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The radio navigation planes use to land safely is insecure and can be hacked

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The radio navigation planes use to land safely is insecure and can be hacked

Radios that sell for $600 can spoof signals planes use to find runways.

Dan Goodin - 5/15/2019, 1:00 PM

A plane in the researchers' demonstration attack as spoofed ILS signals induce a pilot to land to the right of the runway.
Enlarge / A plane in the researchers' demonstration attack as spoofed ILS signals induce a pilot to land to the right of the runway.
Sathaye et al.
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Just about every aircraft that has flown over the past 50 years—whether a single-engine Cessna or a 600-seat jumbo jet—is aided by radios to safely land at airports. These instrument landing systems (ILS) are considered precision approach systems, because unlike GPS and other navigation systems, they provide crucial real-time guidance about both the plane’s horizontal alignment with a runway and its vertical angle of descent. In many settings—particularly during foggy or rainy night-time landings—this radio-based navigation is the primary means for ensuring planes touch down at the start of a runway and on its centerline.

Like many technologies built in earlier decades, the ILS was never designed to be secure from hacking. Radio signals, for instance, aren’t encrypted or authenticated. Instead, pilots simply assume that the tones their radio-based navigation systems receive on a runway’s publicly assigned frequency are legitimate signals broadcast by the airport operator. This lack of security hasn’t been much of a concern over the years, largely because the cost and difficulty of spoofing malicious radio signals made attacks infeasible.

Now, researchers have devised a low-cost hack that raises questions about the security of ILS, which is used at virtually every civilian airport throughout the industrialized world. Using a $600 software defined radio, the researchers can spoof airport signals in a way that causes a pilot’s navigation instruments to falsely indicate a plane is off course. Normal training will call for the pilot to adjust the plane’s descent rate or alignment accordingly and create a potential accident as a result.


One attack technique is for spoofed signals to indicate that a plane’s angle of descent is more gradual than it actually is. The spoofed message would generate what is sometimes called a “fly down” signal that instructs the pilot to steepen the angle of descent, possibly causing the aircraft to touch the ground before reaching the start of the runway.

The video below shows a different way spoofed signals can pose a threat to a plane that is in its final approach. Attackers can send a signal that causes a pilot’s course deviation indicator to show that a plane is slightly too far to the left of the runway, even when the plane is perfectly aligned. The pilot will react by guiding the plane to the right and inadvertently steer over the centerline.

Wireless Attacks on Aircraft Landing Systems.

The researchers, from Northeastern University in Boston, consulted a pilot and security expert during their work, and all are careful to note that this kind of spoofing isn't likely to cause a plane to crash in most cases. ILS malfunctions are a known threat to aviation safety, and experienced pilots receive extensive training in how to react to them. A plane that’s misaligned with a runway will be easy for a pilot to visually notice in clear conditions, and the pilot will be able to initiate a missed approach fly-around.

Another reason for measured skepticism is the difficulty of carrying out an attack. In addition to the SDR, the equipment needed would likely require directional antennas and an amplifier to boost the signal. It would be hard to sneak all that gear onto a plane in the event the hacker chose an onboard attack. If the hacker chose to mount the attack from the ground, it would likely require a great deal of work to get the gear aligned with a runway without attracting attention. What's more, airports typically monitor for interference on sensitive frequencies, making it possible an attack would be shut down shortly after it started.

In 2012, Researcher Brad Haines, who often goes by the handle Rendermanexposed vulnerabilities in the automatic dependent surveillance broadcast—the broadcast systems planes use to determine their location and broadcast it to others. He summed up the difficulties of real-world ILS spoofing this way:

If everything lined up for this, location, concealment of gear, poor weather conditions, a suitable target, a motivated, funded and intelligent attacker, what would their result be? At absolute worst, a plane hits the grass and some injuries or fatalities are sustained, but emergency crews and plane safety design means you're unlikely to have a spectacular fire with all hands lost. At that point, airport landings are suspended, so the attacker can't repeat the attack. At best, pilot notices the misalignment, browns their shorts, pulls up and goes around and calls in a maintenance note that something is funky with the ILS and the airport starts investigating, which means the attacker is not likely wanting to stay nearby.

So if all that came together, the net result seems pretty minor. Compare that to the return on investment and economic effect of one jackass with a $1,000 drone flying outside Heathrow for 2 days. Bet the drone was far more effective and certain to work than this attack.

Still, the researchers said that risks exist. Planes that aren’t landing according to the glide path—the imaginary vertical path a plane follows when making a perfect landing—are much harder to detect even when visibility is good. What’s more, some high-volume airports, to keep planes moving, instruct pilots to delay making a fly-around decision even when visibility is extremely limited. The Federal Aviation Administration’s Category III approach operations, which are in effect for many US airports, call for a decision height of just 50 feet, for instance. Similar guidelines are in effect throughout Europe. Those guidelines leave a pilot with little time to safely abort a landing should a visual reference not line up with ILS readings.

“Detecting and recovering from any instrument failures during crucial landing procedures is one of the toughest challenges in modern aviation,” the researchers wrote in their paper, titled Wireless Attacks on Aircraft Instrument Landing Systems, which has been accepted at the 28th USENIX Security Symposium. “Given the heavy reliance on ILS and instruments in general, malfunctions and adversarial interference can be catastrophic especially in autonomous approaches and flights.”

What happens with ILS failures

Several near-catastrophic landings in recent years demonstrate the danger posed from ILS failures. In 2011, Singapore Airlines flight SQ327, with 143 passengers and 15 crew aboard, unexpectedly banked to the left about 30 feet above a runway at the Munich airport in Germany. Upon landing, the Boeing 777-300 careened off the runway to the left, then veered to the right, crossed the centerline, and came to a stop with all of its landing gear in the grass to the right of the runway. The image directly below shows the aftermath. The image below that depicts the course the plane took.

An instrument landing system malfunction caused Singapore Airlines flight SQ327 to slide off the runway shortly after landing in Munich in 2011.
Enlarge / An instrument landing system malfunction caused Singapore Airlines flight SQ327 to slide off the runway shortly after landing in Munich in 2011.
The path Singapore Airlines flight SQ327 took after landing.
Enlarge / The path Singapore Airlines flight SQ327 took after landing.

An incident report published by Germany’s Federal Bureau of Aircraft Accident Investigation said that the jet missed its intended touch down point by about 1,600 feet. Investigators said one contributor to the accident was localizer signals that had been distorted by a departing aircraft. While there were no reported injuries, the event underscored the severity of ILS malfunctions. Other near-catastrophic accidents involving ILS failures are an Air New Zealand flight NZ 60 in 2000 and a Ryanair flight FR3531 in 2013. The following video helps explain what went wrong in the latter event.

Animation - Stick shaker warning and Pitch-up Upsets.

Vaibhav Sharma runs global operations for a Silicon Valley security company and has flown small aviation airplanes since 2006. He is also a licensed Ham Radio operator and volunteer with the Civil Air Patrol, where he is trained as a search-and-rescue flight crew and radio communications team member. He’s the pilot controlling the X-Plane flight simulator in the video demonstrating the spoofing attack that causes the plane to land to the right of the runway.

Sharma told Ars:

This ILS attack is realistic but the effectiveness will depend on a combination of factors including the attacker's understanding of the aviation navigation systems and conditions in the approach environment. If used appropriately, an attacker could use this technique to steer aircraft towards obstacles around the airport environment and if that was done in low visibility conditions, it would be very hard for the flight crew to identify and deal with the deviations.

He said the attacks had the potential to threaten both small aircraft and large jet planes but for different reasons. Smaller planes tend to move at slower speeds than big jets. That gives pilots more time to react. Big jets, on the other hand, typically have more crew members in the cockpit to react to adverse events, and pilots typically receive more frequent and rigorous training.

The most important consideration for both big and small planes, he said, is likely to be environmental conditions, such as weather at the time of landing.

“The type of attack demonstrated here would probably be more effective when the pilots have to depend primarily on instruments to execute a successful landing,” Sharma said. “Such cases include night landings with reduced visibility or a combination of both in a busy airspace requiring pilots to handle much higher workloads and ultimately depending on automation.”

Aanjhan Ranganathan, a Northeastern University researcher who helped develop the attack, told Ars that GPS systems provide little fallback when ILS fails. One reason: the types of runway misalignments that would be effective in a spoofing attack typically range from about 32 feet to 50 feet, since pilots or air traffic controllers will visually detect anything bigger. It’s extremely difficult for GPS to detect malicious offsets that small. A second reason is that GPS spoofing attacks are relatively easy to carry out.

“I can spoof GPS in synch with this [ILS] spoofing,” Ranganathan said. “It’s a matter of how motivated the attacker is.”

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An ILS primer

Tests on ILS began as early as 1929, and the first fully operational system was deployed in 1932 at Germany’s Berlin Tempelhof Central Airport.

ILS remains one of the most effective navigation systems for landing. Alternative approach systems such as VHF Omnidirectional Range, Non-Directional Beacon, global positioning system, and similar satellite navigation are referred to as non-precision because they provide only horizontal or lateral guidance. ILS, by contrast, is considered a precision approach system because it gives both horizontal and vertical (i.e. glide path) guidance. In recent decades, use of non-precision approach systems has decreased. ILS, meanwhile, has increasingly been folded into autopilot and autoland systems.

An overview of ILS, showing localizer, glideslope, and marker beacons.
Enlarge / An overview of ILS, showing localizer, glideslope, and marker beacons.
Sathaye et al.

There are two key components to ILS. A “localizer” tells a pilot if the plane is too far to the left or right of the runway centerline, while a “glideslope” indicates if the angle of descent is too big to put the plane on the ground at the start of the runway. (A third key component is known as “marker beacons.” They act as checkpoints that enable the pilot to determine the aircraft’s distance to the runway. Over the years, marker beacons have gradually been replaced with GPS and other technologies.)

The localizer uses two sets of antennas that broadcast two tones—one at 90Hz and the other at 150Hz—on a frequency that’s publicly assigned to a given runway. The antenna arrays are positioned on both sides of the runway, usually beyond the departure end, in such a way that the tones cancel each other out when an approaching plane is positioned directly over the runway centerline. The course deviation indicator needle will present a vertical line that’s in the center.

If the plane veers to the right, the 150Hz tone grows increasingly dominant, causing the course deviation indicator needle to move off-center. If the plane veers to the left of the centerline, the 90Hz tone grows increasingly dominant, and the needle will move to the right. While a localizer isn’t an absolute substitute for visually monitoring a plane’s alignment, it provides key, highly intuitive guidance. Pilots need only keep the needle in the center to ensure the plane is directly over the centerline.

Sathaye, et al.

A glideslope works in much the same way except it provides guidance about the plane’s angle of descent relative to the start of the runway. When an approaching plane’s descent angle is too little, a 90Hz tone becomes dominant, causing instruments to indicate the plane should fly down. When the descent is too fast, a 150Hz tone indicates the plane should fly higher. When a plane stays on the prescribed glide-path angle of about three degrees, the two sounds cancel each other out. The two glide-slope antennas are mounted on a tower at specific heights defined by the glide-path angle suitable for a particular airport. The tower is usually located near the touchdown zone of the runway.


Seamless spoofing

The Northeastern University researchers’ attack uses commercially available software defined radios. These devices, which cost between $400 and $600, transmit signals that impersonate the legitimate ones sent by an airport ILS. The attacker’s transmitter can be located either onboard a targeted plane or on the ground, as far as three miles from the airport. As long as the malicious signal is stronger than the legitimate one reaching the approaching aircraft, the ILS receiver will lock into the attacker signal and display attacker-controlled alignments to horizontal or vertical flight paths.

The experiment setup.
Enlarge / The experiment setup.
Sathaye et al.
Sathaye et al.

Unless the spoofing is done carefully, there will be sudden or erratic shifts in instrument readings that would alert a pilot to an ILS malfunction. To make the spoofing harder to detect, the attacker can tap into the precise location of an approaching plane using the Automatic Dependent Surveillance–Broadcast, a system that transmits a plane’s GPS location, altitude, ground speed, and other data to ground stations and other aircraft once per second.

Using this information, an attacker can start the spoofing when an approaching plane is either to the left or right of the runway and send a signal that shows the aircraft is aligned. An optimal time to initiate the attack would be shortly after the targeted plane has passed through a waypoint, as shown in the demonstration video near the beginning of this article.

The attacker would then use a real-time offset correction and signal generation algorithm that continuously adjusts the malicious signal to ensure the misalignment is consistent with the actual movements of the plane. Even if attackers don’t have the sophistication to make spoofing seamless, they could still use malicious signals to create denial-of-service attacks that would prevent pilots from relying on ILS systems as they land.

The offset correction algorithm takes into account an aircraft's real-time position to calculate the difference in the spoofed offset and the current offset.
Enlarge / The offset correction algorithm takes into account an aircraft's real-time position to calculate the difference in the spoofed offset and the current offset.
Sathaye et al.

One variety of spoofing is known as an overshadow attack. It sends carefully crafted tones with a higher signal strength that overpower the ones sent by the airport ILS transmitter. A malicious radio on the ground would typically have to transmit signals of 20 watts. Overshadow attacks have the advantage of making seamless takeovers easier to do.

An overshadow attack.
Enlarge / An overshadow attack.
Sathaye et al.

A second spoofing variety, known as a single-tone attack, has the advantage of working by sending a single frequency tone at a signal strength that’s lower than the airport ILS transmitter. It comes with several disadvantages, including requiring an attacker to know specific details about a targeted plane, like where its ILS antennas are located, for the spoofing to be seamless.

A single-tone attack.
Enlarge / A single-tone attack.
Sathaye et al.

No easy fix

So far, the researchers said, there are no known ways to mitigate the threat posed by spoofing attacks. Alternative navigation technologies—including high-frequency omnidirectional range, non-directional beacons, distance measurement equipment, and GPS—all use unauthenticated wireless signals and are therefore vulnerable to their own spoofing attacks. What’s more, only ILS and GPS are capable of providing both lateral and vertical approach guidance.

In the paper, researchers Harshad Sathaye, Domien Schepers, Aanjhan Ranganathan, and Guevara Noubir of Northeastern University’s Khoury College of Computer Sciences went on to write:

Most security issues faced by aviation technologies like ADS-B, ACARS and TCAS can be fixed by implementing cryptographic solutions. However, cryptographic solutions are not sufficient to prevent localization attacks. For example, cryptographically securing GPS signals similar to military navigation can only prevent spoofing attacks to an extent. It would still be possible for an attacker to relay the GPS signals with appropriate timing delays and succeed in a GPS location or time spoofing attack. One can derive inspiration from existing literature on mitigating GPS spoofing attacks and build similar systems that are deployed at the receiver end. An alternative is to implement a wide-area secure localization system based on distance bounding and secure proximity verification techniques [44]. However, this would require bidirectional communication and warrant further investigation with respect to scalability, deployability etc.

Federal aviation administration officials said they didn't know enough about the researchers' demonstration attack to comment.

The attack and the significant amount of research that went into it are impressive, but the paper leaves a key question unanswered—how likely is it that someone would expend the considerable amount of work required to carry out such an attack in the real world? Other types of vulnerabilities that, say, allow hackers to remotely install malware on computers or bypass widely used encryption protections are often easy to monetize. That’s not the case with an ILS spoofing attack. Life-threatening hacks against pacemakers and other medical devices also belong in this latter attack category.

While it is harder to envision the motivation for such hacks, it would be a mistake to rule them out. A report published in March by C4ADS, a nonprofit that covers global conflict and transnational security issues, found that the Russian Federation has engaged in frequent, large-scale GPS spoofing exercises that cause ship navigation systems to show they are 65 or more miles from their true location.

“The Russian Federation has a comparative advantage in the targeted use and development of GNSS spoofing capabilities,” the report warned, referring to Global Navigation Satellite Systems. “However, the low cost, commercial availability, and ease of deployment of these technologies will empower not only states, but also insurgents, terrorists, and criminals in a wide range of destabilizing state-sponsored and non-state illicit networks.”

While ILS spoofing seems esoteric in 2019, it wouldn’t be a stretch to see it become more banal in the coming years, as attack techniques become better understood and software defined radios become more common. ILS attacks don’t necessarily have to be carried out with the intention of causing accidents. They could also be done with the goal of creating disruptions in much the way rogue drones closed London’s Gatwick Airport for several days last December, just days before Christmas, and then Heathrow three weeks later.

“Money is one motivation, but display of power is another,” Ranganathan, the Northeastern University researcher, said. "From a defense perspective, these are very critical attacks. It’s something that needs to be taken care of because there are enough people in this world who want to display power.”


Sursa: https://arstechnica.com/information-technology/2019/05/the-radio-navigation-planes-use-to-land-safely-is-insecure-and-can-be-hacked/

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