CVE-2026-49215: Symfony LiveComponent: CSRF bypass via safelisted header

### Description When using `symfony/ux-live-component`, methods annotated with `#[LiveAction]` are invokable from the browser and mutate server-side state via AJAX. `Symfony\UX\LiveComponent\EventListener\LiveComponentSubscriber::isLiveComponentRequest()` gated these invocations on the presence of `Accept: application/vnd.live-component+html`, with a code comment stating that this acted as a CSRF protection. The `Accept` header is a [CORS-safelisted request header](https://fetch.spec.whatwg.org/#cors-safelisted-request-header), so a cross-origin `fetch()` can set it without triggering a preflight. The header therefore provided no CSRF protection. Any `#[LiveAction]` could be forged cross-origin against a victim's session. In practice the attack is mitigated by `SameSite=Lax` session cookies (Symfony's default), but applications using `SameSite=None`, `credentials: 'include'` with a permissive cookie policy, or that have been pivoted from another same-origin vector remained exposed. ### Resolution `isLiveComponentRequest()` now additionally requires the request header `X-Requested-With: XMLHttpRequest`. This header is **not** CORS-safelisted, so the browser issues a preflight `OPTIONS` request for any cross-origin attempt; Symfony does not advertise CORS for LiveComponent endpoints, the preflight fails, and the real request is blocked before it reaches the application. The bundled Stimulus client already sends `X-Requested-With` on every LiveComponent request (`RequestBuilder.ts`), so standard usage is unaffected. Cross-origin callers must add `X-Requested-With` to their CORS `Access-Control-Allow-Headers` allow-list. The patch for this issue is available [here](https://github.com/symfony/ux/commit/aed7493db2b4b7bf1f9c79b33cda544f06904b27) 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 targeting an organization with a Symfony-based AI model management dashboard creates a malicious webpage and delivers it to an authenticated administrator via spearphishing. When the admin visits the page, JavaScript silently issues a cross-origin fetch() to the Symfony application with the Accept: application/vnd.live-component+html header — CORS-safelisted, so no preflight is triggered. The browser automatically appends the victim's session cookies. The Symfony server, finding the Accept header and lacking X-Requested-With validation in the vulnerable version, treats the request as legitimate and executes the targeted #[LiveAction] method — for example, triggering a model retraining job, elevating a user to admin, or modifying AI pipeline configuration — entirely under the victim's identity and without their awareness.