In the pessimistic case in which were fuzzing at high speeds for a whole week-end and mutations are 100 bytes long on average, thats 24 GB of PDU history. This time, we want to let WinAFL fuzz only the body part of the message. Having the module and offset is already of a huge help in understanding crashes though: start reversing the client where it crashed and work your way backwards. If WinAFL refuses torun, try running it inthe debug mode. If you havent already, check it out now (or after having finished reading this article)! Official, documented Virtual Channels by Microsoft come by dozens: Non-exhaustive list of *Virtual Channels* documented by Microsoft, found in the FreeRDP wiki. From this bug, we learned a golden rule of fuzzing: that it is not only about crashes. I tried logging debug strings from winsta!WinStationVirtualOpenEx with DebugView++. We can find a description of this function in an older RDP reference page: This function closes the client end of a virtual channel. Theres a twist with this channel: its a state machine. This is an interesting approach because sending a sequence of PDUs of different types in a certain order can help the client enter a state in which a bug will be triggered. AFL/WinAFL work by continously sending and mutating inputs to the target program, to make it behave unexpectedly (and hopefully crash). In the function CClipBase::OnLockClipData, this field is used with some kind of smart array object: Eventually, the function DynArray::CCleanType,unsigned long>::Grow is called and performs: My guess is that an array of dynamic length is used to store information, such as a lock tag, about file streams based on their id (if this is really the case, then it is probably poor choice of data structure). Finally, before we start fuzzing, we should enable a little something that will be useful: PageHeap (GFlags). Therefore, the RDP client will receive a lot of different message types, in a rather random order. Indeed, when fuzzing, you dont want to kill and start your target again every execution. Even though they also used WinAFL and faced similar challenges, their fuzzing approach is interesting and somewhat differs from the one I will present in this article. but office don't have symbols (public symbols) which gives too much pain and too hard for tracing or investigating . This article begins my three-part series on fuzzing Microsofts RDP client. On the other hand, as we said, we cant perform fixed message type fuzzing either at all because of state verification. Themaximum code coverage can beachieved by creating asuitable set ofinput files. In order to skip the condition, we need to send a format number that is equal to the last one we sent. 2021-07-23 Microsoft started reviewing and reproducing. It is our harness which runs parallel to the RDP server. There is a second DLL custom_winafl_server.dll that allows winAFL to act as a server and perform fuzzing of client-based applications. I set breakpoints atits beginning andend andsee what happens. 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. Youll get tons of the same crashes in a row, which can heavily slow down fuzzing for certain periods of time. This implies a lot; we will talk about this. more basic blocks than WinAFL, the state-of-the-art fuzzer on Windows. A blind fuzzer, or blackbox fuzzer, is a fuzzer with no knowledge of a program's inner workings. As a result, real bugs in the RDP client will only constitute a subset of the bugs we will find with the patched DLL. If its not in the correct state, it just drops the message and does not do anything. 47 0. This is a critical fact we must take into account for when we are fuzzing later! Concretely, we only lack two elements to start fuzzing: A good lead is to start by reading Microsofts specification (e.g. Code coverage for our RDPSND fuzzing campaign using Lighthouse. It needs to be adapted to our case, which is fuzzing a client in a network context. This function is a virtual extension that can be used to protect per-session data in the virtual channel client DLL. CVE-2018-20250, CVE-2018-20251, CVE-2018-20252, CVE-2018-20253, https://github.com/DynamoRIO/dynamorio/releases, https://github.com/googleprojectzero/winafl/blob/master/readme_pt.md, https://github.com/googleprojectzero/Jackalope/blob/6d92931b2cf614699e2a023254d5ee7e20f6e34b/test.cpp#L41, https://github.com/googleprojectzero/Jackalope/blob/6d92931b2cf614699e2a023254d5ee7e20f6e34b/test.cpp#L111, CVE-2018-12853, CVE-2018-16024, CVE-2018-16023, CVE-2018-15995, CVE-2018-16004, CVE-2018-16005, CVE-2018-16007, CVE-2018-16009, CVE-2018-16010, CVE-2018-16043, CVE-2018-16045, CVE-2018-16046, CVE-2018-19719, CVE-2018-19720, CVE-2019-7045, [CVE-2021-33599, CVE-2021-33602, CVE-2021-40836, CVE-2021-40837, CVE-2022-28875, CVE-2022-28876, CVE-2022-28879, CVE-2022-28881, CVE-2022-28882, CVE-2022-28883, CVE-2022-28884, CVE-2022-28886, CVE-2022-28887 ], (Let me know if you know of any others, and I'll include them in the list), Dynamic instrumentation using DynamoRIO (. We have just talked about how DynamoRIO monitors code coverage; it starts monitoring it when entering the target function, and stops on return. the specific instrumentation mode you are interested in. But should we really just start fuzzing naively with the seeds weve gathered from the specification? the target process is killed and restarted. In order to achieve coverage-guided fuzzing, WinAFL provides several modes to instrument the target binary: Intel PT has limitations within virtualized environments, and there are too many constraints for us to use Syzygy (compilation restrictions). The PDU sub-handling logic is therefore run in a different thread. The logic used inWinAFL has anumber ofsimple requirements tothe target function used for fuzzing. If the array is not big enough when trying to access a certain index, then it is reallocated with sufficient size. Some researchers collect impressive sets offiles by parsing Google outputs. Note that you need a 64-bit winafl.dll build if Please run the Cyber attack scenario, Network Security. Skimming through the functions, we can try to assess whether were satisfied or not with the coverage. We did gather earlier a little list of channels that looked like fruitful targets. I eventually switched to deterministic and noticed it usually happened around 5 minutes of fuzzing. A team of researchers (Chun Sung Park, Yeongjin Jang, Seungjoo Kim and Ki Taek Lee) found an RCE in Microsofts RDP client. WinAFL managed to find a sequence of PDUs which bypasses a certain condition to trigger a crash and we could have very well overlooked it if we were manually searching for a vulnerability. After your target function runs for the specified number of iterations, Even though it finds fewer bugs, theyre usually easier to reproduce. Just opened theprogram, set themaximum number ofoptions for thedocument andsaved it todisk. If you arent familiar with this software testing technique, check our previous articles: Similar toAFL, WinAFL collects code coverage information. Such anapproach allows you toavoid wasting extra time onthe program launch andinitialization andsignificantly increases thefuzzing speed. But thethings dont always run so smoothly. So it seems that it is indeed used, rightfully, for security purposes. Instead ofreversing each ofthem statically, lets use thedebugger tosee which function iscalled toparse files. Additionally, this mode is considered as experimental since we have experienced some problems with stability and performance. Return normally (So that WinAFL can "catch" this return and redirect In this case, we are only fuzzing whats below Header in the following diagram. Your target runs normally until your target function is reached. In this first installment, I set up a methodology for fuzzing Virtual Channels using WinAFL and share some of my findings. CLIPRDR is a static virtual channel dedicated to synchronization of the clipboard between the server and the client. It is opened by default. In summary, we make the following contributions: We identified the major challenges of fuzzing closed-source Windows applications; Since fuzzing campaigns usually last many hours, we cant be there every time the fuzzer restarts the client to click Connect and select a user account. so that the execution jumps back to step 2. When thenumber ofsuch iterations reaches some maximum (you determine it yourself), WinAFL restarts theprogram. If WinAFL will not find the new target process within 10 seconds, it will terminate. 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. Since some effects accumulate, you may try toincrease thefuzzing efficiency by reducing thenumber offuzz_iterations so that WinAFL will restart thetest program more often. Its use around the world is very widespread; some people, for instance, use it often for remote work and administration. However, if there is only the binary program and no source code available, then standard afl-fuzz -n (non-instrumented mode) is not effective. Now that weve chosen our target, where do we begin? Shared memory is faster and can avoid some problems with files (e.g. In-memory fuzzing implementation not only restores register context, but also writes fuzzing input at the process memory pointing PDU buffer. This PDU is used by the server to send a list of supported audio formats to the client. Fuzzing is the generalized process of feeding random inputs to an executable program in order to create a crash. The first one can find interesting bugs, but which sometimes are very hard to analyze. WinAFL includes the windows port of afl-cmin in winafl-cmin.py. For this purpose, it uses three techniques: Lets focus onthe classical first variant since its theeasiest andmost straightforward one. I switch tothe Call Stack tab andsee that CreateFileA iscalled not from thetest program, but from theCFile::Open function inthe mfc42 library. The Art of Fuzzing - Demo 12- Using PageHeap and ApplicationVerifier to find bug. If a program always behaves the same for the same input data, it will earn a score of 100%. WinAFL is a Windows fork of the popular mutational fuzzing tool AFL. It is also home to Martas and . The tool combines usage examples. Let's say that our input binary has a size of 10 kB. AFL was able tosynthesize valid JPEG files without any additional information). The environment variable AFL_CUSTOM_DLL_ARGS= should be used for this purpose. These can happen in parsing logic: in RDPSND (and similarly in many other channels), the Header includes a BodySize field which must be equal to the length of the actual PDU body. 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. Enabling this has been known to cause RDP fuzzing target function often looks like above. I had struggle investigating it by debugging because I didnt know anything about RPC. When I tried to start fuzzing RDPDR, there was a little hardship. They also started reviewing this case for a potential bounty award. What is fuzzing I modified my VC Server to integrate a slow mode. The DynamoRIO instrumentation mode supports dynamically attaching to running processes. What is coverage-guided fuzzing ? This new mutation could snowball into dozens of new paths, including a crash that leads to the next big RCE. "returning" via ExitProcess() and such won't work). There is an important metric in AFL related to coverage: the stability metric. 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 the target process terminates (regardless of the reason), WinAFL will not restart it, but simply try to reattach. To better reproduce the crash, we implemented machine context and call stack dump when crush occurs. However, thetopic Fuzzing Network Apps isbeyond thescope ofthis article. While writing a PoC, I noticed something interesting. Yes i know by doing reverse engineering. But ifyou pay attention tothe arguments, youll realize that thetarget wants toopen some ofits service files, not thetest file. The Art of Fuzzing - Demo 7- How to detect when a PDF finished loading. Figure 4. RDPSND Server Audio Formats and Version PDU structure. Everything works, everything is sunshine and rainbows, maybe weve even been lucky enough to find bugs. This means we cant use the -thread_coverage option anymore if we target DispatchPdu So we cant perform mixed message type fuzzing with reliable coverage anymore. Select theone you need based onthe bitness ofthe program youre going tofuzz. After around a hundred iterations, the fuzzing would become very slow. As you can see, this function meets theWinAFL requirements. Todo so, add the-debug parameter tothe arguments ofthe instrumentation library. Were gonna have to manually reconstruct the puzzle pieces! The following diagram attempts to summarize the fuzzing process in a very much simplified manner, and using WinAFLs no-loop mode. By setting up a malicious RDP server to which they would connect, you could hack them back, assuming you found a vulnerability in the RDP client. In this case, the harness just sends back the mutation it receives as it is (apart from some exceptions such as overwriting a length field, which we will talk about later). location of your DynamoRIO cmake files (either full path or relative to the As said above, thefunction selected for fuzzing shouldnt have side effects. Some CVEs that came out during this period are CVE-2021-34535, CVE-2021-38631 and CVE-2021-41371. To avoid this, replace the SO_REUSEADDR option by SO_LINGER option in the server source code if available. Strings or magic numbers from the specification can also help. Finally, there are two kinds of Virtual Channels : static ones and dynamic ones. 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. receiving desktop bitmaps from the server; sending keyboard and mouse inputs to the server. This bug is less powerful than the CLIPRDR one because it only goes up to a 4 GB allocation. In particular, were doing stateful fuzzing: the RDP client could be modelled by a complex state machine. This is a case of stateful bug in which a sequence of PDUs crashed the client, and we only know the last PDU. following instrumentation modes: These instrumentation modes are described in more detail in the separate The following cmake configuration options are supported: -DDynamoRIO_DIR=..\path\to\DynamoRIO\cmake - Needed to build the Instead, it is preferable to assess fuzzing quality by looking at coverage quality. 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. 2021-07-31 Microsoft acknowledged the RDPDR deserialization bug and started developing a fix. We also notice a few more channels that are blacklisted the same way. Mutations are repeatedly performed on samples which must initially come from what we call a corpus. Introduction II. Copy them andthe folder with DynamoRIO tothe virtual machine you are going touse for fuzzing. Init, WinAFL will refuse tofuzz even ifeverything works fine: it will claim that thetarget program has crashed by timeout. Example with RDPSND: a message comprises a header (SNDPROLOG) followed by a body. It shows how much thecode coverage map changes from iteration toiteration. It contains many dynamic calls that all lead to CTSCoreEventSource::FireASyncNotification. AFL++, libfuzzer and others are great if you have the source code, and it allows for very fast and coverage guided fuzzing. RDPSND PDU handler and dispatch logic in mstscax.dll. iamelli0t. I thought it could be an issue with WTSVirtualChannelOpen specifically, so I tried with its counterpart WTSVirtualChannelOpenEx. If dissecting the payload does not yield anything, maybe its a stateful bug and youre doomed. By giving following options(-F, -G, -H), fuzzing input can be delivered by socket. The second one needs a bit more effort to setup, but allows to go more in depth in each message types logic. 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 is opened by default. WinAFL can recover thesyntax ofthe targets data format (e.g. Crashes from RDP fuzzer is often not reproducible. There was a problem preparing your codespace, please try again. Reverse engineering will focus on the latter, as it holds most of the RDP logic. It is assumed that the target process will be restarted by an external script (or by the system itself). When WinAFL finds a crash, the only thing it pretty much does is save the mutation in the crashes/ folder, under a name such as id_000000_00_EXCEPTION_ACCESS_VIOLATION. Mitigations Team for his contributions! Until current research about RDP fuzzing, server agent was used to send back fuzzing input. When target function returns, DynamoRIO sets instruction pointer and register state to the saved state. -H option is used during in-memory fuzzing, described below. Virtual Channels (or just channels) are an abstraction layer in the Remote Desktop Protocol used to generically transport data. However, it will still restart from time to time: for instance, when reaching the max number of fuzzing iterations (-fuzz_iterations parameter), or simply because of crashes (if we find some). You will learn how to build a fuzzing harness, optimize it for maximum performance, and triage the . Fuzzing level is a subjective scale to assess how much I fuzzed each channel: RDPSND is a static virtual channel that transports audio data from server to client, so that the client can play sound originating from the server. The crash itself is not especially interesting, but I will still detail it because its a great example of stateful bug. The command line for afl-fuzz on Windows is different than on Linux. It was assigned CVE-2021-38666. https://github.com/googleprojectzero/Jackalope/blob/6d92931b2cf614699e2a023254d5ee7e20f6e34b/test.cpp#L111. However, DynamoRIO does not have such a feature, and we cant do it through procdump or MiniDumpWriteDump either because the client is already a debuggee of DynamoRIO (drrun). We thought they achieved encouraging results that deserved to be prolonged and improved. Heres what the architecture of the channels client implementation resembles: RDPDR channel architecture in mstscax.dll. It describes the channels functioning quite exhaustively, as well as: With a good picture of the channel in mind, we can now start reversing the RDP client. unable to overwrite the sample file because a target maintains a lock on it). 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