'] }
  };
  const keyMeta = validKeys[apiKey];
  if (!keyMeta) return res.status(401).send('Invalid key');
  req.keyMeta = keyMeta;
  next();
});

// Zone transfer endpoint with scope enforcement
app.get('/dns/zone/:origin', (req, res) => {
  const { origin } = req.params;
  const keyMeta = req.keyMeta;
  if (!keyMeta.operations.includes('transfer')) {
    return res.status(403).send('Operation not allowed');
  }
  if (!keyMeta.zones.includes(origin)) {
    return res.status(403).send('Zone not permitted');
  }
  // Validate origin to prevent SSRF (e.g., reject internal IPs or non-DNS targets)
  if (!/^([a-z0-9]([a-z0-9\-]{0,61}[a-z0-9])?\.)+[a-z]{2,}$/i.test(origin)) {
    return res.status(400).send('Invalid zone');
  }
  // Safe forward to internal DNS with restricted network access
  dnsClient.query({ qname: origin, qtype: 'ANY', axfr: true }, (err, data) => {
    if (err) return res.status(500).send('DNS error');
    res.json(data);
  });
});

Key improvements:

• Key-to-permissions mapping replaces blanket access.
• Zone scope is explicitly checked against allowed origins.
• Input validation on origin prevents SSRF and regex-based bypasses.
• Transfer operations are opt-in and audited through middleware.

Complementary measures include storing keys in environment variables, rotating them regularly, and monitoring for unusual transfer requests. middleBrick’s Authentication and Property Authorization checks can help verify that keys are not over-privileged and that endpoints enforce context-aware controls.