feat(core): locked server-trusted state fields (A3, #21)

CHANGELOG.md

@@ -18,6 +18,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   inbound state must be a valid token — tampered, unsigned, or raw-dict state
   is rejected with HTTP 400. When disabled, legacy plain-JSON behavior is
   preserved and a one-time warning is logged. See `docs/STATE_SIGNING.md`.
+- **Locked server-trusted state fields (Epic A, A3 — #21)** — components can
+  declare `locked_fields: ClassVar[frozenset[str]]` for top-level state keys
+  the client must never influence (roles, user IDs, pricing). Locked fields
+  are excluded from `dehydrate()` (they never round-trip through the client)
+  and stripped in place from inbound state in `hydrate()` with a warning.
+  Enforcement lives in the core lifecycle, so all adapters are covered
+  without changes. Defaults to empty — existing components are unaffected.
+  See `docs/LOCKED_FIELDS.md`.
 
 ### Security
 
@@ -27,6 +35,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   as a raw JSON object to skip deserialization/verification.
 - Django `ComponentView.handle_error()` now maps client input errors
   (`ValueError`, including corrupt state) to `400` instead of `500`.
+- Locked fields close the **replay/rollback gap** left by A1: even a
+  validly-signed but stale state blob (or any state in signing-disabled
+  deployments) can no longer roll back server-owned fields — inbound locked
+  values are ignored and the server re-derives them each request.
 
 ## [0.5.0b0] - 2026-06-24
 

README.md

@@ -239,6 +239,7 @@ Generated from docstrings by pdoc and deployed to GitHub Pages on every push to
 | [Django Implementation](docs/DJANGO_IMPLEMENTATION.md) | Django adapter setup and patterns |
 | [Class-Based Views](docs/CBV_GUIDE.md) | CBV auth/permission patterns |
 | [State Signing](docs/STATE_SIGNING.md) | HMAC-signed client state: setup per adapter + key rotation |
+| [Locked Fields](docs/LOCKED_FIELDS.md) | Server-trusted state fields the client can never influence (replay/rollback defense) |
 | [E-Commerce Example](docs/examples/ecommerce.md) | Real-time cart + product demo |
 | [Multi-Step Wizard](docs/examples/wizard.md) | FastAPI wizard recipe (enable [state signing](docs/STATE_SIGNING.md) in production) |
 | [CSRF & CSWSH Guide](docs/SECURITY_CSRF.md) | Per-adapter CSRF coverage audit + WebSocket hijacking guidance |

docs/LOCKED_FIELDS.md

@@ -0,0 +1,108 @@
+# Locked Fields (Server-Trusted State)
+
+Component state round-trips through the client on every dispatch. [State
+signing](STATE_SIGNING.md) guarantees *integrity* — the client cannot alter a
+signed blob — but it does not make state trustworthy:
+
+1. **Unsigned deployments have zero integrity.** If signing is not enabled,
+   the client can freely edit any state field before echoing it back.
+2. **Even with signing, replay is possible.** A client can capture an *older*
+   validly-signed state blob and replay it to roll fields back — e.g. a token
+   from before their `is_admin` flag was revoked, a stale `user_id`, or an
+   old price.
+
+Locked fields are the defense for values the **server owns**: fields the
+client must never influence, in either mode.
+
+## Declaring locked fields
+
+```python
+from typing import ClassVar
+
+from component_framework import Component, registry
+
+
+@registry.register("account_panel")
+class AccountPanel(Component):
+    template_name = "account_panel.html"
+    locked_fields: ClassVar[frozenset[str]] = frozenset({"is_admin", "user_id"})
+
+    def mount(self):
+        super().mount()
+        self.state["user_id"] = self.params["user"].id
+        self.state["is_admin"] = self.params["user"].is_staff
+        self.state["tab"] = "profile"
+
+    def before_render(self):
+        # Re-derive server-trusted values on EVERY request — hydrated
+        # requests do not run mount().
+        user = self.params.get("user")
+        self.state["is_admin"] = bool(user and user.is_staff)
+```
+
+Any iterable of strings works (`frozenset`, `list`, `tuple`); it is
+normalized internally. The default is empty — existing components are
+unaffected.
+
+## Enforcement semantics
+
+Locked fields are enforced at the core lifecycle choke points, so every
+adapter (FastAPI, Flask, Litestar, Django FBV/CBV, WebSocket, SSE streaming)
+is covered without adapter changes:
+
+- **Outbound (`dehydrate()`):** locked fields are excluded from the
+  serialized state — they never reach the client at all. `self.state` keeps
+  them server-side for the rest of the request.
+- **Inbound (`hydrate()`):** locked fields present in client-supplied state
+  are stripped *before* `self.state` is updated, and a warning is logged
+  naming the fields (never the values). Because outbound state never
+  contains locked fields, any inbound occurrence is anomalous — crafted
+  state, or a replayed blob from before the field was locked.
+
+The inbound dict is sanitized **in place**, so `hydrate()` overrides that
+read the raw argument after `super().hydrate(state)` (e.g.
+`DjangoModelMixin.hydrate` reading `state["pk"]`) cannot see locked values
+either.
+
+## Re-establishing locked values
+
+Since locked values never round-trip, the component must (re)derive them
+server-side on every request. Pick whichever fits:
+
+| Pattern | When to use |
+|---------|-------------|
+| `before_render()` | Value only needed for rendering |
+| `hydrate()` override (set after `super().hydrate(state)`) | Value needed by event handlers (handlers run *before* `before_render()`) |
+| Server-supplied `params` (adapter/dependency injection) | Value comes from the request context (current user, tenant, …) |
+| Look up inside the handler | Value only needed by one handler |
+
+## Interaction with signing
+
+Locked fields work identically in both modes and complement signing:
+
+- **Signing disabled:** locked fields are the only integrity guarantee for
+  the fields they cover. Everything else in state remains client-editable —
+  enable signing for production.
+- **Signing enabled:** signing blocks tampering; locked fields additionally
+  block **replay/rollback** of stale-but-validly-signed values for the
+  covered fields.
+
+## Caveats
+
+- **Top-level keys only.** `locked_fields` matches top-level state keys.
+  Nested values (e.g. one field inside `FormComponent`'s
+  `state["form_data"]` dict) cannot be individually locked — keep
+  server-trusted values in their own top-level keys, not inside form data.
+- **Event payloads are not covered.** Locked fields guard the *state*
+  channel. Event payloads (including `FormComponent`'s `submit` payload,
+  which writes `form_data` directly) are separate inputs — validate them
+  with your Pydantic `schema` and authorization checks in handlers.
+- **`ModelFormComponent` / `DjangoModelComponent`:** the bound instance is
+  loaded server-side, but `pk` round-trips by design so the mixin can reload
+  the instance. Locking `pk` therefore breaks instance reloading — instead,
+  authorize access in `get_instance()` (scope the queryset to the current
+  user) rather than trusting the client-supplied `pk`.
+- **Don't put secrets in state.** Locked or not, state design guidance from
+  [State Signing](STATE_SIGNING.md) still applies; locked fields are about
+  *trust*, not confidentiality (in fact they never leave the server, which
+  also helps confidentiality — but that is a side effect, not a contract).

docs/STATE_SIGNING.md

@@ -145,4 +145,6 @@ bundled adapters map it to:
   readable by the client. Never put secrets in component state.
 - **Does not prevent replay of a stale-but-valid token** for the same
   component. Treat state as untrusted input in handlers and enforce
-  authorization server-side (see Epic A3, locked fields, for the follow-up).
+  authorization server-side. For fields the server owns (roles, IDs,
+  pricing), declare them as [locked fields](LOCKED_FIELDS.md) — they never
+  round-trip through the client, so replayed blobs cannot roll them back.

src/component_framework/core/component.py

@@ -47,6 +47,16 @@ class Component:
     permission_classes: ClassVar[list[type["BasePermission"]]] = []
     slots: ClassVar[list[str]] = []
     """Slot names this component accepts. Empty list means *any* slot name is accepted."""
+    locked_fields: ClassVar[frozenset[str] | list[str] | tuple[str, ...]] = frozenset()
+    """Server-trusted top-level state fields the client must never influence.
+
+    Locked fields are excluded from :meth:`dehydrate` (they never reach the
+    client) and stripped from inbound state in :meth:`hydrate` (a replayed or
+    forged value is ignored, with a warning). Components must (re)establish
+    locked values server-side on every request — in a ``hydrate()`` override,
+    in ``before_render()``, or from server-supplied ``params``.
+    See ``docs/LOCKED_FIELDS.md``.
+    """
 
     def __init__(self, **params):
         self.params = params
@@ -67,13 +77,56 @@ def mount(self):
         self._mounted = True
 
     def hydrate(self, state: dict):
-        """Restore component from serialized state."""
+        """Restore component from serialized state.
+
+        Fields declared in :attr:`locked_fields` are removed from *state*
+        (in place, so ``hydrate()`` overrides that read the raw argument
+        after ``super().hydrate()`` cannot see them either) and a warning is
+        logged — locked fields are server-trusted and never accepted from
+        the client.
+        """
+        self._strip_locked_fields(state)
         self.state.update(state)
         self._mounted = True
 
     def dehydrate(self) -> dict:
-        """Serialize component state for persistence."""
-        return self.state.copy()
+        """Serialize component state for persistence.
+
+        Fields declared in :attr:`locked_fields` are excluded — they never
+        round-trip through the client. ``self.state`` itself is untouched,
+        so locked values remain available server-side for the rest of the
+        request.
+        """
+        state = self.state.copy()
+        for field in self._locked_field_set().intersection(state):
+            del state[field]
+        return state
+
+    @classmethod
+    def _locked_field_set(cls) -> frozenset[str]:
+        """Return :attr:`locked_fields` normalized to a frozenset."""
+        return frozenset(cls.locked_fields)
+
+    def _strip_locked_fields(self, state: dict) -> None:
+        """Remove locked fields from inbound client *state*, in place.
+
+        Logs a warning naming the stripped fields (values are deliberately
+        not logged). No-op when :attr:`locked_fields` is empty.
+        """
+        locked = self._locked_field_set()
+        if not locked:
+            return
+        stripped = sorted(locked.intersection(state))
+        if not stripped:
+            return
+        for field in stripped:
+            del state[field]
+        logger.warning(
+            "Ignoring locked state field(s) %s in inbound client state for %s — "
+            "locked fields are server-trusted and never accepted from the client.",
+            stripped,
+            self.__class__.__name__,
+        )
 
     def before_render(self):
         """Called before rendering. Use for derived state computation."""