[] 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. A solution could be to save the entire history of PDUs that were sent to the client. source directory). This function is a virtual extension that can be used to protect per-session data in the virtual channel client DLL. It is also the base channel that hosts several sub-extensions such as the smart card extension, the printing extension or the ports extension. I would like to thank Thalium for giving me the opportunity to work on this subject which I had a lot of fun with, and that also allowed me to skill up in Windows reverse engineering and fuzzing. 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. to use Codespaces. Themaximum code coverage can beachieved by creating asuitable set ofinput files. Based onthe CFile::Open prototypes from theMSDN documentation, thea1 anda2 variables are file paths. until something breaks. Likewise, I covered it in depth in a dedicated article: Remote Deserialization Bug in Microsofts RDP Client through Smart Card Extension. You will learn how to build a fuzzing harness, optimize it for maximum performance, and triage the . 2021-08-03 Microsoft acknowledged the RDPDR heap leak bug and started developing a fix. 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. Now lets do some fuzzing! WTSVirtualChannelOpenEx(WTS_CURRENT_SESSION. the module containing functions you want tofuzz must not becompiled statically. Sending fuzzer input to server agent involves socket communication, and it is implemented at write_to_testcase@afl-fuzz.c. in Kollective Kontiki listed above). Were gonna have to manually reconstruct the puzzle pieces! Heres what our fuzzing architecture resembles now. I set breakpoints atits beginning andend andsee what happens. But inreal life, developers often forget toadd such perfect functions totheir programs, andyou have todeal with what you have. 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. Note that inIDA, thefile path ispassed tothe CFile::Open function as thesecond argument because thiscall isused. The first one can find interesting bugs, but which sometimes are very hard to analyze. This talk describes our journey to make a traditional coverage-guided fuzzer (WinAFL) fuzz a complex network protocol - RDP. Such aset offiles can besubsequently minimized using the[winafl-cmin.py](http://winafl-cmin.py) script available inthe WinAFL repository. This is accomplished by selecting a target function (that the 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. In case of server fuzzing, if the server socket has the SO_REUSEADDR option set like the following code, then this may case 10055 error after some time fuzzing due to the accumulation of TIME_WAIT sockets when WinAFL restart the fuzzing process. Each channel behaves independently, has a different protocol parser, different logic, lots of different structures, and can hide many bugs! Some WinAFL features that can facilitate (or hinder) thefuzzing process are addressed below. Are you sure you want to create this branch? In particular, they found a bug by fuzzing the Virtual Channels of RDP using WinAFL. This can be enabled by giving -s option to afl-fuzz.exe. You are able to reproduce the crash manually. It is opened by default. V. Pham, M. Bhme, and A. Roychoudhury, "AFLNET: a greybox fuzzer for network protocols," in Proceedings of . This new mutation could snowball into dozens of new paths, including a crash that leads to the next big RCE. Selecting tools for reverse engineering. 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 DRDYNVC is really banned from being opened through the WTS API! after the target function returns is never reached. You cannot tell WinAFL to have constraints on your mutations, such as these two bytes should reflect the length of this buffer. so that the execution jumps back to step 2. We could look at code coverage for a certain fuzzing campaign, and judge whether we are satisfied with it or not. AFL was developed tofuzz programs that parse files. To avoid this, replace the SO_REUSEADDR option by SO_LINGER option in the server source code if available. No luck. Work fast with our official CLI. One ofthe approaches used toselect afunction for fuzzing isto find afunction that isone ofthe first tointeract with theinput file. Type the following commands. A team of researchers (Chun Sung Park, Yeongjin Jang, Seungjoo Kim and Ki Taek Lee) found an RCE in Microsofts RDP client. Do we really need that? RDPWrap tampers with the server in order to allow local connections, and even concurrent sessions. on the specific instrumentation mode you are interested in. But ifyou look closely, this library contains only jmp tothe respective functions ofkernelbase.dll. The client will try to allocate too much at once, and malloc will return ERROR_NOT_ENOUGH_MEMORY. It would be painfully slow, especially with the RDP client, which can sometimes take 10 or 20 seconds to connect. Todo so, add the-debug parameter tothe arguments ofthe instrumentation library. Lighthouse is an IDA plugin to visualize code coverage. Sometimes strange stuff just happens, like WinAFL itself randomly crashing and stopping the fuzzing in the middle of a week-end or something. end of each heap allocation. But you still need to make the client allocate enough memory to reach death by swap. Since were fuzzing a network client, we want our harness to act like a server that sends mutations to the client over the network. the specific instrumentation mode you are interested in. Use Git or checkout with SVN using the web URL. But for abnormal targets, like system service or kernel module, SpotFuzzer can switch to agent mode, and inject an agent to the target for fuzzing. WinAFL will attach to the target process, and fuzz it normally. It is also integrated inside many products of the Microsoft / Windows ecosystem such as Office itself, Outlook and Office Online. WinAFL Fuzzing AFL is a popular fuzzing tool for coverage-guided fuzzing. 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. Not using thread coverage is basically relying on luck to trigger new paths in your target function. Fuzzing process with WinAFL in "no-loop" mode. However, we found this option very useful and managed to find several vulnerabilities in network-based applications (e.g. Dont forget todisable thedebug mode! Well, Im not sure myself it is not documented (at least at the time I am writing this article). Risk-wise, this is a case of remote system-wide denial of service. 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). Perhaps multithreading affects it, too. WinAFL will save all the basic blocks encountered at each fuzzing iteration in a temporary buffer (in the thread of interest). Tofind out whats theproblem, you can manually emulate thefuzzers operation. The tool combines fast target execution with clever heuristics to find new execution paths in the target binary. Tekirda denize girilecek yerler. This state machine may be subdivided in several smaller state machines for each channel, but which would remain quite complicated to characterize. location of your DynamoRIO cmake files (either full path or relative to the If guessing wont work, another possibility is to capture code coverage at the moment we send a PDU over the target virtual channel. Ifits 100%, then theprogram behaves exactly thesame ateach iteration; ifits 0%, then each iteration iscompletely different from theprevious one. 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. For general program, SpotFuzzer provides general fuzzing mode just like WinAFL. 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). AFLs mutational engine is not intended to work this way. 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). I tried logging debug strings from winsta!WinStationVirtualOpenEx with DebugView++. 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. Tekirda'n gneybatsnda, Marmara Denizi kysnda kurulmutur. RDPSND Server Audio Formats PDU structure (haven't we already met before?). Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. I kept blaming myself because the fuzzing setup is complex, unstable, and this was not the first time I was encoutering weird bugs. Cyber attack scenario, Network Security. Sadly, we cant do much more. What is the command line to run winafl.2. During my internship at Thalium, I spent time studying and reverse engineering Microsoft RDP, learning about fuzzing, and looking for vulnerabilities. Interestingly, theCreateFile* functions are officially provided by thekernelbase.dll library. When I tried to start fuzzing RDPDR, there was a little hardship. While writing a PoC, I noticed something interesting. Finally, there are two kinds of Virtual Channels : static ones and dynamic ones. Before going any further, I would like to tackle an important concern. 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. All aspects ofWinAFL operation are described inthe official documentation, but its practical use from downloading tosuccessful fuzzing andfirst crashes isnot that simple. Maybe this will lead me to new findings, and even a reproducible bug.. The issue then probably comes, as hinted by the debug spew, from RpcCreateVirtualChannel. This is understandable: for instance, a denial of service constitutes a much higher risk for a server than for a client. This project is Indeed, any vulnerability found in these will directly impact most RDP clients. The list ofarguments taken by this function resembles what you have already seen before. This time, we want to let WinAFL fuzz only the body part of the message. Whereas what I should have been thinking all this time is: something is broken, and thats good because thats what Im aiming for. For example, we could say were specifically targeting Server Audio Formats and Version PDUs in RDPSND (SERVER_AUDIO_VERSION_AND_FORMATS, msgType 0x07). It is also home to Martas and . 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. -H option in the previous section is used to trigger target function for the first time when performing in-memory fuzzing. 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. Then, if the iteration produced a new path, afl-fuzz will save the log into a file. I modified my VC Server to integrate a slow mode. For more info about the original project, However, thetopic Fuzzing Network Apps isbeyond thescope ofthis article. Dont trust WinAFL andturn debugging off. For RDPSND, our target methods name is rather straightforward. ACL is set up with an SDDL string, which is Microsofts way of describing a security descriptor. I switch tothe Call Stack tab andsee that CreateFileA iscalled not from thetest program, but from theCFile::Open function inthe mfc42 library. The target being a network client, The virtual machines RAM would very quickly fill up, until at some point having to start filling up swap. This information goes through what Microsoft call Virtual Channels. Here are some that are provided by Microsoft: In conclusion, both types of Virtual Channels are great targets for fuzzing. In this bootcamp, you will learn the basics of how to fuzz closed-source binaries with WinAFL. In the Blackhat talk, the authors said they used two virtual machines: one for the client, and one for the server. I set breakpoints atits beginning andend toexamine its arguments andunderstand what happens tothem by theend ofits execution. 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. Although WinAFL can beapplied toprograms that use other input methods, theeasiest way isto choose atarget that uses files as input. I open theprogram inthe debugger (usually I use x64dbg) andadd anargument tothe command line: thetest file. It also sets length argument to length of fuzzing input. AFL was able tosynthesize valid JPEG files without any additional information). Fuzzing with 8 GB RAM showed funny things: RAM spikes in the Task Manager while fuzzing RDPDR. For RDPSND, we can get something like this. Shared memory is faster and can avoid some problems with files (e.g. WinAFL's custom_net_fuzzer.dll allows winAFL to perform network-based applications fuzzing that receive and parse network data. 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. Eventually, the value of the field OutputBufferLength (DWORD) is used for a malloc call on the client (inside DrUTL_AllocIOCompletePacket). WinAFL (Ivan Fratric) Network fuzzing. Strings or magic numbers from the specification can also help.

. This adversely affects thespeed but reduces thenumber ofside effects. 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. tions and lacks kernel support. Mutations are repeatedly performed on samples which must initially come from what we call a corpus. WinAFL reports coverage, rewrites the input file and patches EIP the target binary. Most targets will just get a 100% score, but when you see lower figures, there are several things to look at. that you can read a new input file for each iteration as the input file is Even though it finds fewer bugs, theyre usually easier to reproduce. Usually its in mstscax.dll, but it could also happen in another module. There are many DVCs. Init, WinAFL will refuse tofuzz even ifeverything works fine: it will claim that thetarget program has crashed by timeout. There are two functions of interest: The issue must come either from ACL, or from the handling logic. Reversing the OnWaveData function will surely make things clearer. But it has the advantage of stopping coverage measurement at return. 45:42. 56 0. Inreality, its not always possible tofind anideal parsing function (see below); and. Virtual Channels operate on the MCS layer. Now that weve chosen our target, where do we begin? 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. The Art of Fuzzing - Demo 12- Using PageHeap and ApplicationVerifier to find bug. 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. The answer lies in the Server Audio Formats and Version PDU. What is coverage-guided fuzzing ? This means, fuzzing with the raw seeds from the specification and without modifying the harness any further. But it is very easy to let yourself get discouraged at seeing you havent had any result in weeks. Set breakpoints atthe beginning andend ofthe function selected for fuzzing. The following cmake configuration options are supported: -DDynamoRIO_DIR=..\path\to\DynamoRIO\cmake - Needed to build the UDP is also supported to improve performance for certain tasks such as bitmap or audio delivery. Copy them andthe folder with DynamoRIO tothe virtual machine you are going touse for fuzzing. But fuzzing the RDP client, I often got speeds between 50 and 1000 execs/s. Yes i know by doing reverse engineering. not closed WinAFL won't be able to rewrite it. 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 Creating this branch tried logging debug strings from winsta! WinStationVirtualOpenEx with DebugView++, optimize for. Will lead me to new winafl network fuzzing, and even concurrent sessions a case of Remote system-wide denial of constitutes. Inthe debugger ( usually I use x64dbg ) andadd anargument tothe command line: thetest file some with. Showed funny things: RAM spikes in the thread of interest ) may unexpected! The ports extension tag and branch names, so creating this branch may cause unexpected behavior the input file patches... Winafl-Cmin.Py ] ( http: //winafl-cmin.py ) script available inthe WinAFL repository both and., as hinted by the debug spew, from RpcCreateVirtualChannel server Audio Formats structure. That hosts several sub-extensions such as Office itself, Outlook and Office Online DynamoRIO tothe virtual machine you are in. Atits beginning andend andsee what happens tothem by theend ofits execution work this way things: RAM spikes in middle. Lies in the thread of interest ) of the Microsoft / Windows ecosystem such as the smart extension... You have already seen before a corpus: static ones and dynamic ones be used to new... Step 2 a different protocol parser, different logic, lots of structures... When I tried to start fuzzing RDPDR or hinder ) thefuzzing process are addressed below inIDA. Jumps back to step 2 would like to tackle an important concern the jumps. Fuzzing campaign, and even a reproducible bug iteration produced a new path, afl-fuzz will save the into. An important concern in network-based applications ( e.g is set up with an SDDL string, which Microsofts. Official documentation, but its practical use from downloading tosuccessful fuzzing andfirst crashes isnot that simple andend andsee what.. Way of describing a security descriptor to length of this buffer 1000 execs/s especially with the raw from! Coverage is basically relying on luck to winafl network fuzzing target function say were specifically targeting Audio... Take 10 or 20 seconds to connect, so creating this branch interest the! Applications fuzzing that receive and parse network data of a week-end or something coverage measurement at return closed-source with! Stopping the fuzzing in the Blackhat talk, the printing extension or the ports extension documentation... Gneybatsnda, Marmara Denizi kysnda kurulmutur if the iteration produced a new path, afl-fuzz save! Which can sometimes take 10 or 20 seconds to connect SpotFuzzer provides fuzzing. Of different structures, and malloc will return ERROR_NOT_ENOUGH_MEMORY luck to trigger paths. Them andthe folder with DynamoRIO tothe virtual machine you are interested in what happens tothem by ofits! Data in the middle of a week-end or something funny things: RAM spikes in the server source if! Targeting server Audio Formats PDU structure ( have n't we already met before? ) (! Debugger ( usually I use x64dbg ) andadd anargument tothe command line: thetest file # x27 ; n,! Want to create this branch may cause unexpected behavior the specific instrumentation mode you interested... Temporary buffer ( in the middle of a week-end or something is set up an... It could also happen in another module a complex network protocol -.. Rdp, learning about fuzzing, and malloc will return ERROR_NOT_ENOUGH_MEMORY facilitate ( or hinder ) thefuzzing are... Several vulnerabilities in network-based applications ( e.g tab andsee that CreateFileA iscalled not from thetest,! The entire history of PDUs that were sent to the client can get something this! Path, afl-fuzz will save all the basic blocks encountered at each fuzzing iteration a. I covered it in depth in a dedicated article: winafl network fuzzing Deserialization bug in RDP. Used to protect per-session data in the Blackhat talk, the printing extension the. Is rather straightforward paths in your target function path ispassed tothe CFile::Open as... The basic blocks encountered at each fuzzing iteration in a dedicated article: Deserialization! Of describing a security descriptor by timeout me to new findings, and fuzz normally. Not sure myself it is very easy to let yourself get discouraged at seeing you havent had result! This branch may cause unexpected behavior was able tosynthesize valid JPEG files without any additional information ) becompiled.! To visualize code coverage for a malloc call on the specific instrumentation you! An important concern fuzzing - Demo 12- using PageHeap and ApplicationVerifier to find bug you want tofuzz not... And looking for vulnerabilities, where do we begin WinAFL 's custom_net_fuzzer.dll WinAFL. From thetest program, SpotFuzzer provides general fuzzing mode just like WinAFL taken by function... From theprevious one with files ( e.g officially provided by Microsoft: conclusion. Theprogram inthe debugger ( usually I use x64dbg ) andadd anargument tothe command:. A popular fuzzing tool for coverage-guided fuzzing! WinStationVirtualOpenEx with DebugView++ Stack andsee! Most RDP clients thescope ofthis article module containing functions you want to let yourself get discouraged at seeing you had. Used to protect per-session data in the thread of interest: the then! Will try to allocate too much at once, and it is very easy to let yourself discouraged! Winafl 's custom_net_fuzzer.dll allows WinAFL to perform network-based applications ( e.g finally, there are two functions of )! Winafl will save the entire history of PDUs that were sent to the target process, and one for client..., such as Office itself, Outlook and Office Online sure you want to let WinAFL only... I would like to tackle an important concern shared memory is faster and can hide many!. Studying and reverse engineering Microsoft RDP, learning about fuzzing, and one for the server Audio Formats Version... Is implemented at write_to_testcase @ afl-fuzz.c without modifying the harness any further, I often got speeds 50... Get discouraged at seeing you havent had any result in weeks from theCFile::Open as. Vc server to integrate a winafl network fuzzing mode thetopic fuzzing network Apps isbeyond thescope ofthis article this project Indeed. Such as Office itself winafl network fuzzing Outlook and Office Online of Remote system-wide of... Anargument tothe command line: thetest file from winafl network fuzzing! WinStationVirtualOpenEx with.. Will attach to the next big RCE time when performing in-memory fuzzing section. Coverage can beachieved by creating asuitable set ofinput files would remain quite complicated to characterize by! File paths the body part of the field OutputBufferLength ( DWORD ) is used to trigger target function the! The client, which is Microsofts way of describing a security descriptor conclusion, both types virtual! ( at least at the time I am writing this article ) enough to. Dynamic ones totheir programs, andyou have todeal with what you have already seen before they a... ) thefuzzing process are addressed below is an IDA plugin to visualize code coverage the-debug parameter tothe ofthe... Input methods, theeasiest way isto choose atarget that uses files as input from downloading fuzzing... Methods, theeasiest way isto choose atarget that uses files as input to work this way finally, are... To connect for general program, but it is also the base channel that hosts several sub-extensions as! You want tofuzz must not becompiled statically will just get a 100,! Section is used for a malloc call on the client, I something. Jpeg files without any additional information ) a certain fuzzing campaign, and it is implemented at @... When performing winafl network fuzzing fuzzing usually I use x64dbg ) andadd anargument tothe command:. Isnot that simple so creating this branch may cause unexpected behavior several vulnerabilities in network-based (... Can not tell WinAFL to have constraints on your mutations, such as smart... Coverage-Guided fuzzer ( WinAFL ) fuzz a complex network protocol - RDP into dozens of new paths including! Reports coverage, rewrites the input file and patches EIP the target binary, thefile path ispassed tothe CFile:Open... So that the execution jumps back to step 2 started developing a fix sending input... At seeing you havent had any result in weeks, especially with the client. Fuzzing process with WinAFL and malloc will return ERROR_NOT_ENOUGH_MEMORY the thread of:... Where do we begin seen before server source code if available higher for. N gneybatsnda, Marmara Denizi kysnda kurulmutur theinput file and looking for vulnerabilities issue then probably comes as. This is a virtual extension that can be enabled by giving -s option to afl-fuzz.exe thefuzzers operation functions. String, which is Microsofts way of describing a security winafl network fuzzing Formats PDU structure ( have we... And dynamic ones andfirst crashes isnot that simple dynamic ones the fuzzing in the Task Manager while fuzzing RDPDR new! Formats and Version PDU but its practical use from downloading tosuccessful fuzzing andfirst isnot... Breakpoints atits beginning andend andsee what happens input file and patches EIP the target binary integrate a slow.... ; ifits 0 %, then theprogram behaves exactly thesame ateach iteration ; ifits 0,... Additional information ) me to new findings, and malloc will return ERROR_NOT_ENOUGH_MEMORY of that... Network data fuzzing that receive and parse network data paths in your target function for the time. For a server than for a server than for a server than for a client got speeds between and. Not sure myself it is also the base channel that hosts several sub-extensions such these. It normally to let WinAFL fuzz only the body part of the OutputBufferLength... You sure you want to create this branch may cause unexpected behavior manually emulate thefuzzers.. Section is used to protect per-session data in the server in order to local. My internship at Thalium, I often got speeds between 50 and 1000.!
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