Craft CMS 5.0.0 through 5.10.12 (fixed in 5.10.13) contains a broken access control flaw in the nested-elements reorder endpoint. When an authenticated control panel user with viewEntries and viewPee…

Summary

Craft CMS 5.0.0 through 5.10.12 (fixed in 5.10.13) contains a broken access control flaw in the nested-elements reorder endpoint. When an authenticated control panel user with viewEntries and viewPeerEntries (but without savePeerEntries) opens another author's entry in read-only mode, Craft unconditionally grants that session a `manageNestedElements::<ownerId>::field:<handle>` authorization flag for the entry's Matrix/Address fields. Unlike the corresponding delete endpoint, actions/nested-elem…