CVE-2026-82741: Ash.Type.Union with :map_with_tag does not force the tag on dump, enabling tag confusion

Improper Validation of Specified Type of Input vulnerability in ash-project ash lets an attacker confuse the stored type tag of an Ash.Type.Union value that uses storage: :mapwithtag, bypassing that member's validation and any tag-based authorization.

For a union with storage: :mapwithtag, each member is identified in storage by a configured tag and tagvalue. Ash.Type.Union.dumptonative/2 (lib/ash/type/union.ex) did not force the configured tag when writing the value, so a tag carried in the submitted value was persisted verbatim. An attacker can therefore store a value whose data belongs to one member but whose tag names a different member. On read the value is re-selected by its tag and treated as the incompatible member (a type confusion), bypassing the real member's constraints and any logic or policy that branches on the union tag. The fix drops any incoming tag and forces the configured tag value on dump.

This issue affects ash: from 2.14.18 before 3.32.2.