Improperly Controlled Modification of Dynamically-Determined Object Attributes vulnerability in ash-project ash allows a user to set the value of a private action argument on the bulk destroy and bulk update paths.

Action arguments declared with public?: false are meant to be set only by trusted server-side code (for example via Ash.Changeset.set_private_argument/3) and must not be settable from end-user input. CVE-2026-55736 fixed the non-bulk changeset path to strip private arguments from user-supplied parameter maps, but the bulk destroy and bulk update paths were not covered.

Ash.Actions.Destroy.Bulk.base_changeset/5 and Ash.Actions.Update.Bulk.base_changeset/5 match every key in the caller-supplied parameter map against all of the action's arguments with no public? check, then apply the matches to the base changeset. A caller who can submit parameters to a bulk destroy or bulk update action (for example through AshJsonApi, AshGraphql, or a controller that forwards request parameters to Ash.bulk_destroy/4 or Ash.bulk_update/4) can therefore set any private argument of that action, including one referenced by an arg(...) template in the action's changes or validations. Depending on how the application uses the argument (for example an acting_user_id driving authorization or record ownership, or audit metadata), this can lead to an integrity violation or privilege escalation.

The fix requires public? in the argument matching on both bulk paths; private arguments remain settable server-side via the :private_arguments option.

This issue affects ash: from 2.17.15 before 3.33.11.