Budibase: Chat-Link Handoff Identity Confusion (Same-Tenant Account-Link CSRF)

Affected versions: master at commit 3c8d1b4023.

Reachable over HTTP by: the GET and POST are on publicRoutes (no auth-group middleware). The POST requires ctx.isAuthenticated at runtime (controllers/ai/chatIdentityLinks.ts:142) -> so the victim must be logged into Budibase when the POST fires (achievable via standard CSRF if cookies are sent cross-origin, or via phishing that lures the victim to submit).

Root cause

Issue 1 -> The handoff routes are public and unauthenticated at the middleware layer.

// packages/server/src/api/routes/chat.ts:23, 27
publicRoutes.get("/api/chat-links/:instance/:token/handoff", ai.handoffChatLinkSession)
publicRoutes.post("/api/chat-links/:instance/:token/handoff", ai.confirmChatLinkSession)

publicRoutes has no group middleware (endpointGroups/standard.ts:24-25 calls endpointGroupList.group() with no middleware and then .lockMiddleware()). The routes do not have a per-route authorized(...). There is no CSRF middleware on these routes -> Budibase's CSRF synchroniser token is enforced only for state-changing verbs on authenticated routes that are not in NO_CSRF_ENDPOINTS; the public-routes path bypasses it.

Issue 2 -> The confirmation token is exposed to anyone who views the GET page.

// packages/server/src/api/controllers/ai/chatIdentityLinks.ts:40-60
const renderLinkConfirmationPage = (session, action) => {
  ...
  return `<!doctype html>...
    <form method="post" action="${helpers.escapeHtml(action)}">
      <input type="hidden" name="confirmationToken" value="${helpers.escapeHtml(session.confirmationToken)}">
      <button type="submit">Confirm</button>
    </form>
  ...`
}

The confirmationToken (a newid() UUIDv4 generated by prepareChatIdentityLinkSessionConfirmation) is rendered into a hidden form input. Anyone who loads the GET page sees the token in the page source.

Issue 3 -> The POST binds the currently-authenticated user to the external identity based solely on the confirmation token.

// packages/server/src/api/controllers/ai/chatIdentityLinks.ts:124-177
export async function confirmChatLinkSession(ctx) {
  ...
  if (!ctx.isAuthenticated) { throw new HTTPError("Authentication is required to link chat identity", 401) }
  if (!session.confirmationToken || ctx.request.body?.confirmationToken !== session.confirmationToken) {
    throw new HTTPError("Link confirmation is invalid or has expired", 400)
  }

  const currentGlobalUserId = getCurrentGlobalUserId(ctx)        // <- victim's ID
  const consumedSession = await sdk.ai.chatIdentityLinks.consumeChatIdentityLinkSession(token)
  ...
  await sdk.ai.chatIdentityLinks.upsertChatIdentityLink({
    provider: consumedSession.provider,
    externalUserId: consumedSession.externalUserId,              // <- attacker's chat identity
    externalUserName: consumedSession.externalUserName,
    ...
    globalUserId: currentGlobalUserId,                            // <- bound to victim
    linkedBy: currentGlobalUserId,
  })
  ...
}

There is no binding between the session and the requester's Budibase identity at preparation time. prepareChatIdentityLinkSessionConfirmation (sdk/workspace/ai/chatIdentityLinks.ts:173-188) stores the confirmationToken keyed by token with no user-id field. Whoever is authenticated when the POST fires (and supplies the correct confirmationToken) gets bound.

Issue 4 -> assertSessionMatchesInstance only checks workspace ID, not user.

// packages/server/src/api/controllers/ai/chatIdentityLinks.ts:17-27
const assertSessionMatchesInstance = ({ workspaceId, instance }) => {
  if (!workspaceId || workspaceId !== instance) {
    throw new HTTPError("Link token is not valid for this workspace", 400)
  }
}

The check confirms the session belongs to the same workspace as the URL instance param -> useful for preventing cross-workspace confusion but useless against same-tenant identity confusion.

Reproduction

Step-by-step attack

1. Attacker (tenant T) provisions a chat-link session for their own external identity. Via the agent channel provisioning flow (e.g. POST /api/agent/:agentId/slack/provision or via the Discord/MS Teams/Telegram provisioning endpoints), the attacker obtains a chat-link token bound to their own Slack user ID. The session is stored in Redis keyed by token, scoped to tenant T.

2. Attacker triggers prepareChatIdentityLinkSessionConfirmation. Either by GETting the handoff page themselves (if they have a Budibase session) or via an internal API. The confirmationToken is generated and stored.

3. Attacker crafts a phishing/auto-submit page that POSTs to /api/chat-links/<instance>/<token>/handoff with the leaked confirmationToken in the body. Example:

   <form id="f" method="post" action="https://victim.budibase.app/api/chat-links/app_xxx/linktoken/handoff">
     <input name="confirmationToken" value="<leaked-token>">
   </form>
   <script>document.getElementById('f').submit()</script>
   

4. Victim (a Budibase user in tenant T, e.g. an admin) is lured to the attacker page while logged into Budibase. Their browser sends the POST cross-origin. If the Budibase auth cookie is sent on cross-site requests (SameSite=Lax by default allows top-level POST navigations, which a form-submit is), the request is authenticated as the victim.

5. The victim's globalUserId is now bound to the attacker's Slack identity. When the attacker DMs the AI agent from Slack, the agent's operations run as the victim user -> with the victim's permissions on agent operations, knowledge sources, file uploads, and any downstream automation steps keyed off the linked identity.

Mitigating factors:

• SameSite=Lax cookies block cross-site POSTs from sub-resources (fetch / XHR) but allow top-level form-POST navigations. A phishing page that submits the form via document.form.submit() (a top-level navigation) succeeds. So the CSRF works with the default cookie policy.
• The attack is same-tenant only (session.tenantId !== context.getTenantId() is checked in the sdk).
• The attacker must induce the victim to click a link (standard phishing). No silent drive-by.

Impact

The severity depends on what the agent can do as the victim. For an admin victim, this is full tenant administration via chat. For a regular user, it is impersonation within the agent subsystem. The bound identity persists until manually unlinked, so the attacker retains ongoing access.

This is a same-tenant, user-interaction-required primitive. It is below the "unauthenticated RCE" threshold but above "hardening" -> it is a real authentication-confusion vulnerability on a sensitive identity-binding operation.

Fix

Recommended layered fixes:

1. Bind the confirmation token to the requester's Budibase session at preparation time. In prepareChatIdentityLinkSessionConfirmation, store the globalUserId of the requester alongside the confirmationToken. In confirmChatLinkSession, verify that getCurrentGlobalUserId(ctx) matches the stored requester. This breaks the CSRF / cross-user confusion.

2. Add a CSRF token to the confirmation POST. The standard Budibase CSRF synchroniser token (x-csrf-token header, validated against the session's csrfToken) should be enforced on POST /api/chat-links/.../handoff. Move the route out of publicRoutes to an authenticated route group so CSRF applies (the GET can remain public for the redirect-to-login flow; the POST should be authenticated + CSRF-protected).

3. Add a per-session nonce that is bound to the requester's browser (e.g. a signed cookie set on the GET, validated on the POST) to prevent cross-origin submission even when SameSite=Lax allows it.

4. Require user re-authentication (re-entry of password / step-up auth) for sensitive identity-binding operations, similar to how password change requires re-auth.