Trust isn't black and white. Of course you can't 100% verify any update without personally decompiling and analysing exactly what it contains, but you can take reasonable precautions before installing them. You might be willing to trust that a security patch described as fixing a specific vulnerability such as the one we're discussing here is at least trying to do what it says if no-one has reported otherwise after a while, without being willing to trust Microsoft as a whole to push only genuine security updates without supervision.