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ATMs consist of various devices with their own firmware. Application Control solutions fall into the type of software that appeals to our interests the most. These are now widely available on the market, but attacks on ATMs have so far been successfully pursued.
We have become interested in how this software actually protects ATMs and does it make it harder for hackers to get to the cash. We will regard an ATM simply as a safe deposit, which is controlled by a computer.
Currency is put into boxes, which are loaded into two devices in the safe: The computer is connected to a card processing server through an isolated network. There are many people involved in the making of, installing, and operating ATMs.
Potentially, they can exploit their access for theft. These people and their capabilities are reflected in a typical model of threats to a bank owning ATMs. Our expertise covers threats coming impacting programmable components of a device.
Exploitation of these vulnerabilities should lead to arbitrary code execution at the highest level of the execution environment. The benefit of executing your own code on a built-in computer is that it allows sending commands to the dispenser, what usually happens each time we insert a card. However, our code, unlike built-in software, leaves out all irrelevant details, such as entering a PIN or requesting balance. It is just about cash — and all at once. One of the key points of ensuring a secure operation of an ATM is to protect the integrity of the system.
Information Security System ISS suppresses any attempts to modify the system or add external components to it: The goal is to prevent any extraneous code from running on the device. The system, in an unchanged way, must implement only its own functionality. That sounds reasonable, because, as practice shows, there are many ways to upload malware to the ATM PC:.
In addition, access control is equally important. Access to devices, processes, file system, registry objects, and other elements are controlled by the system, and with a correct configuration an attacker only has limited privileges at the initial stage of the attack. Having bypassed the integrity control described above, an attacker will still be dealing with the needto escalate privileges to access critical resources, such as the dispenser.
The market offers a number of products designed to mitigate risk for the banks which operate ATMs. Many of these are made by well-known AV-vendors and include technologies they have already developed: AV-scanner with signatures, heuristics, emulators, and unpackers; HIPS proactive protection, behavior analysis ; firewall.
Given the specific security requirements for this class of devices, the products also have more severe countermeasures such as application whitelisting and device control — generally described above as integrity control tools — as well as secondary access control tools as related to the OS native tools.
We have studied some of these ISS components and reached intriguing conclusions about their work. In its minimal configuration, the solution performs system integrity control.
There are two modules to accomplish this task:. Optionally, you can add Real-Time File Protection — antivirus protection. According to its signature bases and heuristics, the AV scanner will check files when they are accessed. For more information about the product, see the presentation: Kaspersky Embedded Systems Security. We will try to bypass this security system and achieve privileged execution of our code on the machine, which should allow us to use the connected dispenser.
According to the structure of the solutions offered in the product, our attack strategy has the following stages:. If this feature is enabled and correctly configured, it prevents attackers from using their tools.
The same goes for access through a Remote Desktop. Thus, obvious ways to enter data on the machine are left. Adding a keyboard to the whitelist is not likely to happen because it complicates maintenance service.
SYS driver captures launch of executables, scripts, and library sections loads. If it is a component from outside, the use of it will not be allowed.
There are built-in interpreters in the OS, such as WScript, which can be used despite the prohibition of arbitrary script execution. For example, it can be done like this:. The code from the file will be executed in the interpreter, and you can read and write files and registry keys, start processes, etc. It is interesting, but it will not be enough to work with the dispenser. A path to run the native code is needed. PowerShell is much more flexible in this regard, and it has direct access to the system API.
You can solve the problem of script execution prohibition the same way:. Here you can use the following bypass. By default, a rule is created that allows running binaries with digital signatures trusted in the OS. This applies to all Windows components. For example, you can use a debugger signed by Microsoft to inject a shellcode into a legitimate process. Recently, an attack scenario for this component of KESS was published.
It aimed to exhaust system resources by running a large number of binary instances in parallel, which do not have to pass the whitelist to be run. This attack would overload the verification module so that system calls to start processes were not processed within the timeout period, and after that the process was allowed to start without waiting for the KESS decision.
This way, you can achieve arbitrary code execution bypassing Device Control and Applications Launch Control whitelists in local or remote access to the PC. The next step is to escalate privileges to bypass the OS access control. The function is called at a request to the filter port; it generates a list of modules, which are loaded into the address space of the process from the query parameters.
Then, the buffer is sent back to the KESS service process to scan those modules. Pay close attention to the call of the ExAllocatePoolWithTag allocator which has a slightly odd calculation of the sz parameter before it. The logic of the function is this:. The first idea was to replace the InMemoryOrderModuleList pointer while executing the vulnerable function so that the value of sz would be taken to overflow it; and in the second run, of copying, the list would already be of a more appropriate size to fill the allocated memory.
This is possible to achieve, taking advantage of the fact that the function implements double-fetch in relation to our data. It is not possible to seize the moment appropriate for the replacement, so we can just run the cycle, change one value to another and hope for luck.
It works, but it is very unstable. Another convenient way was found by chance. We have noticed that the system has not crashed into the blue screen once when it was copying data string into the buffer from the invalid pointer to the module path. The point is that it cannot be seen in a decompiled listing of the function, but there is a handler for the exception:.
This way, we control length and content of the overflow accurately and reliably. It was suggested that this function could be called by the KESS service in the event of discovery of a certain malware, that could inject its libraries into some processes.
However, when scanning a large array of different samples, no such triggers were found. However, a user without administrator rights cannot do it. We have reversed kavshell. The function is called for each process at the start of KESS, but you also need rights to stop it. The time to use the scan is when the system begins. The exploit can be put into the autorun area which is available to the current user, and then it will restart the machine.
As soon as the exploit is run, the scan will work and impact our process too. However, it will take more time to make a spray in the pool to control the order of allocations; and, as the experiments have shown, we will not be able to make it during this time window.
We will do the following: The exploit launches calculators in suspended mode, and we are buying the time that the KESS kernel needs to work with these processes. To place the object that we are controlling following the overflowing buffer, we will try to create the desired kernel memory state. We need to understand that the vulnerable buffer is being allocated in a paged pool.
This is a little inconvenient for the following reasons:. To understand further details, it is highly recommended that you familiarize yourself with the Windows kernel allocator architecture. By connecting the debugger and inspecting the PoolDescriptor.
ListHeads lists of paged pools, one notices that at the time of the KESS memory scan, after Windows boots, there are allocation sizes that have not yet been involved in intensive system initialization processes.
You can allocate such blocks in paged pools by creating named objects, such as events. We can set the size of the vulnerable buffer and the size of the strings with event names, to get them into same PoolDescriptor. We also need to allocate an array of blocks in several paged pools. We add a small delay after each th object is created in our exploit and this time is usually enough to switch the pool index in the allocator.
All pools are being filled. Next, we make single-block holes in a spray, destroying some of the objects that have been created earlier. Free blocks of the chosen size are returned to the PoolDescriptor. There are different techniques of exploiting overflows in a pool. We will overwrite the Poolindex in the next block. When a block is freed, this value is adopted as an index of PoolDescriptor, to the corresponding ListHead of which the freed block will be added.
When PoolIndex is rewritten with a value higher than there are pools created by the system in our case, 5 , a call for ExDeferredFreePool will adopt NULL value — uninitialized address from the array, where addresses for PoolDescriptor are added when pools are created.
On these pages, we carefully build a fake PoolDescriptor with appropriate values for all members so that ExDeferredFreePool works without errors.