SSRF vulnerability in `/api/upload` - client-controlled upload destination

[BadAtCaptchas]

Description

The /api/upload endpoint accepts a localUploadUrl from the client, allowing file uploads to arbitrary destinations.

Affected File

app/api/upload/route.ts

Current Behavior

const localUploadUrl = formData.get("localUploadUrl") as string || "http://localhost:9000"
const response = await fetch(`${localUploadUrl}/tost/${filename}`, {
  method: "PUT",
  body: file,
})

Security Impact

An attacker could upload files to internal services or overwrite data on accessible endpoints.

Suggested Fix

const LOCAL_UPLOAD_URL = process.env.LOCAL_UPLOAD_URL || 'http://localhost:9000';
// Don't accept localUploadUrl from client