GHSA-8jj7-4v57-frf5

### Summary The `move_plugin` admin endpoint does not prevent a plugin from being reparented under itself or one of its own descendants. Doing so creates a cycle in the plugin tree, after which the recursive descendant/ancestor SQL queries loop without terminating, stalling the request worker.

### Details `move_plugin` (in `cms/admin/placeholderadmin.py`) accepts a `plugin_parent` POST parameter and, for an in-placeholder move, sets the plugin's parent to the target without any cycle/ancestor check. If the target parent is a descendant of the moved plugin, the resulting `parent_id` graph contains a cycle.

Descendant and ancestor traversal is implemented with `WITH RECURSIVE` CTEs (`_get_descendants_cte` / `_get_ancestors_cte` in `cms/models/pluginmodel.py`) that have no cycle clause or depth limit. On a cyclic tree these recurse indefinitely (PostgreSQL/SQLite) or error at the recursion limit (MySQL). `get_descendants()` is invoked while building the move response and on subsequent operations on the affected subtree.

### Impact An authenticated staff user with permission to change plugins in at least one placeholder can corrupt that placeholder's plugin tree, causing requests that traverse it (rendering, copy, delete) to hang and consume application workers (denial of service). The tree is also left in a corrupted state.

Requires `CMS_PERMISSION`/plugin-change permission on a placeholder.

### Patches Fixed in 5.0.8: `move_plugin` now rejects (HTTP 400) any move that would place a plugin inside itself or one of its descendants, before any tree mutation or traversal.

### Workarounds None. Upgrade is recommended.

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