In Chakra, function calls can sometimes take an extra internal argument, using the flag CallFlags_ExtraArg. The global eval function makes assumptions about the type of this extra arg, and casts it to a FrameDisplay object. If eval is called from a location in code where an extra parameter is added, for example, a Proxy function trap, and the extra parameter is of a different type, this can lead to type confusion. A full PoC is as follows and attached:

var p = new Proxy(eval, {});
p("alert(\"e\")"); 

This bug is subject to a 90 day disclosure deadline. If 90 days elapse
without a broadly available patch, then the bug report will automatically
become visible to the public.
 

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