Budibase: S3 presigned URL endpoint authorization regression in v3.39.4 allows BASIC users to obtain S3 PutObject presigned URLs

Overview

Impact

In Budibase v3.39.4, a regression in the authorization level for the S3 attachment upload endpoint allows any BASIC app user to obtain S3 PutObject presigned URLs. The endpoint uses TABLE/WRITE permission level instead of the intended BUILDER level defined in v3.39.3. Additionally, the controller does not pin the target bucket to the datasource's configured bucket, allowing writes to any S3 bucket the stored IAM credentials can access.

Reproduction

1. As a BASIC app user, discover or obtain a valid S3 datasource ID within the app
2. Send a POST request:

POST /api/attachments/<datasourceId>/url
Content-Type: application/json
x-budibase-app-id: <appId>

{"bucket": "target-bucket", "key": "malicious-file.html"}

3. The response contains a valid S3 PutObject presigned URL
4. Use the presigned URL to upload arbitrary content to any writable S3 bucket in the IAM credential scope

Root Cause

The /api/attachments/:datasourceId/url route was changed from authorized(BUILDER) in v3.39.3 to authorized(PermissionType.TABLE, PermissionLevel.WRITE) in v3.39.4. BASIC users have TABLE/WRITE permissions by default, so they can call this endpoint. The controller at packages/server/src/api/controllers/static/index.ts:614-632 accepts the bucket parameter directly from the request body and passes it to getSignedUrl without validating against the datasource's configured bucket.

Evidence

The test suite at packages/server/src/api/routes/tests/static.spec.ts:218-235 confirms this behavior. The test authenticates as a BASIC role user and successfully generates a signed upload URL, verifying HTTP 200 and a defined res.body.signedUrl.

Remediation

1. Restore the authorized(BUILDER) middleware on the route
2. Add paramResource("datasourceId") to ensure the datasource belongs to the caller's app
3. Pin the S3 bucket to datasource.config.bucket in the controller, ignoring the caller-supplied bucket value

Affected packages

• @budibase/server <= 3.38.1

Remediation

Refer to the advisory for the patched release.