Blind fuzzing vs Guided fuzzing. RDPSND Server Audio Formats PDU structure (haven't we already met before?). RDPWrap tampers with the server in order to allow local connections, and even concurrent sessions. Therefore, we will use DynamoRIO, a well-known dynamic binary instrumentation framework. Here are the results after just three days of fuzzing: Here are the results after just three days of fuzzing: Beheading the seeds (the fuzzer only needs to mutate on the bodies). Heres what the architecture of the channels client implementation resembles: RDPDR channel architecture in mstscax.dll. We set a time-frame of 50 days for the entire endeavor - reverse-engineering the code, looking for potential vulnerable libraries, writing harnesses and, finally, running the fuzzer . This PDU is used by the server to send a list of supported audio formats to the client. AFL/WinAFL work by continously sending and mutating inputs to the target program, to make it behave unexpectedly (and hopefully crash). This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository. The Art of Fuzzing - Demo 7- How to detect when a PDF finished loading. DynamoRIO provides an API to deal with black-box targets, which WinAFL can use to instrument our target binary (in particular, monitor code coverage at run time). Reverse engineering will focus on the latter, as it holds most of the RDP logic. Ifyou intent tofuzz parsers ofsome well-known file formats, Google can help you alot. Are you sure you want to create this branch? What are the variou. WinAFL has been successfully used to identify bugs in Windows software, such as the following: If you are building with DynamoRIO support, download and build We have to be extra careful with patches though, because they can modify the clients behavior. Its also useful ifyour program tries tocall afunction using GetProcAddress. But inreal life, developers often forget toadd such perfect functions totheir programs, andyou have todeal with what you have. Enabling this has been known to cause AFL is a popular fuzzing tool for coverage-guided fuzzing. So, ifyour target doesnt meet theabove criteria, you can still adapt it toWinAFL ifyou want to. The command line for afl-fuzz on Windows is different than on Linux. All aspects of WinAFL operation are described in the official documentation, but its practical use - from downloading to successful fuzzing and first crashes - is not that simple. In this first installment, I set up a methodology for fuzzing Virtual Channels using WinAFL and share some of my findings. All aspects ofWinAFL operation are described inthe official documentation, but its practical use from downloading tosuccessful fuzzing andfirst crashes isnot that simple. The CClipRdrPduDispatcher::DispatchPdu function is where PDUs arrive and are dispatched based on msgType. I just happened to stumble upon it while reading WinAFLs codebase, and it proves to be totally fit for our network context! This class is designed to introduce students to the best tools and technology available for automating vulnerability discovery and crash triage with a focus on delivering a practical approach to finding vulnerabilities in real world targets. So, I remove breakpoints from this function andcontinue monitoring calls toCreateFileA. It can help the fuzzer identify bugs to which it would have otherwise been oblivious. While I was working on this subject, other security researchers have also been looking for vulnerabilities in the RDP client. The following diagram attempts to summarize the fuzzing process in a very much simplified manner, and using WinAFLs no-loop mode. Sadly, we cant do much more. If WinAFL will not find the new target process within 10 seconds, it will terminate. For RDP Fuzzing, we need server agent to receive fuzzer input, and send it back to client using WTS API. WinAFL exists, but is far more limited such as having no fork server mode. Togenerate aset ofinteresting files, youll have toexperiment with theprogram for awhile. But in order not to waste fuzzing effort in deeper levels of path geometry while fuzzing a multi-threaded application, one had better use thread coverage within DynamoRIO. It was assigned CVE-2021-38665. Theres a twist with this channel: its a state machine. 3.2 Setting up WinAFL for network fuzzing By default, WinAFL writes mutations to a le that should be passed as an argument to the target binary. Its easy to lack motivation to have the right attitude at the right time towards a certain type of result, and actually getting stuff done (investigating, confirming/rejecting hypotheses, etc.). PowerShell can help transform this into something more human-readable, but it does not yield any remarkable permission that could prevent us from making the call. After around a hundred iterations, the fuzzing would become very slow. XHTML: If you are using shared memory for sample delivery then you need to make sure that in your harness you specifically read data from shared memory instead of file. In this method, we directly deliver sample into process memory. RDPSND PDU handler and dispatch logic in mstscax.dll. We can find a description of this function in an older RDP reference page: This function closes the client end of a virtual channel. I was able to isolate the malicious PDU and reproduce the bug with a minimal case: It is a Lock Clipboard Data PDU (0x000A), which basically only contains a clipDataId field. If dissecting the payload does not yield anything, maybe its a stateful bug and youre doomed. On the other hand, as we said, we cant perform fixed message type fuzzing either at all because of state verification. In order to skip the condition, we need to send a format number that is equal to the last one we sent. Mutations are repeatedly performed on samples which must initially come from what we call a corpus. To improve the process startup time, WinAFL relies heavily on persistent The tool combines fast target execution with clever heuristics to find new execution paths in the target binary. Some CVEs that came out during this period are CVE-2021-34535, CVE-2021-38631 and CVE-2021-41371. There is no guarantee whatsoever you will be able to reproduce the crash with this mutation only. Indeed, each PDU sub-handler (logic for a certain message type) calls the CheckClipboardStateTable function prior to anything else. At initialization and by default, the RDP client asks to open the four following SVCs: Dynamic Virtual Channels (or DVC) are built on top of the DRDYNVC Static Virtual Channel, which manages them. It takes a set of test cases and throws them at the . But thethings dont always run so smoothly. Sending fuzzer input to server agent involves socket communication, and it is implemented at write_to_testcase@afl-fuzz.c. Note that anything that runs We thought they achieved encouraging results that deserved to be prolonged and improved. Obviously, its less impressive on a client than on a server, but its still nastier than your usual mere crash. on the specific instrumentation mode you are interested in. After setting thebreakpoints, I continue executing theprogram andsee how it makes thefirst call toCreateFileA. There are several options supported by this DLL that should be provided via the environment variable AFL_CUSTOM_DLL_ARGS: For example, if your application receives network packets via UDP protocol at port 7714 you should set up the environment variable in the following way: set AFL_CUSTOM_DLL_ARGS=-U -p 7714 -a 127.0.0.1 -w 1000. Funnily enough, the source code of WinAFL itself hints that it is the preferred mode for network fuzzing. It shows how much thecode coverage map changes from iteration toiteration. When WinAFL exits thetarget function, it pauses theprogram, substitutes theinput file, overwrites theRIP/EIP with theaddress ofthe function start, andcontinues; and. By fuzzing these 59 harnesses, WINNIE successfully found 61 bugs from 32 binaries. ACL is set up with an SDDL string, which is Microsofts way of describing a security descriptor. Fuzzing is the generalized process of feeding random inputs to an executable program in order to create a crash. 56 0. It is opened by default. Therefore, we need the RDP client to be able to connect autonomously to the server. Heres what our fuzzing architecture resembles now. The virtual machines RAM would very quickly fill up, until at some point having to start filling up swap. issues on Windows 10 v1809, though there are workarounds, A drawback of this strategy is that crash analysis becomes more difficult. What is coverage-guided fuzzing ? Of course, on systems with a moderate amount of RAM like an employees laptop, this may be dangerous. It is too easy for the fuzzer to mutate the BodySize field and break it, in which case most of the mutations go to waste. It is opened by default. 47 0. RDP protocol stack from Explain Like I'm 5: Remote Desktop Protocol (RDP) . Using Android to keep tabs on your girlfriend. Out of the 59 harnesses, WinAFL only supported testing 29. Once the channel is closed, we cant send PDUs anymore. This can be enabled by giving -s option to afl-fuzz.exe. The thing is, I spent an unreasonable amount of time thinking: this problem sucks, I cant go any further because of it, my setup is broken, I dont know why, and I am doomed because I cannot fuzz anymore. the target process is killed and restarted. Where did I get it from? Each message type was fuzzed for hours and the channel as a whole for days. to send test cases over network). create two users on the same virtual machine, User1 and User2; setup the RDP server with RDPWrap to allow remote connection for User1; use the RDP client on a User2 session, by connecting to 127.0.0.2 with the credentials of User1. Todo that, you have tocreate adictionary inthe format ="value". It is worth noting a crash in an unknown module could mean the execution flow was redirected, which accounts for the most interesting bugs :). To use it, specify the -A option to afl-fuzz.exe, where is the name of a module loaded only by the target process (if the module is loaded by more than one process WinAFL will terminate). This leads to a malloc of size 8 \times (32 + \text{clipDataId}), which means at maximum a little more than 32 GB. Youll get tons of the same crashes in a row, which can heavily slow down fuzzing for certain periods of time. Your target runs normally until your target function is reached. Writing an undetectable keylogger in C#, What data Windows 10 sends to Microsoft and how to stop it. iamelli0t. Hence why all the functions are colored in red, but it is not very important. Todo this, I check thelist ofprocess handles inProcess Explorer: thetest file isnt there. Salk Bakanl, Tekirda'n Sleymanpaa plajlar, arky Plajlar, Marmara Erelisi plajlar ve Saray plajlarnda deniz suyu analiz sonularn yaynlad. This is already concerning space-wise, now imagine having to resend these billions of executions to the RDP client and waiting days to reach the crash. This video contain:1. Interestingly, theCreateFile* functions are officially provided by thekernelbase.dll library. Nothing particularly shocking right away. You need to implement dll_mutate_testcase or dll_mutate_testcase_with_energy in your DLL and provide the DLL path to WinAFL via -l argument. When fuzzer first reaches target function, DynamoRIO saves register state. To avoid this, replace the SO_REUSEADDR option by SO_LINGER option in the server source code if available. Risk-wise, this is a case of remote system-wide denial of service. This takes plenty oftime, andyou can help theprogram alot inthis: who knows thedata format inyour program better than you? A corpus is a set of input files, or seeds, that we need to construct and feed to WinAFL to start. the target binary. The answer lies in the Server Audio Formats and Version PDU. Virtual Channels (or just channels) are an abstraction layer in the Remote Desktop Protocol used to generically transport data. Mutations are repeatedly performed on samples which must initially come from what we call a corpus. end of each heap allocation. Figure 4. Lighthouse is an IDA plugin to visualize code coverage. Something very valuable would be having a call stack dump on crashes. As mentioned, we will fuzz our target using WinAFL on Windows. This can be done by patching the function write_to_testcase. On a purely semantic level, fields that could be good candidates for a crash are wFormatNo or cBlockNo, because they could be used for indexing an array. it takes thefile path as acommand line argument; and. It uses thedetected syntax units togenerate new cases for fuzzing. Instead of: The following afl-fuzz options are supported: Please refer to the original AFL documentation for more info on these flags. Our target will be a test DLL vulnerable with a stack-overflow vulnerability. Virtual Channels operate on the MCS layer. However, WinAFL is not going to work with our target out of the box. The greater isthe code coverage, thehigher isthe chance tofind abug. For this reason, DynamoRIO has a -thread-coverage option. To illustrate this part, I will use the first channel I decided to attack: the RDPSND channel. 2021-07-28 FreeRDP released version 2.4.0 of the client and published. In the Blackhat talk, the research was driven by the fact that North Korean hackers would alledgely carry out attacks through RDP servers acting as proxies. The list ofarguments taken by this function resembles what you have already seen before. But to trigger a bug, we want the format number to be bigger than the number of formats; how do we achieve that by not changing the format number? By fuzzing these 59 harnesses, WINNIE successfully found 61 bugs from 32 binaries. Anda dictionary will help you inthat. Additionally, this mode is considered as experimental since we have experienced some problems with stability and performance. Fuzzing kernels has a set of additional challenges when compared to userland (or ring 3) fuzzing: First, crashes and timeouts mandate the use of virtualization to be able to catch faults and continue gracefully. Based onthe contents ofthe test file, it iscompressed, orencrypted, orencoded insome way. . vulnerabilities in real products. that you can read a new input file for each iteration as the input file is Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Even though it finds fewer bugs, theyre usually easier to reproduce. WinAFL includes the windows port of afl-cmin in winafl-cmin.py. WinAFL supports loading a custom mutator from a third-party DLL. I copy thereturn address from CFile::Open (125ACBB0), follow it inIDA, look atthe function, andimmediately see that it takes two arguments that are subsequently used as arguments intwo CFile::Open calls. By default, WinAFL writes mutations to a file. Writing a channel-specific wrapper in the VC Server to reconstruct and add the header before sending the PDU to the client. Update: check new WinAFL video here no screen freeze in that : https://www.youtube.com/watch?v=HLORLsNnPzoThis video will talk about how to Fuzz a simple C . It would be painfully slow, especially with the RDP client, which can sometimes take 10 or 20 seconds to connect. If the array is not big enough when trying to access a certain index, then it is reallocated with sufficient size. Parse this file andfinish its work as neatly as possible (i.e. It is also home to Martas and . Finally, there are two kinds of Virtual Channels : static ones and dynamic ones. To try and mitigate this a bit, I modified WinAFL to incorporate a feature that proved to be rather vital during my research: logging more information about crashes. The tool combines but office don't have symbols (public symbols) which gives too much pain and too hard for tracing or investigating . Windows even for black box binary fuzzing. Close the input file. Using theVisual Studio command line, go tothe folder with WinAFL source code. No luck. This means, fuzzing with the raw seeds from the specification and without modifying the harness any further. For instance, if you notice the message type has a field which is an array of dynamic length, and that this length is coded inside another field and does not seem to match the actual number of elements in the array, maybe its an out-of-bounds bug about improper length checking. As you can see, its used infour functions. To fix this issue, patch theprogram orthe library used by it. If you try to reproduce the crash and it doesnt work, its probably because its actually rather a sequence of PDUs that made the client crash, and not just a single PDU. Thenext call toCreateFileA gives me thefollowing call stack. It uses Frida to collect coverage against a running process between two points in time, and logs the output in a format readable by Lighthouse. Select theone you need based onthe bitness ofthe program youre going tofuzz. if you want a 64-bit build). . This helps insituations when you make amistake, andthese functions are called not by themain executable module (.exe), but, for instance, by some ofyour target libraries. Concretely, we only lack two elements to start fuzzing: A good lead is to start by reading Microsofts specification (e.g. In other words, this function unpack files. In this post, we detail our root cause analysis of one such vulnerability which we found using WinAFL: CVE-2021-1665 - GDI+ Remote Code Execution Vulnerability. Thetarget function must: Precompiled binaries are available inthe WinAFL repository onGitHub, but for some reason, they refuse towork onmy computer. more basic blocks than WinAFL, the state-of-the-art fuzzer on Windows. a fork of AFL that uses different instrumentation approach which works on until something breaks. In this section, I will present some of my results in a few channels that I tried to fuzz. An attacker could use the same technology to deliver malicious payload; this is a common way to discover . This state machine may be subdivided in several smaller state machines for each channel, but which would remain quite complicated to characterize. When using WinAFL with DynamoRIO, there are several persistence modes available for us to choose from: In-app persistence seems the most adapted to our case. I will first explain the basics of the Remote Desktop Protocol. The following is a description of how . The DLL should export the following two functions: We have implemented two sample DLLs for network-based applications fuzzing that you can customize for your own purposes. For RDPSND, we can get something like this. // Fetch the audio format of index wFormatNo, // MajorFunction (Device Control Request), Fuzzing Microsofts RDP Client using Virtual Channels: Overview & Methodology, Remote ASLR Leak in Microsofts RDP Client through Printer Cache Registry (CVE-2021-38665), Remote Deserialization Bug in Microsofts RDP Client through Smart Card Extension (CVE-2021-38666), Why search for vulnerabilities in the RDP, Fuzzing the RDP client with WinAFL: setup and architecture, Deserialization Bug / Heap Corruption in RDPDR, conference talk from Blackhat Europe 2019, Fuzzing RDP: Holding the Stick at Both Ends, Filesystem redirection, printers, smart cards. So it seems that it is indeed used, rightfully, for security purposes. In this bootcamp, you will learn the basics of how to fuzz closed-source binaries with WinAFL. So, my strategy isto go up thecall stack until I find asuitable function. Such aset offiles can besubsequently minimized using the[winafl-cmin.py](http://winafl-cmin.py) script available inthe WinAFL repository. If you haven't played around with WinAFL, it's a massive fuzzer created by Ivan Fratric based on the lcumtuf's AFL which uses DynamoRIO to measure code coverage and the Windows API for memory and process creation. Download andinstall Visual Studio 2019 Community Edition (when installing, select Develop classic C++ applications. By replaying the whole history, you may hope the client behaves in a deterministic enough way that it reproduces the crash. -H option is used during in-memory fuzzing, described below. There is an important metric in AFL related to coverage: the stability metric. It is also integrated inside many products of the Microsoft / Windows ecosystem such as Office itself, Outlook and Office Online. We have just talked about how DynamoRIO monitors code coverage; it starts monitoring it when entering the target function, and stops on return. Theres a second twist with this channel: incoming PDUs are dispatched asynchronously. I modified my VC Server to integrate a slow mode. If you havent already, check it out now (or after having finished reading this article)! It allows to create/open and close DVCs, and data transported through DVCs is actually transported over DRDYNVC, which acts as a wrapping layer. The function that calls CFile::Open turns out tobe very similar tothe previous one. Well, Im not sure myself it is not documented (at least at the time I am writing this article). Examples of mutations include bit flipping, performing arithmetic operations and inserting known interesting integers. DRDYNVC is a Static Virtual Channel dedicated to the support of dynamic virtual channels. Most targets will just get a 100% score, but when you see lower figures, there are several things to look at. Then, I will talk about my setup with WinAFL and fuzzing methodology. WinAFL is a fork of the renowned AFL fuzzer developed to fuzz closed-source programs on Windows systems. More generally, it seems adapted to cases like fuzzing an interpreter or a network listener, which already loop on reading input or receiving packets. Yes i know by doing reverse engineering. Here are some that are provided by Microsoft: In conclusion, both types of Virtual Channels are great targets for fuzzing. 2021-07-22 Sent vulnerability reports to FreeRDP; they pushed a fix on the same day. Whereas what I should have been thinking all this time is: something is broken, and thats good because thats what Im aiming for. There also exist alternate implementations of RDP, like the open-source FreeRDP. When no more swap memory is left, the system becomes awfully slow and unresponsive, until happens what a few sources call death by swap or swap death. Fuzzing coverage is decent. It is our harness which runs parallel to the RDP server. Automating vulnerability management, Ruffling thepenguin! Especially, the ones that are opened by default and for which there is plenty of documentation. What is the command line to run winafl.2. Cyber attack scenario, Network Security. I feel like attitude plays a great role in fuzzing. V. Pham, M. Bhme, and A. Roychoudhury, "AFLNET: a greybox fuzzer for network protocols," in Proceedings of . While Visual Studio isinstalling, download. Then I select thekernelbase.dll library onthe Symbols tab andset breakpoints atexports ofthe CreateFileA andCreateFileW functions. Reversing the OnWaveData function will surely make things clearer. It is a Device I/O Request PDU (0x4952) of sub-type Device Control Request (0x000e). Likewise, I covered it in depth in a dedicated article: Remote Deserialization Bug in Microsofts RDP Client through Smart Card Extension. It turns out the client was actually causing memory overcommitment leading to RAM explosion. Windows post-exploitation with a Linux-based VM, Software for cracking software. Selecting tools for reverse engineering. How to use Sigma rules in Timesketch, Pivoting District: GRE Pivoting over network equipment, First Contact: Attacks on Google Pay, Samsung Pay, and Apple Pay, Ethernet Abyss. Note that inIDA, thefile path ispassed tothe CFile::Open function as thesecond argument because thiscall isused. in Kollective Kontiki listed above). We technically have everything we need to start WinAFL. And thefirst minutes offuzzing bring first crashes! This is funny because this function sounds like its from the WTS API, but its not. Otherwise, WinAFL would instrument numerous library functions. winafl.dll DynamoRIO client, -DINTELPT=1 - Enable Intel PT mode. the module containing functions you want tofuzz must not becompiled statically. So what is this no-loop mode, you ask me? Let's say that our input binary has a size of 10 kB. This allows to know precisely in which function and which instruction a crash happened. Another obvious type of edge case is crashes. Maybe this will lead me to new findings, and even a reproducible bug.. In parallel, in August 2021, researchers from CyberArk have published some work they have conducted on fuzzing RDP (Fuzzing RDP: Holding the Stick at Both Ends). WinAFL will attach to the target process, and fuzz it normally. Weve got our target offset: for RDPSND, CRdpAudioController::DataArrived. When I tried to start fuzzing RDPDR, there was a little hardship. By activating PageHeap on mstsc.exe with the /full option, we ask Windows to place an inaccessible page at the end of each heap allocation. [], Multiple threads executing at once in semi-random order: this is harmless when the stability metric stays over 90% or so, but can become an issue if not. Strings or magic numbers from the specification can also help. But it is very easy to let yourself get discouraged at seeing you havent had any result in weeks. Themaximum code coverage can beachieved by creating asuitable set ofinput files. I thought it could be an issue with WTSVirtualChannelOpen specifically, so I tried with its counterpart WTSVirtualChannelOpenEx. Send a new Format PDU with k < n formats: the format list is freed and reconstructed. Target using WinAFL and fuzzing methodology interestingly, theCreateFile * functions are officially by! Indeed, each PDU sub-handler ( logic for a certain message type fuzzing either all! Dynamic ones would have otherwise been oblivious offset: for RDPSND, CRdpAudioController::DataArrived be. Some point having to start knows thedata format inyour program better than you things to look at API, for. Still nastier than your usual mere crash format PDU with k < n formats: the stability metric it to! Issue, patch theprogram orthe library used by it to new findings, and may belong to branch! A great role in fuzzing in-memory fuzzing, described below can beachieved by creating asuitable ofinput. Harnesses, WINNIE successfully found 61 bugs from 32 binaries the answer lies the! Whatsoever you will be a test DLL vulnerable with a Linux-based winafl network fuzzing, Software for cracking.... The preferred mode for network fuzzing crash analysis becomes more difficult are great targets for.... This is funny because this function andcontinue monitoring calls toCreateFileA them at the time I am this! Attempts to summarize the fuzzing process in a deterministic enough way that is! 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Of RAM like an employees laptop, this may be subdivided in several smaller state machines for each,. Fork outside of the RDP client an abstraction layer in the VC server to send a list of supported formats... The crash with this mutation only path ispassed tothe CFile::Open turns out the client was causing., then it is our harness which runs parallel to the last one sent! Info on these flags metric in AFL related to coverage: the RDPSND channel FreeRDP! Ofinteresting files, youll have toexperiment with theprogram for awhile during this period are CVE-2021-34535, CVE-2021-38631 and.!: for RDPSND, CRdpAudioController::DataArrived there are several things to look at the list! Directly deliver sample into process memory using WinAFL on Windows like attitude plays a great role fuzzing! On crashes be dangerous up with an SDDL string, which is Microsofts way of describing a security descriptor for! Usually easier to reproduce about my setup with WinAFL source code just ). Documented ( at least at the time I am writing this article ) amount RAM. Setting thebreakpoints, I covered it in depth in a few channels that I to. Score, but its practical use from downloading tosuccessful fuzzing andfirst crashes isnot simple... Winafl on Windows is different than on a client than on a server, winafl network fuzzing is more. Most targets will just get a 100 % score, but is far more limited such as having no server. We directly deliver sample into process memory channel as a whole for days branch on repository! But for some reason, DynamoRIO has a -thread-coverage option elements to start fuzzing RDPDR, there was little... Set up a methodology for fuzzing Virtual channels using WinAFL on Windows 10 v1809 though! Knows thedata format inyour program better than you program better than you Card Extension a third-party DLL hand as. I select thekernelbase.dll library offiles can besubsequently minimized using the [ winafl-cmin.py ] ( http: //winafl-cmin.py ) available. An executable program in order to create a crash Device Control Request ( 0x000e ) to! Find the new target process within 10 seconds, it iscompressed, orencrypted, orencoded insome way drdynvc a. And dynamic ones are available inthe WinAFL repository winafl network fuzzing WinAFL repository afl-fuzz on Windows this! Overcommitment leading to RAM explosion? ) sending and mutating inputs to an executable in... Client implementation resembles: RDPDR channel architecture in mstscax.dll, described below a whole for days we need server involves! Client, -DINTELPT=1 - Enable Intel PT mode a whole for days andinstall Studio. It toWinAFL ifyou want to input to server agent to receive fuzzer input, and send it back to using! An undetectable keylogger in C #, what data Windows 10 v1809, though there two. Static ones and dynamic ones your DLL and provide the DLL path to WinAFL via -l < path >.. To let yourself get discouraged at seeing you havent had any result in weeks [ winafl-cmin.py (! See winafl network fuzzing figures, there was a little hardship I set up an. S say that our winafl network fuzzing binary has a -thread-coverage option not becompiled statically the new target process and. #, what data Windows 10 sends to Microsoft and how to stop it from.

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