EEF-CVE-2026-96581

An upsert on the primary key in Ash ignores attribute multitenancy, so one tenant can read, overwrite and take over another tenant's records

## Summary Authorization Bypass Through User-Controlled Key vulnerability in ash-project ash allows a user of one tenant to read, overwrite and take over another tenant's records via an upserting create action. Under attribute multitenancy, `Ash.Actions.Create` (`lib/ash/actions/create/create.ex`) and `Ash.Actions.Create.Bulk` (`lib/ash/actions/create/bulk.ex`) upserted on the primary key, or on an `all_tenants?` identity, without scoping the conflicting record to the request's tenant. A caller of a create action with `upsert? true` that accepts the primary key, supplying a record id from another tenant, updated that record's upsert fields and received all of its attributes in the result. With the default upsert fields the tenant attribute was written too, moving the record into the caller's tenant. Read and update policies do not run for upserts, and tenant-scoped create policies pass because the tenant is the caller's own. This issue affects ash: from 1.22.0 before 3.37.2. ## Impact A signed-in user of one tenant posts another tenant's record id to an upserting create action, receives that record's attributes, overwrites its upsert fields, and with the default upsert fields moves it into their own tenant. ## Workarounds Add `upsert_condition expr(<tenant attribute> == ^tenant())` to every create action that upserts on the primary key or on an `all_tenants?` identity, and do not combine it with `return_skipped_upsert?`. An upsert into another tenant's record then fails with `Ash.Error.Changes.StaleRecord` (or is skipped in a bulk create) and leaves that record unchanged. This does not help on the Mnesia data layer, which ignores upsert conditions in affected versions. ## Configurations Reachable when a resource uses attribute multitenancy and exposes to callers of one tenant a create action with `upsert? true` that either upserts on the primary key (no `upsert_identity`) and accepts it, or upserts on an `all_tenants?` identity, and the caller knows or can enumerate a key belonging to another tenant (for example a serial integer id, or an id from a shared link or export).