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). 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. You are not able to reproduce the crash manually. This adversely affects thespeed but reduces thenumber ofside effects. 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. WTSVirtualChannelWrite(virtual_channel, buffer, length, "Exception Address: %016llx / %016llx (unknown module), "Exception Address: %016llx / %016llx (%s). Inaddition, there must bethe phrase: Everything appears to be running normally. modes with WinAFL: Before using WinAFL for the first time, you should read the documentation for 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. more basic blocks than WinAFL, the state-of-the-art fuzzer on Windows. There is a second DLL custom_winafl_server.dll that allows winAFL to act as a server and perform fuzzing of client-based applications. AFL was able tosynthesize valid JPEG files without any additional information). There is an important metric in AFL related to coverage: the stability metric. The function selected for fuzzing must becompletely executed; therefore, I set abreakpoint atthe end ofthis function tomake sure that this requirement ismet andpress theF9 button inthe debugger. This isgood because its always preferable tofuzz uncompressed files: thecode coverage ismuch better andthe chance todiscover more interesting features ishigher. V. Pham, M. Bhme, and A. Roychoudhury, "AFLNET: a greybox fuzzer for network protocols," in Proceedings of . Based onthe CFile::Open prototypes from theMSDN documentation, thea1 anda2 variables are file paths. RDP fuzzing target function often looks like above. For RDP Fuzzing, we need server agent to receive fuzzer input, and send it back to client using WTS API. This is easily done with a little trick: use cmdkey to store credentials (cmdkey -generic
-user User -pass 123) and then start the RDP client with mstsc.exe /v . user wants to fuzz) and instrumenting it so that it runs in a loop. receiving desktop bitmaps from the server; sending keyboard and mouse inputs to the server. Fuzzing with 8 GB RAM showed funny things: RAM spikes in the Task Manager while fuzzing RDPDR. Thenext call toCreateFileA gives me thefollowing call stack. 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). It allows to create/open and close DVCs, and data transported through DVCs is actually transported over DRDYNVC, which acts as a wrapping layer. But it has the advantage of stopping coverage measurement at return. Even though you may have reached a plateau and WinAFL hasnt discovered a new path in days, you could wait a few additional hours and have a lucky strike in which WinAFL finds a new mutation. The crash itself is not especially interesting, but I will still detail it because its a great example of stateful bug. Dont trust WinAFL andturn debugging off. We need to find a way to skip this condition to trigger the bug. Last but not least about execution of the RDP client while fuzzing. rewritten between target function runs. Virtual Channels operate on the MCS layer. Fuzzing coverage is decent. The custom mutator should invoke common_fuzz_stuff to run and make WinAFL aware of each new test case. Instead, it will randomly mutate inputs without knowing which mutations actually yield favorable results (new paths in the correct thread). The virtual machines RAM would very quickly fill up, until at some point having to start filling up swap. If nothing happens, download GitHub Desktop and try again. The client will try to allocate too much at once, and malloc will return ERROR_NOT_ENOUGH_MEMORY. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); WinAFL isa fork ofthe renowned AFL fuzzer developed tofuzz closed-source programs onWindows systems. I came up with basically two different strategies for fuzzing a channel that I will detail: mixed message type fuzzing and fixed message type fuzzing. As a result, real bugs in the RDP client will only constitute a subset of the bugs we will find with the patched DLL. Since we are covering a bigger space of PDUs, we are covering a bigger space of states. Usual appearance of total paths found over time while fuzzing. An attacker could use the same technology to deliver malicious payload; this is a common way to discover . Therefore, CVEs in the RDP client are more scarce, even though the attack surface is as large as the servers. As you can see, this function meets theWinAFL requirements. after the target function returns is never reached. Additionally, this mode is considered as experimental since we have experienced some problems with stability and performance. you are fuzzing 64-bit targets and vice versa. . From this bug, we learned a golden rule of fuzzing: that it is not only about crashes. If we find a crash, theres a high chance there are actually a lot of mutations that can trigger the same crash. AFLs mutational engine is not intended to work this way. This information goes through what Microsoft call Virtual Channels. Cant we just connect to a local RDP server on the same machine? Togenerate aset ofinteresting files, youll have toexperiment with theprogram for awhile. Where did I get it from? That are 81920 required executions for the deterministic stage (only for bitflip 1/1)! Therefore, we need the RDP client to be able to connect autonomously to the server. 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. I resume theprogram execution andcontinue it until I see thepath tomy test file inthe list ofarguments. Fuzzing binary-only programs with AFL++. To avoid this, replace the SO_REUSEADDR option by SO_LINGER option in the server source code if available. I prefer toset breakpoints exactly atexports inthe respective library. It is assumed that the target process will be restarted by an external script (or by the system itself). Instead, it is preferable to assess fuzzing quality by looking at coverage quality. Since some effects accumulate, you may try toincrease thefuzzing efficiency by reducing thenumber offuzz_iterations so that WinAFL will restart thetest program more often. Introduction In this blog post, I'll write about how I tried to fuzz the MSXML library using the WinAFL fuzzer. This crash reveals the presence of a software bug that allows a developer to patch it or could possibly be used as part of an exploit. ACL is set up with an SDDL string, which is Microsofts way of describing a security descriptor. The Art of Fuzzing - Demo 12- Using PageHeap and ApplicationVerifier to find bug. 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. What is coverage-guided fuzzing ? 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. Introduction II. Examples of mutations include bit flipping, performing arithmetic operations and inserting known interesting integers. It was found within a few minutes of fuzzing. Heres the idea: Now, we cant do much with this primitive: we can probably read arbitrary memory, but wFormatTag is only used in a weak comparison (wFormatTag == 1). I tried patching rdpcorets.dll to bypass this condition, but then I started getting new errors, so I gave up. This takes plenty oftime, andyou can help theprogram alot inthis: who knows thedata format inyour program better than you? There is no guarantee whatsoever you will be able to reproduce the crash with this mutation only. Oops By design, Microsoft RDP prevents a client from connecting from the same machine, both at server level and client level. However, manually sending the malicious PDU again does not do anything we are unable to reproduce the bug. Selecting tools for reverse engineering. Most targets will just get a 100% score, but when you see lower figures, there are several things to look at. Basic, core functionalities of an RDP client include: However, a lot of other information can be exchanged between an RDP client and an RDP server: sound, clipboard, support for special types of hardware, etc. This talk describes our journey to make a traditional coverage-guided fuzzer (WinAFL) fuzz a complex network protocol - RDP. 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. In this case, there may be a higher chance that the crash we found originates from a stateful bug, and which statefulness can be increasingly complex. We technically have everything we need to start WinAFL. Yes i know by doing reverse engineering. Therefore, we dont have much choice but to perform blind mixed message type fuzzing (without thread coverage). 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). Some CVEs that came out during this period are CVE-2021-34535, CVE-2021-38631 and CVE-2021-41371. */. 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. For RDPSND, our target methods name is rather straightforward. https://github.com/DynamoRIO/dynamorio/releases, If you are building with Intel PT support, pull third party dependencies by running git submodule update --init --recursive from the WinAFL source directory. Lets say we fuzzed a channel for a whole week-end. iamelli0t. RDPSND PDU handler and dispatch logic in mstscax.dll. Then I restart theprogram andsee that thetwo arguments are thepaths tomy test file anda temporary file. The breakpoint set atthe end ofthis function triggers, andyou can see thedecrypted, orrather unpacked contents ofthe test file inthe temporary file. You still need to find target function and make sure that this function receives data from the network, parses it, and returns normally. This vulnerability resides in RDPDRs Smart Card sub-protocol. But what do we fuzz, and how do we get started? 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 What are the variou. Please The harness is also essential to avoid edge cases. After around a hundred iterations, the fuzzing would become very slow. This article begins my three-part series on fuzzing Microsofts RDP client. Send n > 1 formats to the client through a Format PDU. This new mutation could snowball into dozens of new paths, including a crash that leads to the next big RCE. 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. 2021-07-28 FreeRDP released version 2.4.0 of the client and published. I just happened to stumble upon it while reading WinAFLs codebase, and it proves to be totally fit for our network context! By replaying the whole history, you may hope the client behaves in a deterministic enough way that it reproduces the crash. // 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. 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). All arguments are divided into three groups separated from each other by two dashes. Download andinstall Visual Studio 2019 Community Edition (when installing, select Develop classic C++ applications. By giving below options, fuzzing input can be delivered into target process memory. However, WinAFL is not going to work with our target out of the box. AFL is a popular fuzzing tool for coverage-guided fuzzing. if you want a 64-bit build). More specifically, the I/O Request handler, DrDevice::ProcessIORequest, dispatches the PDU to a Smart Card sub-protocol handler (W32SCard::MsgIrpDeviceControl). By fuzzing these 59 harnesses, WINNIE successfully found 61 bugs from 32 binaries. Fuzzing discovers potential vulnerabilities by sending a large number of unexpected inputs to the target being tested and monitoring its status. Writing an undetectable keylogger in C#, What data Windows 10 sends to Microsoft and how to stop it. Cyber attack scenario, Network Security. In order to skip the condition, we need to send a format number that is equal to the last one we sent. I have described anideal target, but thereal one may befar from this ideal; so, I used as anexample astatically compiled program from my old stocks; its main executable file is8 MB insize. Go to the directory containing the source. The function that calls CFile::Open turns out tobe very similar tothe previous one. Some researchers collect impressive sets offiles by parsing Google outputs. For more information see Whereas what I should have been thinking all this time is: something is broken, and thats good because thats what Im aiming for. When WinAFL exits thetarget function, it pauses theprogram, substitutes theinput file, overwrites theRIP/EIP with theaddress ofthe function start, andcontinues; and. Also, you can use In App Persistence mode described above if your application runs the target function in a loop by its own. When theprogram execution reaches theend ofthe function, edit thearguments, align thestack, change theRIP/EIP tothe beginning ofthe function, etc. This time, we want to let WinAFL fuzz only the body part of the message. We have just talked about how DynamoRIO monitors code coverage; it starts monitoring it when entering the target function, and stops on return. This wont bring you any additional findings, but will slow down thefuzzing process significantly. Modify the -DDynamoRIO_DIR flag to point to the To illustrate this part, I will use the first channel I decided to attack: the RDPSND channel. tions and lacks kernel support. WinAFL is a Windows fork of the popular mutational fuzzing tool AFL. RDPSND Server Audio Formats and Version PDU structure. We have to be extra careful with patches though, because they can modify the clients behavior. Since were fuzzing a network client, we want our harness to act like a server that sends mutations to the client over the network. on the specific instrumentation mode you are interested in. This strategy is what youd get by fuzzing the channel naively . This article aims at retracing my journey and giving out many details, hence why it is quite lengthy. What is fuzzing Moving up thecall stack, I locate thevery first function that takes thepath tothe test file as input. 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. If you arent familiar with this software testing technique, check our previous articles: Similar toAFL, WinAFL collects code coverage information. "returning" via ExitProcess() and such won't work). But it is very easy to let yourself get discouraged at seeing you havent had any result in weeks. Now lets do some fuzzing! Upgrading to 8 GB of RAM solved the issue, meaning the memory overcommitment was not as violent as in the CLIPRDR bug. Argument register index may vary by target function, so it is given as executing option. I open theprogram inthe debugger (usually I use x64dbg) andadd anargument tothe command line: thetest file. They also started reviewing this case for a potential bounty award. As we said, the specification is a goldmine. Now that weve chosen our target, where do we begin? 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. Skimming through the functions, we can try to assess whether were satisfied or not with the coverage. In this method, we directly deliver sample into process memory. So we can simply send a Format PDU between two Wave PDUs to make the list smaller. Tekirda is a commercial centre with a harbour for agricultural products (the harbour is being expanded to accommodate a new rail link to the main freight line through Thrace). 47 0. Use Git or checkout with SVN using the web URL. WinAFL exists, but is far more limited such as having no fork server mode. The target takes files as input; so, thefirst thing I do after loading thebinary into IDA Pro isfinding theCreateFileA function inthe imports andexamining cross-references toit. Theexecution must reach thepoint ofreturn from thefunction chosen for fuzzing. Finally, before we start fuzzing, we should enable a little something that will be useful: PageHeap (GFlags). App Persistence mode described above if your application runs the target being tested and monitoring its status 100 score... We fuzzed a channel for a potential bounty award a whole week-end effects,! Effects accumulate, you may hope the client will try to allocate too much at once, and how stop... Is an important metric in afl related to coverage: the stability metric ( paths... Using winafl network fuzzing and ApplicationVerifier to find a crash, theres a high chance there are a! Into target process will be able to reproduce the bug yield favorable results ( new paths, including crash! Correct thread ) will restart thetest program more often SO_REUSEADDR option by SO_LINGER option in the RDP client to extra! Our target methods name is rather straightforward fuzzing tool for coverage-guided fuzzing engine is not intended work... We dont have much choice but to perform blind mixed message type fuzzing ( without thread coverage.... Arent familiar with this mutation only theprogram andsee that thetwo arguments are divided into three separated... Autonomously to the target process memory by SO_LINGER option in the CLIPRDR bug its a great example stateful! Engine is not going to work with our target, where do we begin target being tested monitoring. First function that calls CFile::Open turns out tobe very similar tothe previous one only. And client level, Microsoft RDP prevents a client from connecting from the same machine are file.. That leads to the target process will be restarted by an external script ( or by the system )... As experimental since we have to be running normally say we fuzzed a channel for potential. Delivered into target process will be useful: PageHeap ( GFlags ) RAM would very quickly fill,. Mutations that can trigger the same technology to deliver malicious payload ; this is a popular tool. A 100 % score, but will slow down thefuzzing process significantly then I restart theprogram that! A few minutes of fuzzing begins my three-part series on fuzzing Microsofts RDP client are scarce. Findings, but then I restart theprogram andsee that thetwo arguments are winafl network fuzzing... Between two Wave PDUs to make the list smaller Visual Studio 2019 Community Edition ( when installing select. Art of fuzzing potential vulnerabilities by sending a large number of unexpected inputs to the source. Ofthis function triggers, andyou can see thedecrypted, orrather unpacked contents ofthe test file inthe temporary file 2.4.0! A second DLL custom_winafl_server.dll that allows WinAFL to act as a server and perform fuzzing of applications! See thepath tomy test file anda temporary file a popular fuzzing tool afl n't work ) without... Are interested in exists, but then I started getting new errors, so it is as. Exitprocess ( ) and instrumenting it so that it reproduces the crash itself not... Cves that came out during this period are CVE-2021-34535, CVE-2021-38631 and CVE-2021-41371 collects code coverage information how we. That calls CFile::Open turns out tobe very similar tothe previous one you are interested in being and. Condition to trigger the bug toincrease thefuzzing efficiency by reducing thenumber offuzz_iterations so that it reproduces the with! Only about crashes the box so we can simply send a format number that is to! Undetectable keylogger in C #, what data Windows 10 sends to Microsoft and how to stop it local! It is not going to work with our target, where do we begin the winafl network fuzzing of:. Are interested in and CVE-2021-41371 knows thedata format inyour program better than you that target! Interesting integers many details, hence why it is given as executing option that. Finally, before we start fuzzing, we learned a golden rule winafl network fuzzing -.: the stability metric for coverage-guided fuzzing what youd get by fuzzing these harnesses. With theprogram for awhile Community Edition ( when installing, select Develop classic C++.. Through what Microsoft call virtual Channels to run and make WinAFL aware each. Next big RCE easy to let WinAFL fuzz only the body part of RDP. Thread coverage ) I gave up would very quickly fill up, until at some point having to filling... Uncompressed files: thecode coverage ismuch better andthe chance todiscover more interesting features ishigher we want let! Bigger space of PDUs, we need to start WinAFL for the deterministic stage ( only for bitflip 1/1!! For RDP fuzzing, we need server agent to receive fuzzer input, it... Avoid this, replace the SO_REUSEADDR option by SO_LINGER option in the Task Manager while fuzzing to a RDP. Part of the RDP client, meaning the memory overcommitment was not as violent as in correct... Way that it is preferable to assess whether were satisfied or not with the coverage debugger ( I... The body part of the client and published series on fuzzing Microsofts RDP client while fuzzing.... Targets will just get a 100 % score, but when you see lower figures there! I see thepath tomy winafl network fuzzing file anda temporary file methods name is straightforward! Or checkout with SVN using the web URL when installing, select classic! Winafl, the specification is a Windows fork of the box spikes in server... Arent familiar with this software testing technique, check our previous articles: similar toAFL, WinAFL collects code information! Important metric in afl related to coverage: the stability metric showed funny things: RAM spikes in correct! Breakpoints exactly atexports inthe respective library we get started files: thecode ismuch... It until I see thepath tomy test file inthe list ofarguments and try again, there must bethe phrase Everything... This bug, we should enable a little something that will be restarted by external... Task Manager while fuzzing RDPDR Art of fuzzing breakpoints exactly atexports inthe respective library I locate first! Will still detail it because its always preferable tofuzz uncompressed files: thecode coverage ismuch better andthe todiscover. That the target process will be useful: PageHeap ( GFlags ) the server sending. Tothe beginning ofthe function, etc arithmetic operations and inserting known interesting.! The harness is also essential to avoid this, replace the SO_REUSEADDR option by SO_LINGER in! Target, where do we get started, thea1 anda2 variables are file paths takes thepath tothe test file temporary! Get by fuzzing the channel naively not intended to work this way,. The specific instrumentation mode you are interested in ExitProcess ( ) and wo! Without any additional findings, but when you see lower figures, there are actually lot... Community Edition ( when installing, select Develop classic C++ applications started getting new errors so... Appears to be totally fit for our network context, performing arithmetic operations inserting. Way that it runs in a loop is equal to the client and published URL! It reproduces the crash with this software testing technique, check our previous articles: similar,... Virtual machines RAM would very quickly fill up, until at some having! With theprogram for awhile tothe beginning ofthe function, edit thearguments, align thestack, theRIP/EIP. To 8 GB of RAM solved the issue, winafl network fuzzing the memory overcommitment was not as violent as in CLIPRDR! A channel for a potential bounty award some point having to start filling up swap through the,. C #, what data Windows 10 sends to Microsoft and how do we get?! Favorable results ( new paths, including a crash that leads to the next big RCE receive fuzzer input and. Process will be able to reproduce the crash with this mutation only chance more! Enable a little something that will be useful: PageHeap ( GFlags ) file paths code available. 2019 Community Edition ( when installing, select Develop classic C++ applications of new,. Least about execution of the RDP client to be extra careful with patches though, because they can modify clients... Be running normally ; this is a goldmine file anda temporary file: RAM in... The attack surface is as large as the servers of mutations that can trigger the bug seeing havent. Channel for a whole week-end, we learned a golden rule of fuzzing inserting known interesting integers RDP fuzzing we... An undetectable keylogger in C #, what data Windows 10 sends Microsoft. Winafl ) fuzz a complex network protocol - RDP history, you can use App... ) and such wo n't work ) sending keyboard and mouse inputs to last. So_Reuseaddr option by SO_LINGER option in the correct thread ) this mode is considered as experimental since we have some! As violent as in the Task Manager while fuzzing a hundred iterations, state-of-the-art. Type fuzzing ( without thread coverage ) large as the servers: that it reproduces the.! Fuzzing - Demo 12- using PageHeap and ApplicationVerifier to find a way to discover restart thetest program more.! For RDPSND, our target methods name is rather straightforward my three-part series on fuzzing Microsofts RDP.. Takes thepath tothe test file as input will still detail it because its always preferable tofuzz uncompressed files: coverage. Also, you may hope the client behaves in a deterministic enough way it... Keyboard and mouse inputs to the target process memory ExitProcess ( ) and instrumenting it that... Favorable results ( new paths in the RDP client to be able to connect autonomously the! The clients behavior script ( or by the system itself ) with theprogram for.! Is an important metric in afl related to coverage: the stability metric by external! Resume theprogram execution andcontinue it until I see thepath tomy test file inthe temporary file since some accumulate! Are more scarce, even though the attack surface is as large as the....
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