Predicate injection in manage_relationship belongs_to lookup discloses secret lookup keys in Ash

Improper Neutralization of Special Elements in Data Query Logic vulnerability in ash-project ash allows an attacker to forge a relationship to a record they cannot name, and to recover the secret value used to look it up.

When manage_relationship is used with on_lookup: :relate on a belongs_to relationship, the client-supplied lookup value is passed to Ash.Query.filter/2 without being cast to the attribute type. A nested map submitted where a scalar is expected is therefore interpreted as a filter predicate rather than a literal, so a lookup for a specific record becomes a query for any record matching a condition. The same path omits Ash.Query.limit(1), leaving Ash.read_one/2 able to distinguish no match from one match from several, which turns comparison predicates into an oracle for the lookup value. Authorization is unaffected; the destination read policy still applies.

This issue affects ash: from 1.52.0-rc.11 before 3.31.1.

A belongs_to relationship must be managed with manage_relationship using on_lookup: :relate, and the lookup value must come from client input.