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The breach has reportedly raised fears the information may have been copied and smuggled out of the airport with the purpose of selling it on the dark web to the highest bidder.
The comprehensive security information would be of interest to criminal and terror groups and may have been compiled for that market.
The data may also have been collected by a member of staff for their personal use, but at the very least, the breach represents serious failings in airport data security.
If the data was compiled for legitimate use, information security best practice dictates the USB stick should have been password protected and all the data encrypted.
The USB stick was handed over to The Sunday Mirror, which found the device contained security information such as maps, videos and documents, including details of measures used to protect the Queen and the route she takes when she uses the airport.
According to The Sunday Mirror, there were at least 174 documents on the USB stick, and although some were marked as “confidential” or “restricted”, they were not encrypted or password protected.
Some files detailed the types of ID needed to access restricted areas, a timetable of security patrols, maps showing the locations of CCTV cameras, and the type of security threats the airport could face.
Other files contained details of routes and safeguards for cabinet ministers and foreign dignitaries, as well as details of the ultrasound radar system used to scan runways and the perimeter fence.
One of the maps shows where maintenance tunnels and escape shafts link the airport to the Heathrow Express train line.
Heathrow said in a statement that its security plans had been reviewed, it was taking steps to prevent similar leaks in future, it was “confident” the airport was secure, and that the safety and security of passengers and staff was a “top priority”.
“The UK and Heathrow have some of the most robust aviation security measures in the world, and we remain vigilant to evolving threats by updating our procedures on a daily basis,” the airport said.