EEF-CVE-2026-81853

## Summary

Authorization Bypass Through User-Controlled Key vulnerability in ash-project ash_admin turns a record-lookup URL into an equality oracle over sensitive attributes.

`AshAdmin.Helpers.decode_primary_key/2` decodes the composite-primary-key form (Base64 plus ETF) and returns the decoded map verbatim as the lookup filter, without checking that its keys are the resource's primary-key fields. The deserialization guards bound size, block new atoms and funs, and reject nested expressions, but none restricts which fields come back, and `:safe` still allows any already-interned attribute name. An attacker can therefore encode `%{api_token: "guess"}` and have it spliced into the lookup filter, brute-forcing a sensitive attribute value (API token, reset token) one equality guess at a time; `Map.to_list/1` also accepts structs, yielding a bogus `__struct__` key. The fix rejects any decoded key that is not a real primary-key field.

This issue affects ash_admin: from 0.1.0 before 1.3.1.

## Configurations

The resource has a composite primary key, and the deployment relies on AshAdmin's authorization (a custom actor plug and policies) so that reading otherwise-restricted attribute values crosses a boundary.