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 setattrs 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.

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.

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.

Reproducer + finding writeup retained privately by the maintainer.