CVE-2026-61598: djust: Client mass-assignment of arbitrary view attributes via the default dj-model update_model handler

### Impact `djust.mixins.model_binding.ModelBindingMixin` provides a default `update_model` event handler and is part of the **LiveView base MRO**, so every LiveView exposes it. It `setattr`s a view attribute whose **name is client-supplied** (`field`), gated only by: reject `_`-prefixed names; reject a **14-entry denylist** of framework internals (`FORBIDDEN_MODEL_FIELDS`); optional `allowed_model_fields` which **defaults to None = allow all**; and `hasattr` existence. Result: a client can set **any public, existing view attribute — not just the fields actually bound with `dj-model=` in the rendered template**. The denylist covers framework plumbing but nothing about developer business/authz state, and the allowlist is opt-in (off by default). A developer who binds one `dj-model="search"` input and also keeps `self.account_id` / `self.is_admin` / `self.total_price` as view state does not realize a client can set ALL of them via `{type:event, event:"update_model", params:{field, value}}` over the WebSocket. Type coercion matches the target attribute's type (so `"true"` -> bool True), aiding the attacker. **Severity High** for apps that hold authorization/ownership/business state in public view attributes (the normal djust pattern) -> state tampering / IDOR / authz-flag manipulation; Low otherwise. Default-on across every LiveView. For a public (no-login) view an anonymous client can mass-assign; for an authenticated view a logged-in user can tamper their own session's view state (the IDOR/authz vector when downstream handlers act on it without re-authorizing). Reproduced: a view with `account_id/is_admin/total_price` (none bound with dj-model) had all three set via `update_model` calls. ### Patches Restrict the default handler to fields actually exposed via `dj-model=`: have the template renderer record the bound-field set per render and reject any `field` outside it (preferred, secure + zero-config); or make `allowed_model_fields` fail-closed (required). Keep `FORBIDDEN_MODEL_FIELDS` only as defense-in-depth. Add a regression that a non-`dj-model` public attribute (e.g. `is_admin`) is rejected while a bound field still updates. ### Workarounds Set `allowed_model_fields` explicitly on every view using dj-model (or subclassing LiveView) to the minimal list of bindable fields; do not keep authorization/ownership state in public view attributes that share the view with dj-model bindings. ### References Reproducer + finding writeup retained privately by the maintainer.