EUVD-2026-273973

fkie_cve-2026-27638

Actual is a local-first personal finance tool. Prior to version 26.2.1, in multi-user mode (OpenID), the sync API endpoints (`/sync/*`) don't verify that the authenticated user owns or has access to the file being operated on. Any authenticated user can read, modify, and overwrite any other user's budget files by providing their file ID. Version 26.2.1 patches the issue.

GHSA-qmjj-p7m9-wjrv — @actual-app/sync-server: Missing authorization in sync endpoints allows cross-user budget file access in multi-user mode

Affected: npm: @actual-app/sync-server

In multi-user mode (OpenID), the sync API endpoints (`/sync/*`) don't verify that the authenticated user owns or has access to the file being operated on. Any authenticated user can read, modify, and overwrite any other user's budget files by providing their file ID.

## Affected Code

File: `packages/sync-server/src/app-sync.ts`

The `validateSessionMiddleware` on line 31 confirms the user is authenticated, but individual endpoints only check that the file *exists* (via `verifyFileExists`), never that the requesting user *owns* or *has access to* the file.

Compare with `POST /sync/delete-user-file` (lines 394-430) which correctly checks: ```js const isOwner = file.owner === userId; const isServerAdmin = isAdmin(userId); if (!isOwner && !isServerAdmin) { ... } ```

This check is missing from all other endpoints.

## Affected Endpoints

- `GET /sync/download-user-file` - download any budget file - `POST /sync/upload-user-file` - overwrite any budget file - `POST /sync/sync` - read/write sync messages of any file - `POST /sync/user-get-key` - read encryption key info - `POST /sync/user-create-key` - change encryption key - `POST /sync/reset-user-file` - reset sync state - `POST /sync/update-user-filename` - rename file - `GET /sync/get-user-file-info` - read file metadata

## PoC

Setup: Two users (Alice, Bob) authenticated via OpenID on the same Actual server. Alice has a budget with fileId `abc-123`.

Bob downloads Alice's budget: ```bash curl -X GET 'https://actual.example…

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EUVD-2026-273973

fkie_cve-2026-27638

GHSA-qmjj-p7m9-wjrv — @actual-app/sync-server: Missing authorization in sync endpoints allows cross-user budget file access in multi-user mode