Summary

fetchToken in the OAuth2 SDK makes a POST to a builder-supplied URL with plain node-fetch, skipping the blacklist.isBlacklisted check that every other outbound fetch path in the codebase uses. The Joi schema for the OAuth2 URL has no scheme or host restriction. Alice, a builder, points an OAuth2 config at http://169.254.169.254/... or http://127.0.0.1:5984/; the server connects and returns response-body fragments in the validation result.

Details

packages/server/src/sdk/workspace/oauth2/utils.ts:17-65 defines fetchToken. Near the end:

const resp = await fetch(config.url, fetchConfig)

config.url is whatever the builder stored. fetchConfig has redirect: "follow" (the default), so a public URL that returns 302 to an internal target is also reachable.

The route validation at packages/server/src/api/routes/oauth2.ts:9 accepts any string:

url: Joi.string().required(),

The controller passes the URL into fetchToken through crud.ts. The /api/oauth2/validate endpoint (builder role) is the most direct attack path: it lives on builderRoutes, takes the URL from the body, fires the fetch, and returns a validation envelope that includes the upstream error string.

Compare with every other outbound fetch in the codebase:

• packages/server/src/integrations/rest.ts:754 calls blacklist.isBlacklisted(url) before its fetch (though it does not re-check redirects; see companion advisory for REST-redirect SSRF).
• packages/backend-core/src/utils/outboundFetch.ts:98-100 sets redirect: "manual" and re-validates each hop.
• packages/server/src/automations/steps/outgoingWebhook.ts routes through fetchWithBlacklist.

The default blacklist blocks 127.0.0.0/8, 169.254.0.0/16, 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 (packages/backend-core/src/blacklist/blacklist.ts:6-16). The OAuth2 path never consults it.

Proof of Concept

Tested against Budibase 3.35.8 (built from master at f960e361).

Step 1: Alice, a builder, POSTs an OAuth2 config pointed at CouchDB on the same host as Budibase:

curl -sS -b "$BUILDER_COOKIE" -X POST "$BASE/api/oauth2/validate" \
  -H "Content-Type: application/json" \
  -d '{"url":"http://127.0.0.1:5984/","clientId":"t","clientSecret":"t",
       "method":"BODY","grantType":"client_credentials"}'

Server response:

{"valid":false,"message":"Method Not Allowed"}

Budibase reached CouchDB (which rejects POST at / with 405). Without the blacklist bypass this request would be blocked at the IP check.

Step 2: Probe the cloud metadata range:

curl -sS -b "$BUILDER_COOKIE" -X POST "$BASE/api/oauth2/validate" \
  -H "Content-Type: application/json" \
  -d '{"url":"http://169.254.169.254/latest/meta-data/","clientId":"t","clientSecret":"t","method":"BODY","grantType":"client_credentials"}'

Server response:

{"valid":false,"message":"invalid json response body at http://169.254.169.254/latest/meta-data/ reason: Unexpected token 'N', \"Not Found\" is not valid JSON"}

The "Not Found" substring is the upstream body; the server reached the link-local metadata endpoint and leaked the first bytes of the response into the validation error.

Impact

Two concrete paths, both reachable from any builder account (free-tier signup on Budibase Cloud is enough):

1. Cross-tenant data read on Cloud. Budibase Cloud multi-tenants on a shared CouchDB; each tenant gets its own <tenantId>_global-db and app_<id> databases on the same port 5984. The blacklist is what keeps a builder from talking to CouchDB directly. With that bypassed, Alice can GET http://127.0.0.1:5984/_all_dbs via a 302 redirector and enumerate every other tenant's databases, then read their _users, app definitions, and datasource configs (which include third-party credentials). None of this traffic goes through Budibase's tenant isolation layer, so standard app-level access controls do not apply.
2. IAM credential exfiltration. Alice points the URL at http://169.254.169.254/latest/meta-data/iam/security-credentials/<role>/ and receives the instance role credentials in the validation error path. Those credentials carry whatever AWS permissions the Budibase instance role holds.

Self-hosted deployments face the same CouchDB/Redis/MinIO access plus any other service reachable on the host or pod network. The blacklist was explicitly added to prevent exactly this, and every other outbound fetch path uses it.

Recommended Fix

Call blacklist.isBlacklisted before the fetch and set redirect: "manual" on fetchConfig, matching the pattern in outboundFetch.ts:

import { blacklist } from "@budibase/backend-core"

async function fetchToken(config: { url: string; /* ... */ }) {
  config = await processEnvironmentVariable(config)
  if (await blacklist.isBlacklisted(config.url)) {
    throw new Error("OAuth2 token URL is blocked.")
  }
  const fetchConfig: RequestInit = {
    method: "POST",
    headers: { "Content-Type": "application/x-www-form-urlencoded" },
    body: new URLSearchParams({ grant_type: "client_credentials" }),
    redirect: "manual",
  }
  // ...
}

Alternatively, replace the fetch call with fetchWithBlacklist, which handles both checks and re-validates redirect targets.

Found by aisafe.io

References

• GHSA-4q6h-8p4v-67vq
• https://nvd.nist.gov/vuln/detail/CVE-2026-48153