GHSA-hhg7-c65m-h7ff

### Description

`symfony/html-sanitizer` lets applications sanitise untrusted HTML. `UrlAttributeSanitizer` is the visitor responsible for validating URL-valued attributes and stripping dangerous schemes from them; it runs on every element regardless of configuration. Whether an attribute is *kept* is decided by the element/attribute allow-list; validating the *scheme* of a URL attribute is solely `UrlAttributeSanitizer`'s responsibility.

`UrlAttributeSanitizer::getSupportedAttributes()` returned only `['src', 'href', 'lowsrc', 'background', 'ping']`. The HTML URL-valued attributes `action` (`<form>`), `formaction` (`<button>`, `<input type=image>`), `poster` (`<video>`) and `cite` (`<blockquote>`, `<q>`, `<del>`, `<ins>`) were missing from that list, so `DomVisitor` never invoked scheme validation for them. As a result, when a configuration admits one of those attributes, a `javascript:` URI in it survived sanitisation.

### Conditions for exploitation

`allowSafeElements()` is **not** affected: `<form>` and the `formaction` attribute are both flagged unsafe in `W3CReference`, and `allowElement('form')` resets the element's attribute list. Reaching the vulnerable attributes requires a deliberately permissive configuration, for example:

* `<form>` + `action`: `allowElement('form', '*')`, `allowElement('form', ['action', …])`, `allowElement('form')->allowAttribute('action', 'form')`, or the `allowStaticElements()` preset (whose docblock already warns the output "may still contain other dangerous behaviors"); * `<button>` / `<input type=image>` + `formaction`: `allowElement(…, '*')`, `allowAttribute('formaction', …)`, or `allowStaticElements()`; * `<blockquote>` / `<q>` / `<del>` / `<ins>` + `cite`, or `<video>` + `poster`: similarly via `'*'`, `allowAttribute()`, or `allowStaticElements()`.

For the `action` / `formaction` cases the victim must additionally submit the form or click the button.

### Resolution

`UrlAttributeSanitizer` now also handles `action`, `formaction`, `cite` and `poster`. `action` / `formaction` / `cite` are validated against the link schemes (like `<a href>`, so `javascript:` is rejected and `data:` is dropped too); `poster` is validated against the media schemes (so `data:` images keep working). The behaviour of `<a href>` and `<img src>` is unchanged.

One behaviour change to be aware of: a relative `action="/submit"` on an allowed `<form>` is now dropped by default (the same as `<a href>` / `<img src>` today); `->allowRelativeLinks()` re-enables it.

The patch for this issue is available [here](https://github.com/symfony/symfony/commit/26a598fcfc4f903cc55ff202f642ee621839825e) for branch 6.4.

### Credits

Symfony would like to thank Himanshu Anand and Rémi Pelloux for reporting the issue and Nicolas Grekas for providing the fix.