CVE-2026-50136: Budibase: Unauthenticated S3 signed upload URL generation allows arbitrary writes with stored datasource credentials

The application server exposes an unauthenticated endpoint that generates S3 PutObject presigned URLs using credentials stored in a workspace datasource. The route is protected only by the recaptcha middleware and does not require authentication, table permission, datasource permission, or builder access. A public caller who knows a workspace ID and S3 datasource ID can request a signed upload URL for attacker-controlled bucket and key values.

Details

The static route registers the signed upload URL endpoint with only recaptcha before the controller:

- packages/server/src/api/routes/static.ts:44-48

ts 44: .post( 45: "/api/attachments/:datasourceId/url", 46: recaptcha, 47: controller.getSignedUploadURL 48: )

The controller loads the datasource by datasourceId with enriched secret values:

- packages/server/src/api/controllers/static/index.ts:590-598

ts 590:export const getSignedUploadURL = async function ( 591: ctx: Ctx<GetSignedUploadUrlRequest, GetSignedUploadUrlResponse> 592:) { 593: // Ensure datasource is valid 594: let datasource 595: try { 596: const { datasourceId } = ctx.params 597: datasource = await sdk.datasources.get(datasourceId, { enriched: true }) 598: if (!datasource) {

The request body controls bucket and key, and the server signs a PUT URL using the stored datasource credentials:

- packages/server/src/api/controllers/static/index.ts:609-629

ts 609: if (datasource?.source === "S3") { 610: const { bucket, key } = ctx.request.body || {} 611: if (!bucket || !key) { 612: ctx.throw(400, "bucket and key values are required") 613: } 614: try { 615: let endpoint = datasource?.config?.endpoint 616: if (endpoint && !utils.urlHasProtocol(endpoint)) { 617: endpoint = https://${endpoint} 618: } 619: const s3 = new S3({ 620: region: awsRegion, 621: endpoint: endpoint, 622: credentials: { 623: accessKeyId: datasource?.config?.accessKeyId as string, 624: secretAccessKey: datasource?.config?.secretAccessKey as string, 625: }, 626: }) 627: const params = { Bucket: bucket, Key: key } 628: signedUrl = await getSignedUrl(s3, new PutObjectCommand(params)) 629: if (endpoint) {

The endpoint returns the signed URL and public URL to the caller:

- packages/server/src/api/controllers/static/index.ts:630-639

ts 630: publicUrl = ${endpoint}/${bucket}/${key} 631: } else { 632: publicUrl = https://${bucket}.s3.${awsRegion}.amazonaws.com/${key} 633: } 634: } catch (error: any) { 635: ctx.throw(400, error) 636: } 637: } 638: 639: ctx.body = { signedUrl, publicUrl }

Because no authorization middleware is applied, the API trusts public input to choose where the stored S3 credentials will write.

PoC

Non-destructive validation approach:

1. Create or identify a workspace with an S3 datasource. 2. Obtain the production workspace ID and S3 datasource ID. 3. Send an unauthenticated request with the workspace ID header and attacker-controlled bucket/key:

http POST /api/attachments/<datasourceId>/url HTTP/1.1 x-budibase-app-id: app<workspace-id> content-type: application/json

{"bucket":"attacker-controlled-or-permitted-bucket","key":"poc/budibase.txt"}

4. Observe that the response contains a signed PUT URL. 5. Upload harmless content to the returned signedUrl and confirm the object is created using the datasource's stored S3 credentials.

Impact

This allows unauthenticated arbitrary object writes wherever the stored S3 datasource credentials have PutObject access. Depending on the datasource permissions, this can corrupt application data, overwrite public assets, place attacker-controlled objects in trusted buckets, consume storage, or abuse an organization's cloud credentials.

— GitHub