livewire-markdown-editor has arbitrary file upload that allows stored XSS via attachment handler

Summary

### Impact All versions of `mckenziearts/livewire-markdown-editor` prior to **v1.3** contain a critical arbitrary file upload vulnerability in the `MarkdownEditor::updatedAttachments()` Livewire handler. The handler calls `$file->store()` with no server-side validation of MIME type, extension, or file content.

Any authenticated user with access to a page embedding `<livewire:markdown-editor>` can upload files of any type (`.html`, `.svg`, `.js`, `.php`, `.exe`, etc.) to the disk configured by `livewire-markdown-editor.disk`. When that disk is a public cloud bucket (S3, DigitalOcean Spaces, Cloudflare R2, Scaleway Object Storage — the common configuration when `FILESYSTEM_DISK` points to such a disk), uploaded files are served publicly with a guessed `Content-Type` header.

The consequences include:

- **Stored XSS** on the storage domain via uploaded `.html` or `.svg` files - **Phishing page hosting** on the application's own storage domain (trust laundering) - **Malware distribution** from a domain users associate with the application - **Markdown injection** in the editor output via crafted filenames (the client-supplied `getClientOriginalName()` value was inserted verbatim into the markdown)

A real-world exploitation of this vulnerability was observed in production on a community platform using this package.

### Patches

Upgrade to **v1.3** or later.

### Workarounds

If developers cannot upgrade immediately, disable the upload UI on every instance of the editor by passing `:show-upload="false"`:

```blade <livewire:markdown-editor wire:model="content" :show-upload="false" /> ```

This hides the file input and prevents the vulnerable code path from being reached.

### Resources

- Patch commit: https://github.com/mckenziearts/livewire-markdown-editor/pull/12 - Release: https://github.com/mckenziearts/livewire-markdown-editor/releases/tag/v1.3 - CWE-434: https://cwe.mitre.org/data/definitions/434.html - CWE-79: https://cwe.mitre.org/data/definitions/79.html