CVE-2026-54356: Budibase authenticated arbitrary S3 signed upload URL issuance via `/api/attachments/:datasourceId/url`

Budibase 3.39.7 allows a low-privilege authenticated published-app user with the built-in BASIC role to obtain arbitrary S3 pre-signed upload URLs backed by a workspace datasource’s stored server-side credentials.

The affected endpoint is:

POST /api/attachments/:datasourceId/url

The caller can control:

bucket
key

and receives:

signedUrl
publicUrl

This lets a low-privilege published-app user mint S3 PUT URLs using server-side datasource credentials for attacker-chosen object destinations.

Steps:

1. Log in as an admin user.
2. Create a new app/workspace.
3. In the development app context, create an S3 datasource with valid credentials.
4. Publish the app.
5. Create a low-privilege user with the built-in BASIC role on the published production app ID.
6. Log in as that BASIC user.
7. Send: POST /api/attachments/<datasourceId>/url

with:

{"bucket":"foo","key":"bar"}

and the published app header:

x-budibase-app-id: <published_app_id>

Observe a successful response containing:

signedUrl
publicUrl