Craft CMS is a content management system (CMS). In versions 5.6.0 through 5.9.14, the `actionSavePermissions()` endpoint allows a user with only `viewUsers` permission to remove arbitrary users from all user groups. While `_saveUserGroups()` enforces per-group authorization for additions, it performs no equivalent authorization check for removals, so submitting an empty `groups` value removes all existing group memberships. Version 5.9.15 contains a patch.

Affected: Packagist: craftcms/cms

## Summary

The `actionSavePermissions()` endpoint allows a user with only `viewUsers` permission to remove arbitrary users from all user groups. While `_saveUserGroups()` enforces per-group authorization for additions, it performs no equivalent authorization check for removals, so submitting an empty `groups` value removes all existing group memberships.

## Affected Versions

- Craft CMS 5.6.0 through 5.9.14 (latest release at time of report) - Regression introduced in 5.6.0 when the `viewUsers` permission was added - Prior to 5.6.0, `editedUser()` required `editUsers`, which implicitly protected this endpoint - Requires Pro edition or higher (the vulnerable code path is gated by `CmsEdition::Pro`)

## Vulnerability Details

### Root Cause

This is a **regression** introduced in Craft CMS 5.6.0 when the `viewUsers` permission was added. Before that change, `editedUser()` required `editUsers` permission for accessing other users’ data, which implicitly protected `actionSavePermissions()`. After the change, `actionSavePermissions()` became reachable for users with read-only access to other users, but the underlying group-saving logic still lacked authorization for group removals.

The vulnerability has two components:

1. **`actionSavePermissions()` reachable with read-only access**: The action only requires a control panel request and delegates to `editedUser()`, which now only checks `viewUsers` — a permission explicitly documented as "read-only access to user elements."…