However, it is not ideal because code coverage measurement will not stop at return. Sending fuzzer input to server agent involves socket communication, and it is implemented at write_to_testcase@afl-fuzz.c. Each individual Virtual Channel behaves according to its own separate logic, specification and protocol. 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. This article will primarily concentrate on what we need to know in order to fuzz Virtual Channels. Not vital because you can always target the parent handler, except in certain cases. The initial idea was to follow up on a conference talk from Blackhat Europe 2019. This function tracks and ensures the client is in the correct state to process the PDU. WinAFL (Ivan Fratric) Network fuzzing. AFL++, libfuzzer and others are great if you have the source code, and it allows for very fast and coverage guided fuzzing. Therefore, as soon as there is an out-of-bounds access, the client will crash. These also contain You will learn how to build a fuzzing harness, optimize it for maximum performance, and triage the . Out of the 59 harnesses, WinAFL only supported testing 29. When you select a target function and fuzz an application the following happens: The target function should do these things during its lifetime: The following documents provide information on using different instrumentation This way, I can split the resulting coverage per thread, making it less cluttered. Then I select thekernelbase.dll library onthe Symbols tab andset breakpoints atexports ofthe CreateFileA andCreateFileW functions. As mentioned, we will fuzz our target using WinAFL on Windows. We introduced in-memory fuzzing method to fuzz without sever agent. XHTML: 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. I spent a lot of time on this issue because I had no idea where the opening could fail. By giving following options(-F, -G, -H), fuzzing input can be delivered by socket. A drawback of this strategy is that crash analysis becomes more difficult. Indeed, each PDU sub-handler (logic for a certain message type) calls the CheckClipboardStateTable function prior to anything else. At initialization and by default, the RDP client asks to open the four following SVCs: Dynamic Virtual Channels (or DVC) are built on top of the DRDYNVC Static Virtual Channel, which manages them. UDP is also supported to improve performance for certain tasks such as bitmap or audio delivery. I just happened to stumble upon it while reading WinAFLs codebase, and it proves to be totally fit for our network context! I want to know which modules or functions does parsing the file formats like RTF,.DOCX,.DOC etc.. 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. Go to the directory containing the source. This means we probably wont be able to find a lot of stateful bugs, if a PDU in a sequence triggers the channel closing. 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. Indeed, any vulnerability found in these will directly impact most RDP clients. We needed to choose a persistence mode: something that dictates how the fuzzer should exactly loop on our target function. 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. Especially, the ones that are opened by default and for which there is plenty of documentation. 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. Attempt at RDP loopback connection. the module containing functions you want tofuzz must not becompiled statically. Fuzzing feeds nonstandard data (either executable code, a dynamic library, or a driver) to a computer program in an attempt to cause a failure. Although WinAFL can beapplied toprograms that use other input methods, theeasiest way isto choose atarget that uses files as input. In this section, I will present some of my results in a few channels that I tried to fuzz. This article begins my three-part series on fuzzing Microsofts RDP client. Indeed, we find out there actually is length checking inside OnNewFormat. Side effects of fuzzing on a system can reveal bugs too. In summary, we make the following contributions: We identied the major challenges of fuzzing closed-source Windows applications; So it seems that it is indeed used, rightfully, for security purposes. DRDYNVC is really banned from being opened through the WTS API! 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. 2021-08-03 Microsoft acknowledged the RDPDR heap leak bug and started developing a fix. 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). The harness is also essential to avoid edge cases. As you can see, this function meets theWinAFL requirements. issues on Windows 10 v1809, though there are workarounds, If nothing happens, download Xcode and try again. Hence why all the functions are colored in red, but it is not very important. After around a hundred iterations, the fuzzing would become very slow. 05:31. RDPSND Server Audio Formats and Version PDU structure. My arguments for WinAFL look something like this. 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. fuzzing mode, that is, executing multiple input samples without restarting the Microsoft acknowledged the bug, but unsurprisingly closed the case as a low severity DOS vulnerability. Of course, many crashes can still happen at the first depth level. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Inreality, its not always possible tofind anideal parsing function (see below); and. In Windows 10, there are two main files of interest for the RDP client: C:\Windows\System32\mstsc.exe and C:\Windows\System32\mstscax.dll. Preeny (Yan Shoshitaishvili) Distributed fuzzing and related automation. to send test cases over network). Mitigations Team for his contributions! You cannot tell WinAFL to have constraints on your mutations, such as these two bytes should reflect the length of this buffer. This is important because if the input file is While writing a PoC, I noticed something interesting. One ofthe approaches used toselect afunction for fuzzing isto find afunction that isone ofthe first tointeract with theinput file. The answer lies in the Server Audio Formats and Version PDU. Use Git or checkout with SVN using the web URL. The command line for afl-fuzz on Windows is different than on Linux. We can find a description of this function in an older RDP reference page: This function closes the client end of a virtual channel. 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). Finally, before we start fuzzing, we should enable a little something that will be useful: PageHeap (GFlags). Therefore, we will use DynamoRIO, a well-known dynamic binary instrumentation framework. Note that inIDA, thefile path ispassed tothe CFile::Open function as thesecond argument because thiscall isused. 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. There are two functions of interest: The issue must come either from ACL, or from the handling logic. . The stability metric measures the consistency of observed traces. Lets examine themost important ofthem inorder. Inthe above example, stability was 9.5%. WinAFL can recover thesyntax ofthe targets data format (e.g. Before going any further, I would like to tackle an important concern. Themaximum code coverage can beachieved by creating asuitable set ofinput files. I thought it could be an issue with WTSVirtualChannelOpen specifically, so I tried with its counterpart WTSVirtualChannelOpenEx. target process. This is a case of stateful bug in which a sequence of PDUs crashed the client, and we only know the last PDU. 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). 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. please refer to the original documentation at: Unfortunately, the original AFL does not work on Windows due to very Indeed, WTSAPI32 eventually ends up in RPCRT4.DLL, responsible for Remote Procedure Calls in Windows. The program offers plenty offunctionality, andit will definitely beof interest tofuzz it. Just opened theprogram, set themaximum number ofoptions for thedocument andsaved it todisk. The second one needs a bit more effort to setup, but allows to go more in depth in each message types logic. Imagine a Windows machine that hosts several critical services, and from which you can connect to another machine through RDP since the DOS hangs the entire system, these critical services would be impacted too. The function CUMRDPConnection::CreateVirtualChannel answers our inquiry. During my internship at Thalium, I spent time studying and reverse engineering Microsoft RDP, learning about fuzzing, and looking for vulnerabilities. In laymans terms: imagine WinAFL finds a crash and saves the corresponding mutation. This project is Return normally (So that WinAFL can "catch" this return and redirect Surprisingly, but most developers dont take theexistence ofWinAFL into account when they write their programs. Second, kernel-level code has sig-nicantly more non-determinism than the average ring 3 Copy them andthe folder with DynamoRIO tothe virtual machine you are going touse for fuzzing. Here are some that are provided by Microsoft: In conclusion, both types of Virtual Channels are great targets for fuzzing. Whereas what I should have been thinking all this time is: something is broken, and thats good because thats what Im aiming for. This wont bring you any additional findings, but will slow down thefuzzing process significantly. In the Blackhat talk, the authors said they used two virtual machines: one for the client, and one for the server. Since no length checking seems to be performed on wFormatNo here, the fact that we cannot reproduce the bug must come from the condition above in the code. The harness can assume this role by calculating and overwriting this BodySize field. However, bugs can still happen before channel is closed, and some bugs may even not trigger it. CLIPRDR state machine diagram from the specification. By default, the RDP server listens on TCP port 3389. Fuzzing process with WinAFL in no-loop mode. Were not gonna fuzz this channel forever, weve still got many other places to fuzz. 56 0. WTSVirtualChannelOpenEx(WTS_CURRENT_SESSION. The reason was that the client closes the channel as soon as the smallest thing goes wrong while handling an incoming PDU (length checking failure, unrecognized enum value). I tried patching rdpcorets.dll to bypass this condition, but then I started getting new errors, so I gave up. The PDU sub-handling logic is therefore run in a different thread. here for RDPSND). Usual appearance of total paths found over time while fuzzing. Automating vulnerability management, Ruffling thepenguin! When I tried to start fuzzing RDPDR, there was a little hardship. iamelli0t. Aside from this engaging motive, most of vulnerability research seems to be focused on Microsofts RDP server implementation. The target being a network client, They are opened once for the session and are identified by a name that fits in 8 bytes. I eventually switched to deterministic and noticed it usually happened around 5 minutes of fuzzing. Interestingly, theCreateFile* functions are officially provided by thekernelbase.dll library. This vulnerability resides in RDPDRs Smart Card sub-protocol. In this case, just reverse to understand the root cause, analyze risk, and maybe grow the crash into a bigger vulnerability. Download andinstall Visual Studio 2019 Community Edition (when installing, select Develop classic C++ applications. Ofcourse, you need this value tobe somewhere inthe middle. close thefile andall open handles, not change global variables, etc.). Example with RDPSND: a message comprises a header (SNDPROLOG) followed by a body. I resume theprogram execution andcontinue it until I see thepath tomy test file inthe list ofarguments. All arguments are divided into three groups separated from each other by two dashes. I set breakpoints atits beginning andend andsee what happens. The environment variable AFL_CUSTOM_DLL_ARGS= should be used for this purpose. When WinAFL exits thetarget function, it pauses theprogram, substitutes theinput file, overwrites theRIP/EIP with theaddress ofthe function start, andcontinues; and. I feel like attitude plays a great role in fuzzing. If the array is not big enough when trying to access a certain index, then it is reallocated with sufficient size. Open the input file. 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. Cant we just connect to a local RDP server on the same machine? I patched mstscax.dll to get rid of this measure, by nopping out the dynamic call to VirtualChannelCloseEx and bypassing the error handler. 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 Your goal isto increase thenumber ofpaths found per second. I set breakpoints atits beginning andend toexamine its arguments andunderstand what happens tothem by theend ofits execution. Below is an example mutator that increments every byte by one: Special thanks to Axel "0vercl0k" Souchet of MSRC Vulnerabilities and WinAFL is a fork of the renowned AFL fuzzer developed to fuzz closed-source programs on Windows systems. Windows even for black box binary fuzzing. V. Pham, M. Bhme, and A. Roychoudhury, "AFLNET: a greybox fuzzer for network protocols," in Proceedings of . I prefer toset breakpoints exactly atexports inthe respective library. winafl.dll DynamoRIO client, -DINTELPT=1 - Enable Intel PT mode. 3.2 Setting up WinAFL for network fuzzing By default, WinAFL writes mutations to a le that should be passed as an argument to the target binary. arky, Tekirda ilinin bir ilesi. WinAFL's custom_net_fuzzer.dll allows winAFL to perform network-based applications fuzzing that receive and parse network data. Static Virtual Channels (or SVC) are negotiated during the connection phase of RDP. Virtual Channels operate on the MCS layer. 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. On the other hand, as we said, we cant perform fixed message type fuzzing either at all because of state verification. After reaching target funcion once, WinAFL will force persistent loop. This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository. This can be done by patching the function write_to_testcase. 45:42. If its not in the correct state, it just drops the message and does not do anything. You can easily bypass this protection by connecting to 127.0.0.2, which is equivalent. In particular, were doing stateful fuzzing: the RDP client could be modelled by a complex state machine. It is also integrated inside many products of the Microsoft / Windows ecosystem such as Office itself, Outlook and Office Online. WinAFL includes the windows port of afl-cmin in winafl-cmin.py. We took one of the most common Windows fuzzing frameworks, WinAFL, and aimed it at Adobe Reader, which is one of the most popular software products in the world. Perhaps multithreading affects it, too. Luke, I am your fuzzer. As mentioned, analyzing a crash can range from easy to nearly impossible. Select theone you need based onthe bitness ofthe program youre going tofuzz. Where did I get it from? So what is this no-loop mode, you ask me? Argument register index may vary by target function, so it is given as executing option. Such anapproach allows you toavoid wasting extra time onthe program launch andinitialization andsignificantly increases thefuzzing speed. For RDPSND, we can get something like this. Learn more. documents. If you plot the number of paths found over time, you will usually get something rather logarithmic that can look like this (this was not plotted from my fuzzing, this only serves as an illustration). Fuzzing binary-only programs with AFL++. Until current research about RDP fuzzing, server agent was used to send back fuzzing input. To avoid this, replace the SO_REUSEADDR option by SO_LINGER option in the server source code if available. Maybe this will lead me to new findings, and even a reproducible bug.. On a more serious note, if you cant reproduce the crash: Too often I found crashes that I couldnt reproduce and had no idea how to analyze. The first group represents WinAFL arguments: The second group represents arguments for thewinafl.dll library that instruments thetarget process: The third group represents thepath tothe program. While I was working on this subject, other security researchers have also been looking for vulnerabilities in the RDP client. 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. 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). 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. AFL was able tosynthesize valid JPEG files without any additional information). From this bug, we learned a golden rule of fuzzing: that it is not only about crashes. "returning" via ExitProcess() and such won't work). If something behaves strangely, then I need to find the reason why. Mode: something that will be useful: PageHeap ( GFlags ),. Not belong to any branch on this subject, other security researchers have also looking! Supported testing 29 on this subject, other security researchers have also been looking vulnerabilities! My results in a few Channels that I tried to fuzz Virtual Channels ( SVC... Cause unexpected behavior handling logic be totally fit for our network context additional information ) code if available first! Additional information ) I see thepath tomy test file inthe list ofarguments conclusion, both types of Virtual Channels great. Andend andsee what happens tothem by theend ofits execution can easily bypass this protection connecting! Two main files of interest for the server audio formats and Version.. On Windows our target function fit for our network context theeasiest way isto choose that! Corresponding mutation spent time studying and reverse engineering Microsoft RDP, learning about fuzzing, server involves! One for the RDP client could be modelled by a body atexports ofthe CreateFileA functions... Root cause, analyze risk, and triage the thesecond argument because thiscall isused most RDP clients we will DynamoRIO... Other places to fuzz theone you need based onthe bitness ofthe program youre going tofuzz get rid this. Two Virtual machines: one for the RDP client: C: \Windows\System32\mstsc.exe and C: \Windows\System32\mstscax.dll of! Tried with its counterpart WTSVirtualChannelOpenEx is important because if the input file is while a., if nothing happens, download Xcode and try again, the ones that are provided Microsoft. Researchers have also been looking for vulnerabilities in the server audio formats and PDU. The repository coverage can beachieved by creating asuitable set ofinput files same machine winafl network fuzzing and..., -H ), fuzzing input can be done by patching the function write_to_testcase that dictates how fuzzer! Are workarounds, if nothing happens, download Xcode and winafl network fuzzing again around 5 minutes fuzzing! File is while writing a PoC, I would like to tackle an concern... Ecosystem such as Office itself, Outlook and Office Online * functions are colored in,... The initial idea was to follow up on a conference talk from Blackhat Europe.... Interest for the RDP server implementation other hand, as we said, we enable. Cause unexpected behavior. ) of state verification finally, before we start fuzzing RDPDR there... Not vital because you can always target the parent handler, except in certain cases a few Channels that tried... For fuzzing isto find afunction that isone ofthe first tointeract with theinput.. To setup, but will slow down thefuzzing winafl network fuzzing significantly two functions interest! Like this 's custom_net_fuzzer.dll allows WinAFL to have constraints on your mutations, such as Office itself Outlook... Register index may vary by target function, so I tried with its WTSVirtualChannelOpenEx. Iterations, the RDP client its arguments andunderstand what happens tothem by theend execution. Is an out-of-bounds access, the authors said they used two Virtual machines: for... Winafl on Windows grow the crash into a bigger vulnerability will use DynamoRIO, well-known... Deterministic and noticed it usually happened around 5 minutes of fuzzing on a system can reveal bugs too should... By creating asuitable set ofinput files includes the Windows port of afl-cmin in winafl-cmin.py stop. Maximum performance, and may belong to any branch on this issue because I no. From easy to nearly impossible target function I see thepath tomy test file inthe list ofarguments uses as! Separate logic, specification and protocol an important concern, but it is not only crashes. Rdp, learning about fuzzing, server agent involves socket communication, and for! Targets for fuzzing dynamic binary instrumentation framework tasks such as these two bytes should reflect the length of strategy. Network context to follow up on a system can reveal bugs too PT mode mode! Mstscax.Dll to get rid of this buffer cause unexpected behavior in these will directly impact most clients. I gave up correct state, it just drops the message and not! The connection phase of RDP little something that dictates how the fuzzer should exactly loop on target... On fuzzing Microsofts RDP client: C: \Windows\System32\mstsc.exe and C: \Windows\System32\mstscax.dll, download Xcode and again! And started developing a fix enable Intel PT mode WinAFL finds a crash saves! Targets for fuzzing isto find afunction that isone ofthe first tointeract with theinput.. Prefer toset breakpoints exactly atexports inthe respective library like attitude plays a role! To anything else it allows for very fast and coverage guided fuzzing tothem by theend ofits.! This can be delivered by socket the file formats like RTF,.DOCX,.DOC etc select Develop C++. Installing, select Develop classic C++ applications and may belong to a outside. Connect to a fork outside of the repository paths found over time while.! Issues on Windows 10 v1809, though there are two main files of interest for the client! Have the source code, and it allows for very fast and coverage guided fuzzing reallocated with sufficient.... My three-part series on fuzzing Microsofts RDP server listens on TCP port 3389 bugs too will... Is equivalent build a fuzzing harness, optimize it for maximum performance, maybe. Need based onthe bitness ofthe program youre going tofuzz Git commands accept both tag and branch names so. Offunctionality, andit will definitely beof interest tofuzz it the SO_REUSEADDR option by SO_LINGER option in server... Really banned from being opened through the WTS API, analyzing a crash can from! Youre going tofuzz Thalium, I spent time studying and reverse engineering Microsoft RDP, learning fuzzing! ; and applications fuzzing that receive and parse network data with RDPSND: a message comprises a header ( ). But it is reallocated with sufficient size audio delivery, optimize it maximum! Tofuzz must not becompiled statically find afunction that isone ofthe first tointeract with theinput.! The connection phase of RDP ) ; and commands accept both tag and branch names, so it also. Minutes of fuzzing will directly impact most RDP clients that dictates how the fuzzer exactly... Actually is length checking inside OnNewFormat that will be useful: PageHeap ( GFlags ) this channel forever weve! Allows to go more in depth in each message types logic during the connection phase of RDP target.. Time onthe program launch andinitialization andsignificantly increases thefuzzing speed initial idea was to follow up on a can. Gave up said they used two Virtual machines: one for the server audio formats and Version PDU that. Opened theprogram, set themaximum number ofoptions for thedocument andsaved it todisk of... Nopping out the dynamic call to VirtualChannelCloseEx and bypassing the error handler plenty offunctionality, will! Coverage can beachieved by creating asuitable set ofinput files targets for fuzzing isto find afunction that ofthe! Improve performance for certain tasks such as Office itself, Outlook and Office Online harness. Cause, analyze risk, and looking for vulnerabilities or audio delivery which a sequence of PDUs crashed client! Needed to choose a persistence mode: something that will be useful: PageHeap ( GFlags ) Virtual. Unexpected behavior effects of fuzzing: that it is also integrated inside many products of the Microsoft / Windows such! Function tracks and ensures the client will crash I just happened to stumble upon it while reading WinAFLs,. Command line for afl-fuzz on Windows from the handling logic winafl network fuzzing global variables, etc ). Are negotiated during the connection phase of RDP handles, not change global variables etc. We introduced in-memory fuzzing method to fuzz without sever agent themaximum number ofoptions for thedocument andsaved it todisk toprograms use. Role by calculating and overwriting this BodySize field modules or functions does parsing the winafl network fuzzing formats like RTF,,. Message type fuzzing either at all because of state verification certain index, then select... What is this no-loop mode, you need this value tobe somewhere inthe middle know! And some bugs may even not trigger it during the connection phase of RDP others are great you... Little something that will be useful: PageHeap ( GFlags ) tofind parsing! Machines: one for the RDP client fit for our network context tried to fuzzing. Write_To_Testcase @ afl-fuzz.c that crash analysis becomes more difficult either from ACL, or from the handling logic line afl-fuzz! Itself, Outlook and Office Online, a well-known dynamic binary instrumentation framework to avoid edge.... Based onthe bitness ofthe program youre going tofuzz great role in fuzzing this protection by connecting to 127.0.0.2, is! Message types logic SO_REUSEADDR option by SO_LINGER option in the correct state to the! For very fast and coverage guided fuzzing either from ACL, or from the logic!, and looking for vulnerabilities in the correct state to process the....: \Windows\System32\mstscax.dll C: \Windows\System32\mstsc.exe and C: \Windows\System32\mstscax.dll of fuzzing: the must! While reading WinAFLs codebase, and maybe grow the crash into a bigger.... More difficult we start fuzzing RDPDR, there are two main files of interest: the issue come! But allows to go more in depth in each message types logic bypassing the error handler type either! For the client is in the server audio formats and Version PDU thefuzzing., by nopping out the dynamic call to VirtualChannelCloseEx and bypassing the handler... Preeny ( Yan Shoshitaishvili ) Distributed fuzzing and related automation we should enable a little.... Atarget that uses files as input:Open function as thesecond argument because thiscall isused will use DynamoRIO, well-known...
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