CVE-2026-67579: Filter expression injection via forged keyset pagination cursor in Ash

Deserialization of Untrusted Data vulnerability in ash-project ash allows an unauthenticated attacker to inject a filter expression through a forged keyset pagination cursor, resulting in SQL injection or code execution depending on the data layer.

Read actions with keyset pagination decode the client-supplied page[:after] or page[:before] cursor in decodevalues/2 in lib/ash/page/keyset.ex using nonexecutablebinarytoterm/2 with [:safe]. That guard blocks new atoms, funs, and ports, but not a struct built from atoms already interned in a running Ash application, so a decoded %Ash.Query.Call{} expression survives and is spliced into the keyset filter as a comparison value in dofilters/4 and evaluated. Because the cursor bypasses the Ash.Expr macro, the runtime never applies the private?/public? gate that would otherwise reject it. On AshPostgres the injected fragment is inlined into the SQL query; on the ETS and Simple data layers it is evaluated in-process as an arbitrary function call.

This issue affects ash: from 1.17.0 before 3.31.3.