As a result, real bugs in the RDP client will only constitute a subset of the bugs we will find with the patched DLL. Below is an example mutator that increments every byte by one: Special thanks to Axel "0vercl0k" Souchet of MSRC Vulnerabilities and Eventually, the value of the field OutputBufferLength (DWORD) is used for a malloc call on the client (inside DrUTL_AllocIOCompletePacket). Fuzzing binary-only programs with AFL++. Two new ways to hide processes from antiviruses, SIGMAlarity jump. Todo this, I check thelist ofprocess handles inProcess Explorer: thetest file isnt there. In this first installment, I set up a methodology for fuzzing Virtual Channels using WinAFL and share some of my findings. It contains many dynamic calls that all lead to CTSCoreEventSource::FireASyncNotification. 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. Introduction In this blog post, I'll write about how I tried to fuzz the MSXML library using the WinAFL fuzzer. This way, I can split the resulting coverage per thread, making it less cluttered. Since I am just looking for afunction tofuzz, I have tokeep inmind that it must take thepath tothe input file, do something with this file, andterminate as neatly as possible. Strings or magic numbers from the specification can also help. Ifthe program operates normally, it should have thesame numbers oflines In pre_fuzz_handler andIn post_fuzz_handler. If you are interested in that, there are other resources out there that will explain it well, such as articles, or even the official Microsoft specification itself. For instance, my dictionary begins as follows: So, you have found afunction tobe fuzzed, concurrently deciphered theinput file ofthe program, created adictionary, selected arguments andfinally can start fuzzing! Official, documented Virtual Channels by Microsoft come by dozens: Non-exhaustive list of *Virtual Channels* documented by Microsoft, found in the FreeRDP wiki. This can be enabled by giving -s option to afl-fuzz.exe. // 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. The function CUMRDPConnection::CreateVirtualChannel answers our inquiry. Lighthouse is an IDA plugin to visualize code coverage. RDPDR is a Static Virtual Channel dedicated to redirecting access from the server to the client file system. Surprisingly, but most developers dont take theexistence ofWinAFL into account when they write their programs. 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. No luck. We did gather earlier a little list of channels that looked like fruitful targets. More specifically, the I/O Request handler, DrDevice::ProcessIORequest, dispatches the PDU to a Smart Card sub-protocol handler (W32SCard::MsgIrpDeviceControl). Finally, it is probably the most complex and interesting channel Ive had to fuzz among the few ones Ive studied! Sometimes strange stuff just happens, like WinAFL itself randomly crashing and stopping the fuzzing in the middle of a week-end or something. Init, WinAFL will refuse tofuzz even ifeverything works fine: it will claim that thetarget program has crashed by timeout. Windows post-exploitation with a Linux-based VM, Software for cracking software. All arguments are divided into three groups separated from each other by two dashes. You can use these tags: What is coverage-guided fuzzing ? I didnt talk about these because theyre not about the Microsoft client, theyre not the most interesting and the article is getting really long either way, but feel free to look them up: /* We don't need to reload context in case of network-based fuzzing. I eventually identified three bugs. Concretely, we only lack two elements to start fuzzing: A good lead is to start by reading Microsofts specification (e.g. 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. WinAFL (Ivan Fratric) Network fuzzing. This is easily done with the WTS API I mentioned earlier, which allows to open, read from and write to a channel. It turns out the client was actually causing memory overcommitment leading to RAM explosion. V. Pham, M. Bhme, and A. Roychoudhury, "AFLNET: a greybox fuzzer for network protocols," in Proceedings of . 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. The environment variable AFL_CUSTOM_DLL_ARGS= should be used for this purpose. Out of the 59 harnesses, WinAFL only supported testing 29. I set breakpoints atits beginning andend toexamine its arguments andunderstand what happens tothem by theend ofits execution. This takes plenty oftime, andyou can help theprogram alot inthis: who knows thedata format inyour program better than you? I wait until thefunction execution iscompleted andsee that my test file isstill encrypted, while thetemporary file isstill empty. WinAFL is a fuzzer for Windows which can take a corpus of input files, track which code is executed, and generate new inputs to execute new execution paths. not closed WinAFL won't be able to rewrite it. Youll get tons of the same crashes in a row, which can heavily slow down fuzzing for certain periods of time. After reaching target funcion once, WinAFL will force persistent loop. Fuzzing with 8 GB RAM showed funny things: RAM spikes in the Task Manager while fuzzing RDPDR. We have just talked about how DynamoRIO monitors code coverage; it starts monitoring it when entering the target function, and stops on return. Finally, there are two kinds of Virtual Channels : static ones and dynamic ones. They are especially used by developers to create extensions, but also by red teamers to exfiltrate data, bypass firewalls, etc. Are you sure you want to create this branch? However, understanding which sequence of PDUs made the client crash is hard, not to say often a lost cause. It is opened by default. There are two functions of interest: The issue must come either from ACL, or from the handling logic. Then, I will talk about my setup with WinAFL and fuzzing methodology. Therefore, toavoid any issues, lets compile WinAFL together with thelatest DynamoRIO version. In particular, the msgType field will be fixed, so we need to start a fuzzing campaign for each message type (there are 13 in RDPSND). Top 10 Haunting Pictures Taken Seconds Before Disaster. We can find a description of this function in an older RDP reference page: This function closes the client end of a virtual channel. How tofuzz theLinux kernel, synthesize valid JPEG files without any additional information, Herpaderping and Ghosting. In this section, I will present some of my results in a few channels that I tried to fuzz. Each individual Virtual Channel behaves according to its own separate logic, specification and protocol. It is opened by default. This is a case of stateful bug in which a sequence of PDUs crashed the client, and we only know the last PDU. 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 (. Heres the interesting piece: The out-of-bounds read is quite evident: we control wFormatNo (unsigned short). It is also integrated inside many products of the Microsoft / Windows ecosystem such as Office itself, Outlook and Office Online. Using theVisual Studio command line, go tothe folder with WinAFL source code. With her consent, of course! Thanksfully, Windows provides an API called the WTS API to interact with this layer, which allows us to easily open, read from and write to a channel. The crash happened upon receipt of a Wave2 PDU (0x0D), at CRdpAudioController::OnWaveData+0x27D. It is worth noting a crash in an unknown module could mean the execution flow was redirected, which accounts for the most interesting bugs :). tions and lacks kernel support. Hence why all the functions are colored in red, but it is not very important. The first one can find interesting bugs, but which sometimes are very hard to analyze. Cant we just connect to a local RDP server on the same machine? I fuzzed most of the message types referenced in the specification. 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. I patched mstscax.dll to get rid of this measure, by nopping out the dynamic call to VirtualChannelCloseEx and bypassing the error handler. In this article, I will address different fuzzing types and show how to use one of them, WinAFL. More generally, it seems adapted to cases like fuzzing an interpreter or a network listener, which already loop on reading input or receiving packets. This vulnerability resides in RDPDRs Smart Card sub-protocol. After around a hundred iterations, the fuzzing would become very slow. DynamoRIO sources or download DynamoRIO Windows binary package from close thefile andall open handles, not change global variables, etc.). It is a Device I/O Request PDU (0x4952) of sub-type Device Control Request (0x000e). Upgrading to 8 GB of RAM solved the issue, meaning the memory overcommitment was not as violent as in the CLIPRDR bug. Check a simple harness here: https://github.com/googleprojectzero/Jackalope/blob/6d92931b2cf614699e2a023254d5ee7e20f6e34b/test.cpp#L41 Use Git or checkout with SVN using the web URL. ACL is set up with an SDDL string, which is Microsofts way of describing a security descriptor. 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). receiving desktop bitmaps from the server; sending keyboard and mouse inputs to the server. What is more, the four aforementioned SVCs (as well as a few DVCs) being opened by default makes them an even more interesting target risk-wise. Based onthe contents ofthe test file, it iscompressed, orencrypted, orencoded insome way. The list ofarguments taken by this function resembles what you have already seen before. At first, my virtual machine had only 4 GB of RAM, so death by swap (which we know of and are used to by now) would happen. As you can see, this function meets theWinAFL requirements. 2021-07-23 Microsoft started reviewing and reproducing. Research By: Netanel Ben-Simon and Yoav Alon. Although, this requires having reversed engineered the channel enough to have a good depiction of whats going on in mind more specifically, knowing what are all the functions and basic blocks we are interested in. Instead, it will randomly mutate inputs without knowing which mutations actually yield favorable results (new paths in the correct thread). 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. Well, Im not sure myself it is not documented (at least at the time I am writing this article). After installing Visual Studio, youll see inthe Start menu shortcuts opening theVisual Studio command prompt: (1) x86 Native Tools Command Prompt for VS 2019; and(2) x64 Native Tools Command Prompt for VS 2019. I switch tothe Call Stack tab andsee that CreateFileA iscalled not from thetest program, but from theCFile::Open function inthe mfc42 library. Fuzzing is a battle against the binary, but it is also a battle against yourself. WinAFL is doing in-memory fuzzing which means that we don't have to start the application every time, but let's forget this for now so that our discussion does not get too complicated. This issue was fixed in January . The stability metric measures the consistency of observed traces. But ifyou look closely, this library contains only jmp tothe respective functions ofkernelbase.dll. Then I select thekernelbase.dll library onthe Symbols tab andset breakpoints atexports ofthe CreateFileA andCreateFileW functions. Funnily enough, the source code of WinAFL itself hints that it is the preferred mode for network fuzzing. unable to overwrite the sample file because a target maintains a lock on it). arky, Tekirda ilinin bir ilesi. Please CLIPRDR state machine diagram from the specification. Theres a twist with this channel: its a state machine. But inreal life, developers often forget toadd such perfect functions totheir programs, andyou have todeal with what you have. Heres what the architecture of the channels client implementation resembles: RDPDR channel architecture in mstscax.dll. 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. 2021-07-22 Sent vulnerability reports to FreeRDP; they pushed a fix on the same day. Therefore, CVEs in the RDP client are more scarce, even though the attack surface is as large as the servers. When I got started on this channel, I began studying the specification, message types, reversing the client, identifying all the relevant functions Until realizing a major issue: I was unable to open the channel through the WTS API (ERROR_ACCESS_DENIED). There is a second DLL custom_winafl_server.dll that allows winAFL to act as a server and perform fuzzing of client-based applications. For RDPSND, we can get something like this. WinAFL will change @@ tothe full path tothe input file. Mutations are repeatedly performed on samples which must initially come from what we call a corpus. To compile the32-bit version, execute thefollowing commands: In my case, these commands look as follows: After thecompilation, thefolder \build<32/64>\bin\Release will contain working WinAFL binaries. So what is this no-loop mode, you ask me? You are not able to reproduce the crash manually. winafl.dll DynamoRIO client, -DINTELPT=1 - Enable Intel PT mode. This implies a lot; we will talk about this. More specifically, everytime a crash is encountered, WinAFL/DynamoRIO will now log the exception address, module and offset, timestamp, and also exception information (like if theres an access violation on read, which address was tried to be read). Attempt at RDP loopback connection. 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. There is an important metric in AFL related to coverage: the stability metric. Therefore, we dont have much choice but to perform blind mixed message type fuzzing (without thread coverage). But fuzzing the RDP client, I often got speeds between 50 and 1000 execs/s. However, it still accounts for a remote system-wide denial of service for target clients with around 4 GB of RAM on their system. A tag already exists with the provided branch name. RDPSND Server Audio Formats and Version PDU structure. Thus, the two next steps are: With this in mind, I developed what I will call during the rest of this article the VC Server (for Virtual Channel Server). arky ilesinde biri ile merkezi ikisi kasaba olmak zere 3 belediye (Hoky, Mrefte) tekilat vardr.Bunlar dnda ile merkezi 3 mahalleden oluurken, ileye bal 26 ky bulunmaktadr. Virtual Channels operate on the MCS layer. As said above, thefunction selected for fuzzing shouldnt have side effects. As weve seen in the fixed message type fuzzing strategy, the harness can be adapted to calculate the header for a given message type and wrap the headless mutation with this header. Even though I couldnt find any ground-breaking vulnerability such as an RCE with a working exploit, I am very happy with my results, especially as part of an internship. In laymans terms: imagine WinAFL finds a crash and saves the corresponding mutation. more basic blocks than WinAFL, the state-of-the-art fuzzer on Windows. documents. 2 = Quite satisfied with my fuzzing campaigns (but there might be more to fuzz). A blind fuzzer, or blackbox fuzzer, is a fuzzer with no knowledge of a program's inner workings. 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. 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. I want to know which modules or functions does parsing the file formats like RTF,.DOCX,.DOC etc.. Use Winafl to fuzz jpeg2000 with the harness I built above: Looking at the interface Winafl we should be interested in some of the following parameters: - exec speed: the number of test cases that can be executed on 1s - stability: this indicator shows stability during fuzzing. Here are the results after just three days of fuzzing: Here are the results after just three days of fuzzing: To bypass this constraint, there exists a wonderful tool called RDPWrap. Fuzzing is gambling. WinAFL will save all the basic blocks encountered at each fuzzing iteration in a temporary buffer (in the thread of interest). Such aset offiles can besubsequently minimized using the[winafl-cmin.py](http://winafl-cmin.py) script available inthe WinAFL repository. If, like me, you opt for extra challenge, you can try fuzzing network programs. If something behaves strangely, then I need to find the reason why. The custom mutator should invoke common_fuzz_stuff to run and make WinAFL aware of each new test case. Just opened theprogram, set themaximum number ofoptions for thedocument andsaved it todisk. WinAFL reports coverage, rewrites the input file and patches EIP Todo so, add the-debug parameter tothe arguments ofthe instrumentation library. Note that inIDA, thefile path ispassed tothe CFile::Open function as thesecond argument because thiscall isused. This article begins my three-part series on fuzzing Microsofts RDP client. [] If it goes into red, you may be in trouble, since AFL will have difficulty discerning between meaningful and phantom effects of tweaking the input file. Indeed, we find out there actually is length checking inside OnNewFormat. Todo that, you have tocreate adictionary inthe format ="value". Instead, it is preferable to assess fuzzing quality by looking at coverage quality. The client will try to allocate too much at once, and malloc will return ERROR_NOT_ENOUGH_MEMORY. Otherwise, WinAFL would instrument numerous library functions. I had struggle investigating it by debugging because I didnt know anything about RPC. Aside from this engaging motive, most of vulnerability research seems to be focused on Microsofts RDP server implementation. after the target function returns is never reached. Tekirda (pronounced [tecida]) is a city in Turkey.It is located on the north coast of the Sea of Marmara, in the region of East Thrace.In 2019 the city's population was 204,001.

. We thought they achieved encouraging results that deserved to be prolonged and improved. https://github.com/googleprojectzero/winafl/blob/master/readme_pt.md, -DUSE_COLOR=1 - color support (Windows 10 Anniversary edition or higher), -DUSE_DRSYMS=1 - Drsyms support (use symbols when available to obtain Its also useful ifyour program tries tocall afunction using GetProcAddress. Maybe this will lead me to new findings, and even a reproducible bug.. This means, fuzzing with the raw seeds from the specification and without modifying the harness any further. 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. Of course, many crashes can still happen at the first depth level. 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. Themaximum code coverage can beachieved by creating asuitable set ofinput files. AFL is a popular fuzzing tool for coverage-guided fuzzing. Of course, this is specific to RDPSND and such patches should happen in each channel. AFL is a popular fuzzing tool for coverage-guided fuzzing. There also exist alternate implementations of RDP, like the open-source FreeRDP. sign in Second, kernel-level code has sig-nicantly more non-determinism than the average ring 3 A team of researchers (Chun Sung Park, Yeongjin Jang, Seungjoo Kim and Ki Taek Lee) found an RCE in Microsofts RDP client. The no-loop mode lets the program loop by its own, just like in-app persistence. These documentations are an invaluable resource; each channel has its own open specification, and some can span more than a hundred pages. This article will primarily concentrate on what we need to know in order to fuzz Virtual Channels. It would be painfully slow, especially with the RDP client, which can sometimes take 10 or 20 seconds to connect. Mutations are repeatedly performed on samples which must initially come from what we call a corpus. This new mutation could snowball into dozens of new paths, including a crash that leads to the next big RCE. Salk Bakanl, Tekirda'n Sleymanpaa plajlar, arky Plajlar, Marmara Erelisi plajlar ve Saray plajlarnda deniz suyu analiz sonularn yaynlad. It was assigned CVE-2021-38666. afl-analyze.c Remove redundant file API calls (unlink before open, seek before close) last year afl-fuzz.c Add initialization using socket & config changes (-F,G,H) last month afl-showmap.c Remove redundant file API calls (unlink before open, seek before close) last year afl-staticinstr.c Fix a protocol broken issue 3 years ago afl-staticinstr.h 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. It looks more like legacy. DRDYNVC is a Static Virtual Channel dedicated to the support of dynamic virtual channels. I just happened to stumble upon it while reading WinAFLs codebase, and it proves to be totally fit for our network context! Before going any further, I would like to tackle an important concern. Writing an undetectable keylogger in C#, What data Windows 10 sends to Microsoft and how to stop it. 2021-07-31 Microsoft acknowledged the RDPDR deserialization bug and started developing a fix. However, it requires some more preparation: In conclusion, its nice to try both fuzzing approaches for a channel. Here are some that are provided by Microsoft: In conclusion, both types of Virtual Channels are great targets for fuzzing. To see the supported instrumentation flags, please refer to the documentation In practice, this . Inaddition, there must bethe phrase: Everything appears to be running normally. WinAFL can recover thesyntax ofthe targets data format (e.g. issues on Windows 10 v1809, though there are workarounds, In particular, they found a bug by fuzzing the Virtual Channels of RDP using WinAFL. The DLL path to WinAFL via -l winafl network fuzzing path > argument tothe full path tothe input file patches. Of service for target clients with around 4 GB of RAM solved issue... 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Finally, there must bethe phrase: Everything appears to be totally fit for network! From close thefile andall open handles, not change global variables, etc )! Ecosystem such as Office itself, Outlook and Office Online able to reproduce the crash happened upon of. Sometimes take 10 or 20 seconds to connect value '' specification can also.... A Linux-based VM, Software for cracking Software both fuzzing approaches for a channel function meets requirements... My fuzzing campaigns ( but there might be more to fuzz Virtual using... Set breakpoints atits beginning andend toexamine its arguments andunderstand what happens tothem by theend ofits execution fuzz among few! With WinAFL source code Microsofts specification ( e.g input file and patches EIP todo so, add the-debug parameter arguments... Like the open-source FreeRDP thefunction execution iscompleted andsee that my test file, it should have thesame numbers oflines pre_fuzz_handler. Winafl reports coverage, rewrites the input file and patches EIP todo so, add parameter... Themaximum code coverage can beachieved by creating asuitable set ofinput files new mutation could snowball into dozens new... For coverage-guided fuzzing means, fuzzing with 8 GB of RAM on their system Studio!