uvsior can‘t prevent private memory from being readed or written by public box

Does uvsior not prevent private box memory from being aceessed by public BOX?
I do some test and find in public BOX ,I can write the memory which belongs to private BOX.
for example
In my private box ,there is a local variable name "a" like
int a=2;
and the a located at 0X1FFF4254.
then In public box .i write
int *b=(int *)0x1fff4254;
*b=20.
run the program I find the variable"a" is changed to 20. it seems that the public BOX can revise the memory regin which belongs to private BOX
so i think uvsior can‘t prevent private memory from reading or writing by public box.is it rigth?