This way, I could have time to monitor which PDU was guilty and what exactly happened when it was sent. WinAFL is a Windows fork of the popular mutational fuzzing tool AFL. The freezing always happened at a random time since I was fuzzing in non-deterministic mode. 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. 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. Besides, each channel is architectured in a different fashion; there is rarely a common code structure or even naming convention between two channels implementation. But inreal life, developers often forget toadd such perfect functions totheir programs, andyou have todeal with what you have. So, I remove breakpoints from this function andcontinue monitoring calls toCreateFileA. 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. Download andinstall Visual Studio 2019 Community Edition (when installing, select Develop classic C++ applications. Reverse engineering will focus on the latter, as it holds most of the RDP logic. Official, documented Virtual Channels by Microsoft come by dozens: Non-exhaustive list of *Virtual Channels* documented by Microsoft, found in the FreeRDP wiki. [] 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. 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. 56 0. I resume theprogram execution andcontinue it until I see thepath tomy test file inthe list ofarguments. []. These also contain Obviously, its less impressive on a client than on a server, but its still nastier than your usual mere crash. 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. However, if there is only the binary program and no source code available, then standard afl-fuzz -n (non-instrumented mode) is not effective. 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. This function looks very interesting anddeserves adetailed examination. To better reproduce the crash, we implemented machine context and call stack dump when crush occurs. On the other hand, as we said, we cant perform fixed message type fuzzing either at all because of state verification. 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 In the Blackhat talk, the research was driven by the fact that North Korean hackers would alledgely carry out attacks through RDP servers acting as proxies. More specifically, the client calls VCManager::ChannelClose which calls VirtualChannelCloseEx. I was still able to identify a little bug with this fuzzing strategy. If, like me, you opt for extra challenge, you can try fuzzing network programs. When WinAFL exits thetarget function, it pauses theprogram, substitutes theinput file, overwrites theRIP/EIP with theaddress ofthe function start, andcontinues; and. It is also the base channel that hosts several sub-extensions such as the smart card extension, the printing extension or the ports extension. The Art of Fuzzing - Demo 12- Using PageHeap and ApplicationVerifier to find bug. The function CUMRDPConnection::CreateVirtualChannel answers our inquiry. I did mention the function we target should be fuzzed in a loop without restarting the process. after the target function returns is never reached. This is understandable: for instance, a denial of service constitutes a much higher risk for a server than for a client. How tofuzz theLinux kernel, synthesize valid JPEG files without any additional information, Herpaderping and Ghosting. Therefore, for each new path, we have a corresponding basic block trace log. Two new ways to hide processes from antiviruses, SIGMAlarity jump. execution. For instance, you can open a channel this way: All that remains is to modify WinAFL so that instead of writing mutations to a file, it sends them over TCP to our VC Server. "returning" via ExitProcess() and such won't work). By replaying the whole history, you may hope the client behaves in a deterministic enough way that it reproduces the crash. 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 . It is worth noting a crash in an unknown module could mean the execution flow was redirected, which accounts for the most interesting bugs :). However, WinAFL is not going to work with our target out of the box. A team of researchers (Chun Sung Park, Yeongjin Jang, Seungjoo Kim and Ki Taek Lee) found an RCE in Microsofts RDP client. The greater isthe code coverage, thehigher isthe chance tofind abug. We need to find a way to skip this condition to trigger the bug. https://github.com/googleprojectzero/Jackalope/blob/6d92931b2cf614699e2a023254d5ee7e20f6e34b/test.cpp#L111. Now that weve chosen our target, where do we begin? Indeed, when naively measuring code coverage (the trace) in a multi-threaded application, other threads may interfere with the one of interest. This vulnerability resides in RDPDRs Printer sub-protocol. We also notice a few more channels that are blacklisted the same way. For RDPSND, we can get something like this. Learn more. Lighthouse is an IDA plugin to visualize code coverage. As soon as something happens out-of-bounds, the client will then crash. Especially, the ones that are opened by default and for which there is plenty of documentation. If a program always behaves the same for the same input data, it will earn a score of 100%. Hence why all the functions are colored in red, but it is not very important. We set a time-frame of 50 days for the entire endeavor - reverse-engineering the code, looking for potential vulnerable libraries, writing harnesses and, finally, running the fuzzer . In laymans terms: imagine WinAFL finds a crash and saves the corresponding mutation. Lets say we fuzzed a channel for a whole week-end. This option allows to collect coverage only from the thread of interest, which is the one that executed the target function. WinAFL will save all the basic blocks encountered at each fuzzing iteration in a temporary buffer (in the thread of interest). Parse it (so that you can measure coverage of file parsing). Dont forget todisable thedebug mode! Our target will be a test DLL vulnerable with a stack-overflow vulnerability. If something behaves strangely, then I need to find the reason why. WinAFL will change @@ tothe full path tothe input file. We thought they achieved encouraging results that deserved to be prolonged and improved. It is a Device I/O Request PDU (0x4952) of sub-type Device Control Request (0x000e). Anda dictionary will help you inthat. But ifyou look closely, this library contains only jmp tothe respective functions ofkernelbase.dll. 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). Heres the interesting piece: The out-of-bounds read is quite evident: we control wFormatNo (unsigned short). Usually its in mstscax.dll, but it could also happen in another module. However, manually sending the malicious PDU again does not do anything we are unable to reproduce the bug. 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. the specific instrumentation mode you are interested in. As you can see, its used infour functions. Based onthe CFile::Open prototypes from theMSDN documentation, thea1 anda2 variables are file paths. More specifically, the I/O Request handler, DrDevice::ProcessIORequest, dispatches the PDU to a Smart Card sub-protocol handler (W32SCard::MsgIrpDeviceControl). that you can read a new input file for each iteration as the input file is By giving following options(-F, -G, -H), fuzzing input can be delivered by socket. Perhaps multithreading affects it, too. Then I select thekernelbase.dll library onthe Symbols tab andset breakpoints atexports ofthe CreateFileA andCreateFileW functions. Below is an example mutator that increments every byte by one: Special thanks to Axel "0vercl0k" Souchet of MSRC Vulnerabilities and please refer to the original documentation at: Unfortunately, the original AFL does not work on Windows due to very I suppose that this isbecause theprogram was built statically, andsome library functions adversely affect thestability. Static Virtual Channels (or SVC) are negotiated during the connection phase of RDP. 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 Unfortunately, the way channels globally work in RDP is somewhat circuitous and I never got around to fully figuring it out. Heres what a WinAFL command line could look like: However, remember were fuzzing in a network context. We technically have everything we need to start WinAFL. I tried patching rdpcorets.dll to bypass this condition, but then I started getting new errors, so I gave up. 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). When I tried to start fuzzing RDPDR, there was a little hardship. Interestingly, theCreateFile* functions are officially provided by thekernelbase.dll library. An attacker could use the same technology to deliver malicious payload; this is a common way to discover . The initial idea was to follow up on a conference talk from Blackhat Europe 2019. 2021-08-03 Microsoft acknowledged the RDPDR heap leak bug and started developing a fix. 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