Though here, it is rarely >50% because there is a large proportion of error-handling blocks that are never triggered. This way, I could have time to monitor which PDU was guilty and what exactly happened when it was sent. This video contain:1. By replaying the whole history, you may hope the client behaves in a deterministic enough way that it reproduces the crash. They also started reviewing this case for a potential bounty award. Fuzzing should entirely happen without human intervention. We thought they achieved encouraging results that deserved to be prolonged and improved. 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. In this article, I will address different fuzzing types and show how to use one of them, WinAFL. It was assigned CVE-2021-38665. But ifyou pay attention tothe arguments, youll realize that thetarget wants toopen some ofits service files, not thetest file. Another obvious type of edge case is crashes. UDP is also supported to improve performance for certain tasks such as bitmap or audio delivery. target process. RDPSND PDU handler and dispatch logic in mstscax.dll. AFL/WinAFL work by continously sending and mutating inputs to the target program, to make it behave unexpectedly (and hopefully crash). As you can see, this function meets theWinAFL requirements. Nothing particularly shocking right away. This PDU is used by the server to send a list of supported audio formats to the client. AFL++, libfuzzer and others are great if you have the source code, and it allows for very fast and coverage guided fuzzing. Thenext call toCreateFileA gives me thefollowing call stack. 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). Check a simple harness here: https://github.com/googleprojectzero/Jackalope/blob/6d92931b2cf614699e2a023254d5ee7e20f6e34b/test.cpp#L41 You pass theoffset ofthe so called target function contained inthe binary as one ofthe arguments; WinAFL isinjected into theprogram andwaits for thetarget function toexecute; WinAFL starts recording code coverage information. 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. What is coverage-guided fuzzing ? 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. 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. 2021-07-31 Microsoft acknowledged the RDPDR deserialization bug and started developing a fix. 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. Your goal isto increase thenumber ofpaths found per second. The command line for afl-fuzz on Windows is different than on Linux. Todo this, I check thelist ofprocess handles inProcess Explorer: thetest file isnt there. 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. The second one needs a bit more effort to setup, but allows to go more in depth in each message types logic. All you need is to set up the port to listen on for incoming connections from your target application. While writing a PoC, I noticed something interesting. 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. AFL was able tosynthesize valid JPEG files without any additional information). We now have a working harness and are pretty much ready to fuzz. I did mention the function we target should be fuzzed in a loop without restarting the process. Indeed, any vulnerability found in these will directly impact most RDP clients. The program offers plenty offunctionality, andit will definitely beof interest tofuzz it. roving (Richo Healey) Distfuzz-AFL (Martijn Bogaard) AFLDFF (quantumvm) afl-launch (Ben Nagy) AFL Utils (rc0r) AFL crash analyzer (floyd) afl-extras (fekir) afl-fuzzing-scripts (Tobias Ospelt) afl-sid (Jacek Wielemborek) afl-monitor . How tofuzz theLinux kernel, synthesize valid JPEG files without any additional information, Herpaderping and Ghosting. Tofind out whats theproblem, you can manually emulate thefuzzers operation. Finally, there are two kinds of Virtual Channels : static ones and dynamic ones. Of course, many crashes can still happen at the first depth level. Then, I will talk about my setup with WinAFL and fuzzing methodology. It needs to be adapted to our case, which is fuzzing a client in a network context. -target_offset from -target_method). However, bugs can still happen before channel is closed, and some bugs may even not trigger it. 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. However, WinAFL is not going to work with our target out of the box. A corpus is a set of input files, or seeds, that we need to construct and feed to WinAFL to start. Thecreator ofAFL believes that you should aim atsome 85%. Send n > 1 formats to the client through a Format PDU. so that the execution jumps back to step 2. . Select theone you need based onthe bitness ofthe program youre going tofuzz. While I was working on this subject, other security researchers have also been looking for vulnerabilities in the RDP client. The Remote Desktop Protocol (RDP) is a proprietary protocol designed by Microsoft which allows the user of an RDP Client software to connect to a remote computer over the network with a graphical interface. There also exist alternate implementations of RDP, like the open-source FreeRDP. Especially, the ones that are opened by default and for which there is plenty of documentation. Although WinAFL can beapplied toprograms that use other input methods, theeasiest way isto choose atarget that uses files as input. But you still need to make the client allocate enough memory to reach death by swap. If we find a crash, theres a high chance there are actually a lot of mutations that can trigger the same crash. Heres the interesting piece: The out-of-bounds read is quite evident: we control wFormatNo (unsigned short). I just happened to stumble upon it while reading WinAFLs codebase, and it proves to be totally fit for our network context! After setting thebreakpoints, I continue executing theprogram andsee how it makes thefirst call toCreateFileA. Open the input file. 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. By fuzzing these 59 harnesses, WINNIE successfully found 61 bugs from 32 binaries. 2021-07-22 Sent vulnerability reports to Microsoft Security Response Center. This isgood because its always preferable tofuzz uncompressed files: thecode coverage ismuch better andthe chance todiscover more interesting features ishigher. WinAFL Fuzzing AFL is a popular fuzzing tool for coverage-guided fuzzing. This is easily done with the WTS API I mentioned earlier, which allows to open, read from and write to a channel. Fuzzing the Office Ecosystem June 8, 2021 Research By: Netanel Ben-Simon and Sagi Tzadik Introduction Microsoft Office is a very commonly used software that can be found on almost any standard computer. 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 DRDYNVC is really banned from being opened through the WTS API! But it has the advantage of stopping coverage measurement at return. 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. Here are some that are provided by Microsoft: In conclusion, both types of Virtual Channels are great targets for fuzzing. Sometimes strange stuff just happens, like WinAFL itself randomly crashing and stopping the fuzzing in the middle of a week-end or something. To fix this issue, patch theprogram orthe library used by it. 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). If something behaves strangely, then I need to find the reason why. Hence why all the functions are colored in red, but it is not very important. It uses Frida to collect coverage against a running process between two points in time, and logs the output in a format readable by Lighthouse. If you havent already, check it out now (or after having finished reading this article)! Ifyou intent tofuzz parsers ofsome well-known file formats, Google can help you alot. 45:42. This function looks very interesting anddeserves adetailed examination. XHTML: Learn more. REcon 2015 - This Time Font hunt you down in 4 bytes (Peter Hlavaty, Jihui Lu) iamelli0t. It is also the base channel that hosts several sub-extensions such as the smart card extension, the printing extension or the ports extension. You signed in with another tab or window. Go to the directory containing the source. The crash happened upon receipt of a Wave2 PDU (0x0D), at CRdpAudioController::OnWaveData+0x27D. Code coverage for our RDPSND fuzzing campaign using Lighthouse. sign in Themaximum code coverage can beachieved by creating asuitable set ofinput files. You are not able to reproduce the crash manually. This information goes through what Microsoft call Virtual Channels. This can be enabled by giving -s option to afl-fuzz.exe. RDPDR is a Static Virtual Channel dedicated to redirecting access from the server to the client file system. If nothing happens, download GitHub Desktop and try again. I patched mstscax.dll to get rid of this measure, by nopping out the dynamic call to VirtualChannelCloseEx and bypassing the error handler. Therefore, for each new path, we have a corresponding basic block trace log. The PDU sub-handling logic is therefore run in a different thread. With her consent, of course! You are able to reproduce the crash manually. Network pentesting at the data link layer, Spying penguin. Set breakpoints atthe beginning andend ofthe function selected for fuzzing. I spent a lot of time on this issue because I had no idea where the opening could fail. Windows even for black box binary fuzzing. Thetarget function must: Precompiled binaries are available inthe WinAFL repository onGitHub, but for some reason, they refuse towork onmy computer. It is a Device I/O Request PDU (0x4952) of sub-type Device Control Request (0x000e). To use it, specify the -A option to afl-fuzz.exe, where is the name of a module loaded only by the target process (if the module is loaded by more than one process WinAFL will terminate). So, my strategy isto go up thecall stack until I find asuitable function. [] 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. Using theVisual Studio command line, go tothe folder with WinAFL source code. It would be painfully slow, especially with the RDP client, which can sometimes take 10 or 20 seconds to connect. Usually its in mstscax.dll, but it could also happen in another module. CLIPRDR state machine diagram from the specification. I debugged the TermService svchost process and stepped until ending up inside rdpcorets.dll. 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). 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 custom mutator should invoke common_fuzz_stuff to run and make WinAFL aware of each new test case. 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). But in order not to waste fuzzing effort in deeper levels of path geometry while fuzzing a multi-threaded application, one had better use thread coverage within DynamoRIO. It allows to create/open and close DVCs, and data transported through DVCs is actually transported over DRDYNVC, which acts as a wrapping layer. */. 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. In this case: lie down, try not to cry, cry a lot. You could say youre satisfied with your fuzzing once youve found a big vulnerability, but thats obviously a rather poor indicator of fuzzing quality. Note that anything that runs The key question is: are we satisfied with our fuzzing? I resume theprogram execution andcontinue it until I see thepath tomy test file inthe list ofarguments. Shared memory is faster and can avoid some problems with files (e.g. This is accomplished by selecting a target function (that the location of your DynamoRIO cmake files (either full path or relative to the Work fast with our official CLI. This wont bring you any additional findings, but will slow down thefuzzing process significantly. The issue then probably comes, as hinted by the debug spew, from RpcCreateVirtualChannel. When do we stop exactly? 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. By fuzzing these 59 harnesses, WINNIE successfully found 61 bugs from 32 binaries. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); WinAFL isa fork ofthe renowned AFL fuzzer developed tofuzz closed-source programs onWindows systems. However, thetopic Fuzzing Network Apps isbeyond thescope ofthis article. The initial idea was to follow up on a conference talk from Blackhat Europe 2019. This option allows to collect coverage only from the thread of interest, which is the one that executed the target function. If a program always behaves the same for the same input data, it will earn a score of 100%. 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. I eventually switched to deterministic and noticed it usually happened around 5 minutes of fuzzing. If WinAFL will not find the new target process within 10 seconds, it will terminate. However, ifyou (like me) prefer parsers ofproprietary file formats, thesearch engine wont help you much. This adversely affects thespeed but reduces thenumber ofside effects. Most targets will just get a 100% score, but when you see lower figures, there are several things to look at. This file should be passed as an argument to the target binary. AFL/WinAFL work by continously sending and mutating inputs to the target program, to make it behave unexpectedly (and hopefully crash). 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. The tool combines Ifits 100%, then theprogram behaves exactly thesame ateach iteration; ifits 0%, then each iteration iscompletely different from theprevious one. Lets examine themost important ofthem inorder. to use Codespaces. I also make sure that this function closes all open files after thereturn. WTSVirtualChannelWrite(virtual_channel, buffer, length, "Exception Address: %016llx / %016llx (unknown module), "Exception Address: %016llx / %016llx (%s). So it seems that it is indeed used, rightfully, for security purposes. DynamoRIO sources or download DynamoRIO Windows binary package from 1 I am looking for the ways to fuzz Microsoft office, let's say Winword.exe. A blind fuzzer, or blackbox fuzzer, is a fuzzer with no knowledge of a program's inner workings. Instead of instrumenting the code at compilation time, WinAFL supports the The first one can find interesting bugs, but which sometimes are very hard to analyze. During my internship at Thalium, I spent time studying and reverse engineering Microsoft RDP, learning about fuzzing, and looking for vulnerabilities. Virtual Channels (or just channels) are an abstraction layer in the Remote Desktop Protocol used to generically transport data. Risk-wise, this is a case of remote system-wide denial of service. For RDPSND, our target methods name is rather straightforward. Cant we just connect to a local RDP server on the same machine? The thing is, I spent an unreasonable amount of time thinking: this problem sucks, I cant go any further because of it, my setup is broken, I dont know why, and I am doomed because I cannot fuzz anymore. user wants to fuzz) and instrumenting it so that it runs in a loop. It contains many dynamic calls that all lead to CTSCoreEventSource::FireASyncNotification. Indeed, when fuzzing, you dont want to kill and start your target again every execution. Now lets do some fuzzing! Finally, it is probably the most complex and interesting channel Ive had to fuzz among the few ones Ive studied! 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. Sometimes theprogram gets so screwed during fuzzing that it crashes atthe preparatory WinAFL stage, andWinAFL reasonably refuses toproceed further. As for the client application, it seems that only connections to localhost and 127.0.0.1 are blocked. Argument register index may vary by target function, so it is given as executing option. This will greatly help us develop a fuzzing harness. The Remote Desktop Protocol provides multiplexed management of multiple virtual channels. My program was quite talkative anddisplayed pop-up messages claiming that theformat ofinput files iswrong. WinAFL exists, but is far more limited such as having no fork server mode. All in all, this bug is still interesting because it highlights how mixed message type fuzzing can help find new bugs. In this method, we directly deliver sample into process memory. Fuzzing binary-only programs with AFL++. 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 Last but not least about execution of the RDP client while fuzzing. Inthe above example, stability was 9.5%. *nix-specific design (e.g. The client will save this list of formats in this->savedAudioFormats. But inreal life, developers often forget toadd such perfect functions totheir programs, andyou have todeal with what you have. Something very valuable would be having a call stack dump on crashes. Parse it (so that you can measure coverage of file parsing). I open theprogram inthe debugger (usually I use x64dbg) andadd anargument tothe command line: thetest file. Therefore, CVEs in the RDP client are more scarce, even though the attack surface is as large as the servers. More specifically, the I/O Request handler, DrDevice::ProcessIORequest, dispatches the PDU to a Smart Card sub-protocol handler (W32SCard::MsgIrpDeviceControl). 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. To try and mitigate this a bit, I modified WinAFL to incorporate a feature that proved to be rather vital during my research: logging more information about crashes. instrumentation, forkserver etc.). So, I remove breakpoints from this function andcontinue monitoring calls toCreateFileA. For instance, sometimes small out-of-bounds reads will not trigger a crash depending on whats done with the read value, but can still hide a bigger looming threat. Funnily enough, the source code of WinAFL itself hints that it is the preferred mode for network fuzzing. Concretely, we only lack two elements to start fuzzing: A good lead is to start by reading Microsofts specification (e.g. Too bad, custom_net_fuzzer works pretty slowly because it sends network requests toits target, andadditional time isspent ontheir processing. By activating PageHeap on mstsc.exe with the /full option, we ask Windows to place an inaccessible page at the end of each heap allocation. To improve the process startup time, WinAFL relies heavily on persistent The environment variable AFL_CUSTOM_DLL_ARGS= should be used for this purpose. Before going any further, I would like to tackle an important concern. Side effects of fuzzing on a system can reveal bugs too. Anda dictionary will help you inthat. The Art of Fuzzing - Demo 7- How to detect when a PDF finished loading. to send test cases over network). 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. Attempt at RDP loopback connection. Where did I get it from? documents. Automating vulnerability management, Ruffling thepenguin! 05:31. that you can read a new input file for each iteration as the input file is When theprogram execution reaches theend ofthe function, edit thearguments, align thestack, change theRIP/EIP tothe beginning ofthe function, etc. it takes thefile path as acommand line argument; and. I modified my VC Server to integrate a slow mode. The following diagram attempts to summarize the fuzzing process in a very much simplified manner, and using WinAFLs no-loop mode. In laymans terms: imagine WinAFL finds a crash and saves the corresponding mutation. Homemade keylogger. 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. In particular, were doing stateful fuzzing: the RDP client could be modelled by a complex state machine. please refer to the original documentation at: Unfortunately, the original AFL does not work on Windows due to very Blind fuzzing vs Guided fuzzing. The CClipRdrPduDispatcher::DispatchPdu function is where PDUs arrive and are dispatched based on msgType. Return normally. Such anapproach allows you toavoid wasting extra time onthe program launch andinitialization andsignificantly increases thefuzzing speed. Luke, I am your fuzzer. 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. 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. This method brings two advantages. Since we are covering a bigger space of PDUs, we are covering a bigger space of states. When the target process terminates (regardless of the reason), WinAFL will not restart it, but simply try to reattach. 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. If you arent familiar with this software testing technique, check our previous articles: Similar toAFL, WinAFL collects code coverage information. Salk Bakanl, Tekirda'n Sleymanpaa plajlar, arky Plajlar, Marmara Erelisi plajlar ve Saray plajlarnda deniz suyu analiz sonularn yaynlad. "returning" via ExitProcess() and such won't work). Please run the Just opened theprogram, set themaximum number ofoptions for thedocument andsaved it todisk. Then, if the iteration produced a new path, afl-fuzz will save the log into a file. This issue was fixed in January . drAFL: AFL + DynamoRIO = fuzzing binaries with no source code on Linux (spare time) https://github.com/mxmssh/drAFL Contributions: drltrace, winAFL, DynamoRIO, DrMemory, Ponce PhD on vulnerability research in machine code Speaker: 3 Outline I. Sadly, we cant do much more. It has been successfully used to find a large number of winafl.dll DynamoRIO client, -DINTELPT=1 - Enable Intel PT mode. But ifyou look closely, this library contains only jmp tothe respective functions ofkernelbase.dll. Tekirda denize girilecek yerler. Therefore, the RDP client will receive a lot of different message types, in a rather random order. However, it is not ideal because code coverage measurement will not stop at return. Additionally, this mode is considered as experimental since we have experienced some problems with stability and performance. after the target function returns is never reached. Not vital because you can always target the parent handler, except in certain cases. This means we probably wont be able to find a lot of stateful bugs, if a PDU in a sequence triggers the channel closing. The list ofarguments taken by this function resembles what you have already seen before. Parse this file andfinish its work as neatly as possible (i.e. They also started reviewing this case for a potential bounty award. The Remote Desktop Protocol is relevant now more than ever, having almost everyone started working remotely in 2020, and having Microsoft's Azure and Hyper-V platforms using it as the default remote connection protocol. Otherwise, WinAFL would instrument numerous library functions. Heres what our fuzzing architecture resembles now.
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