Defeating the Shoe Scanning Machine at Heathrow Airport

For a while now, Heathrow Airport has had a unique setup for scanning shoes. Instead of taking your shoes off during the normal screening process, as you do in U.S. airports, you go through the metal detector with your shoes on. Then, later, there is a special shoe scanning X-ray machine. You take your shoes off, send them through the machine, and put them on at the other end.

It's definitely faster, but it's an easy system to defeat. The vulnerability is that no one verifies that the shoes you walked through the metal detector with are the same shoes you put on the scanning machine.

Here's how the attack works. Assume that you have two pairs of shoes: a clean pair that passes all levels of screening, and a dangerous pair that doesn't. (Ignore for a moment the ridiculousness of screening shoes in the first place, and assume that an X-ray machine can detect the dangerous pair.) Put the dangerous shoes on your feet and the clean shoes in your carry-on bag. Walk through the metal detector. Then, at the shoe X-ray machine, take the dangerous shoes off and put them in your bag, and take the clean shoes out of your bag and place them on the X-ray machine. You've now managed to get through security without having your shoes screened.

This works because the two security systems are decoupled. And the shoe screening machine is so crowded and chaotic, and so poorly manned, that no one notices the switch.

U.S. airports force people to put their shoes through the X-ray machine and walk through the metal detector shoeless, ensuring that all shoes get screened. That might be slower, but it works.

EDITED TO ADD (12/14): Heathrow Terminal 3, that is. The system wasn't in place in Terminal 4, and I don't know about Terminals 1 and 2.