CVE-2026-49212: symfony/ux-live: HMAC replay bypasses prop integrity

### Description In `symfony/ux-live-component`, a component's server-side state is exposed to the browser as a set of props (`#[LiveProp]`-annotated properties). Props marked `writable: true` can be freely changed by the client. Read-only props are round-tripped to the browser and back, and their integrity is protected by an HMAC so the client cannot tamper with them. Child components additionally receive a `propsFromParent` blob, also HMAC-signed. The HMAC computed by `Symfony\UX\LiveComponent\LiveComponentHydrator` covered only the sorted prop key/value pairs. It didn't include the component name, the slot identifier (`props` vs `propsFromParent`), or any request context, and a single application-wide secret is used for every component. A signed blob the server minted for component A is therefore a valid signature for component B if the key names happen to match, and a `props` blob can be replayed in the `propsFromParent` slot (or the reverse). An attacker can use this to set a read-only prop on a target component to a value they were only ever allowed to choose as a writable prop on another component. ### Resolution The HMAC is now bound to its context: the component name and a slot identifier are included in the pre-image before hashing. Two constants (`CHECKSUM_SLOT_PROPS` and `CHECKSUM_SLOT_PROPS_FROM_PARENT`) name the two slots, and `calculateChecksum()`, `verifyChecksum()`, `addChecksumToData()`, and `ChildComponentPartialRenderer` thread these values through. Cross-component and cross-slot replays no longer verify. The patch for this issue is available [here](https://github.com/symfony/ux/commit/a224b5af3e2e33ee14ac71356ae0e0877900a81c) for branch 2.x (and forward-ported to 3.x). ### Credits Symfony would like to thank Anthropic (via Project Glasswing) for reporting the issue and Hugo Alliaume for providing the fix.

An attacker on a Symfony-based AI platform identifies two live components: a PremiumFeaturesComponent with a read-only prop `subscription_tier: "basic"` and a UserPreferencesComponent with a writable prop of the same key name. The attacker legitimately submits the user preferences form at their `basic` tier, capturing the HMAC-signed props blob the server returns. Because the HMAC excludes the component name and slot identifier, the attacker replays this blob against PremiumFeaturesComponent's props slot in a crafted request. The server validates the HMAC as correct, accepts the tampered state, and the attacker's session is rendered as `premium` tier — unlocking gated AI inference features, higher rate limits, or compliance evidence packs without a paid subscription.