GHSA-v8x2-fjv7-8hjh

Grav has Broken Access Control which allows an Editor to modify the page's YAML Frontmatter to alter form processing actions

### Summary Due to a broken access control vulnerability in the `/admin/pages/{page_name}` endpoint, an editor ( user with full permissions to pages ) can change the functionality of a form after submission. ### Details Due to improper authorization checks when modifying critical fields on a POST request to `/admin/pages/{page_name}`, an editor with only permissions to change basic content on the form is now able to change the functioning of the form through modifying the content of the `data[_json][header][form]` which is the YAML frontmatter which includes the `process` section which dictates what happens after a user submits the form which include some important actions that could lead to further vulnerabilities. ### PoC - Have Admin and Form plugins installed - Connect to panel as admin, create user and give him permission for pages all - Now connect as that user and notice you cant edit any process field in the panel - Change anything in the content of the form and save - Intercept the request:  - Now modify the field `data[_json][header][form] with the following payload URL-encoded not like this: ``` {"name":"ssti-test 2","fields":{"name":{"type":"text","label":"Name","required":true}},"buttons":{"submit":{"type":"submit","value":"Submit"}},"process":[{"message":"{{ evaluate_twig(form.value('name')) }}"}]} ``` - Change the field and forward it:  Request goes through and changes have been made to the form.  ### Impact - Attacker can modify submission logic of the form which leads to changing redirect value, email sending, changing template, breaking out of the Twig sandbox potentially executing code... ### Fix recommendation - Implement proper authorization checks to such requests especially when it contains fields user shouldn't be able to modify based on his role.