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-elements/reorder trusts this session flag alone and never rechecks the caller's save permission for the owner element. As a result, a view-only user can POST to actions/nested-elements/reorder using the ownerElementType, ownerId, ownerSiteId, attribute, elementIds, and offset parameters present in the read-only page source and rewrite the sort order of Matrix blocks or Addresses belonging to content they are explicitly denied save access to.
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