GHSA-4xfr-4p46-gc6p

### Summary The clipboard copy paths of the `copy_plugins` admin endpoint validate only the target (the user's own clipboard) and skip source-side authorization. A staff user can copy plugins out of a placeholder they have no permission on into their clipboard, then read the (secret) content.

### Details In `cms/admin/placeholderadmin.py`, `_copy_plugin_to_clipboard` and `_copy_placeholder_to_clipboard` check `has_copy_plugins_permission`, which only evaluates `request.toolbar.clipboard.has_add_plugins_permission(...)` — the clipboard belongs to the requesting user, and `check_source` is likewise applied only to the clipboard. The source placeholder identified by the attacker-supplied `source_placeholder_id` / `source_plugin_id` is never authorization-checked. (The placeholder-to-placeholder copy path, `has_copy_from_placeholder_permission`, correctly checks both sides.)

### Impact A staff user holding the global add permission for a plugin type, but with no access to a given placeholder/page, can copy that placeholder's plugins into their own clipboard and read content (e.g. link names/URLs, text) they cannot reach through the normal edit endpoints.

Requires `CMS_PERMISSION=True` with per-placeholder/page restrictions.

### Patches Fixed in 5.0.8: the clipboard copy paths now also verify source-side permission (`has_add_plugins_permission` + `check_source` on the source placeholder), matching placeholder-to-placeholder copy.

### Workarounds None. Upgrade is recommended.

### Credits Reported by the security team at the University of Sydney ([@reporter]).