I discovered this exploit while playing on RaysWorks' testing stream. If a player goes into their own creative world and uses a command block (or just the chat) to give themselves special items like barriers, they can save this item in a toolbar. Then, if the player goes to a server (or another world) that allows them to use creative mode (possibly if the player doesn't have operator permissions) and opens that saved toolbar, the content of the toolbar is copied over, and if the player who could not access commands and could not obtain these blocks now can have thousands of them.
This also works for 32k items and things like that.
This is a powerful exploit for any deopped players who cannot usually access standard but very powerful commands.
Thank you for your report!
We're tracking this issue in MC-114692, so this ticket is being resolved and linked as a duplicate.
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