@budibase/backend-core has potential SSRF DNS rebinding bypass in outbound fetch validation

Summary

Authenticated users with automation permissions can bypass Budibase's SSRF blacklist through DNS rebinding.

The outbound fetch flow validates a hostname against the blacklist before the request is sent, but the actual socket connection later performs a separate DNS lookup through node-fetch. Since the validated IPs are never pinned to the connection, an attacker-controlled hostname can return a public IP during validation and a private/internal IP during the real connection.

This results in a non-blind SSRF primitive against internal services reachable from the Budibase host, including loopback, RFC1918 ranges, and cloud metadata endpoints.

Details

The issue comes from the outbound fetch validation flow resolving DNS twice:

During blacklist validation Again during the real socket connection

The first lookup result is discarded after validation, so the second lookup is free to resolve to a different IP.

This creates a classic TOCTOU DNS rebinding issue.

Affected flow in:

packages/backend-core/src/utils/outboundFetch.ts

async function throwIfUnsafe(url: string): Promise<void> {
  const parsed = parseUrl(url)

  if (await isBlacklisted(parsed.hostname)) {
    throw new Error("URL is blocked or could not be resolved safely.")
  }
}

for (let redirects = 0; redirects <= MAX_REDIRECTS; redirects++) {
  await throwIfUnsafe(nextUrl)

  const response = await fetchFn(nextUrl, nextRequest)

  // ...
}

fetchFn uses plain node-fetch with no custom http.Agent / https.Agent, so the underlying socket performs its own independent dns.lookup after validation completes.

The same pattern also exists in:

packages/server/src/automations/steps/utils.ts

await throwIfBlacklisted(nextUrl)

const response = await fetch(nextUrl, nextRequest)

The blacklist implementation resolves hostnames but only returns a boolean:

packages/backend-core/src/blacklist/blacklist.ts

async function lookup(address: string): Promise<string[]> {
  address = parseAddress(address)

  const addresses = await performLookup(address, { all: true })

  return addresses.map(addr => addr.address)
}

export async function isBlacklisted(address: string): Promise<boolean> {
  // ...

  if (!net.isIP(address)) {
    try {
      ips = await lookup(address)
    } catch (e) {
      /* ... */
    }
  } else {
    ips = [address]
  }

  return ips.some(ip => blackList!.check(ip, getIpVersion(ip)))
}

The resolved IPs are discarded, so callers cannot pin the later socket connection to the validated addresses.

An attacker controlling authoritative DNS for a hostname can therefore return:

a public IP during validation a private/internal IP during the actual connection