Budibase: DNS rebinding SSRF bypasses remain in OpenAPI import and REST query execution

Impact

A builder-level user can make Budibase issue server-side HTTP requests to loopback or private-network targets by using DNS rebinding against two outbound fetch paths that are still not pinned to the validated DNS answer.

The first path is OpenAPI query import. It validates the supplied hostname with the blacklist and then performs a separate raw fetch. A hostname that resolves to a public address during validation and to 127.0.0.1 during the request is accepted.

The second path is REST datasource/query execution. It calls the fixed fetchWithBlacklist() helper, but provides a custom undici fetchFn that installs a dispatcher. The helper passes a pinned Node agent, but undici uses the dispatcher and resolves the original hostname again at connection time. This defeats the DNS pinning added for the previous outbound fetch advisory.

Impact is authenticated SSRF from the Budibase server process. An attacker with the relevant builder/query privileges can reach loopback or private HTTP services that should be blocked by Budibase's outbound fetch protections, subject to the normal response handling of each feature.

Reproduction

1. Configure an attacker-controlled hostname so the validation lookup returns a public IP address and the later request lookup returns an internal address such as 127.0.0.1.
2. Start a loopback canary HTTP service on the Budibase host.
3. For OpenAPI query import, submit an import URL such as http://rebind.test:<port>/openapi.json through the builder query import endpoints. In a harness matching the current import code, validation resolved rebind.test to 8.8.8.8, the raw fetch then resolved rebind.test to 127.0.0.1, and the loopback canary returned loopback-canary-hit:/openapi.json.
4. For REST datasource/query execution, configure a REST datasource/query URL using the same rebinding hostname. In a harness matching the current helper plus REST undici custom fetch path, a direct loopback URL was blocked and a pinned Node-agent control did not hit loopback, but the REST undici path re-resolved the hostname to 127.0.0.1 and returned REST-UNDICI-REBIND-CANARY from the loopback service.

Observed OpenAPI import proof:

{
  "outcome": "allowed",
  "body": "loopback-canary-hit:/openapi.json",
  "lookups": [
    {"phase": "guard", "hostname": "rebind.test", "address": "8.8.8.8"},
    {"phase": "fetch", "hostname": "rebind.test", "address": "127.0.0.1"}
  ]
}

Observed REST undici proof:

{
  "directControl": {"outcome": "blocked", "hitsAfter": 0},
  "pinnedNodeFetchControl": {"outcome": "no_loopback_hit", "hitsAfter": 0},
  "undiciBypass": {
    "outcome": "allowed",
    "status": 200,
    "body": "REST-UNDICI-REBIND-CANARY",
    "hitsAfter": 1
  },
  "lookupTrace": [
    {"phase": "guard", "hostname": "rebind.test", "address": "8.8.8.8"},
    {"phase": "fetch", "hostname": "rebind.test", "address": "127.0.0.1"}
  ]
}

Root cause and technical details

The central helper now resolves and validates the target hostname, then passes a pinned Node http.Agent or https.Agent to the request. That protects callers whose HTTP client honors the Node agent option.

OpenAPI query import does not use that helper. In packages/server/src/api/controllers/query/import/index.ts, assertUrlIsSafe() calls blacklist.isBlacklisted(parsed.hostname), but fetchFromUrl() then calls fetch(currentUrl, { redirect: "manual" }). That creates a validate-then-fetch gap where DNS can change between the blacklist lookup and the actual connection.

REST datasource/query execution uses the helper but changes the transport semantics. In packages/server/src/integrations/rest.ts, setDispatcher() creates an undici dispatcher with getDispatcher({ rejectUnauthorized, url: requestUrl }), then the custom fetch function calls fetch(requestUrl, setDispatcher(requestInput, requestUrl)). In packages/backend-core/src/utils/outboundFetch.ts, fetchWithBlacklist() passes the pinned Node agent to fetchFn, but undici fetch uses the supplied dispatcher instead. The actual connection performs a second hostname resolution and can be rebound to a blocked address.

Current affected code is present in latest master and in the latest release tag 3.39.14.

Remediation

Route OpenAPI query import through the pinned outbound fetch helper rather than validating and raw-fetching separately.

Make fetchWithBlacklist() pin the actual connection for every supported transport. For undici callers, provide a dispatcher whose lookup or connect behavior is pinned to the validated IP, or reject custom fetch functions that do not explicitly enforce the pinned address. Add regression coverage for direct loopback blocking, DNS rebinding through OpenAPI import, DNS rebinding through REST undici dispatcher, redirect-to-loopback, IPv4-mapped IPv6, and mixed public/private DNS answers.