Budibase is an open-source low-code platform. Prior to 3.39.9, 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. This vulnerability is fixed in 3.39.9.

Affected: npm: @budibase/backend-core

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 /…

Ein Angreifer kann mehrere Schwachstellen in Budibase ausnutzen, um Sicherheitsvorkehrungen zu umgehen, und um Dateien zu manipulieren.