For the most part, I would argue that the 9/11 attacks did not really affect the security of the US. They clearly showed that the US was not as secure as people thought, but they themselves did not affect the security of the US.
You could argue that the attacks, however, led to responses that have improved the security of the US. After the attacks, the US put into place all sorts of efforts (airport security, Patriot Act, international efforts) to protect against terrorism. You can argue that these efforts have helped make America more secure.
However, you can also argue that the response to the attacks has made the US less secure. You can argue that the wars in Iraq and Afghanistan have sapped our military strength and have, at the same time, created more hatred of the US in the Muslim world.
So, the 9/11 attacks did not really affect US security, but the responses to them arguably have. Different people will, however, have different opinions as to whether the responses have increased or decreased the security of the US.