winafl network fuzzingwho came first, noah or abraham
Of course, on systems with a moderate amount of RAM like an employees laptop, this may be dangerous. Return normally (So that WinAFL can "catch" this return and redirect This article begins my three-part series on fuzzing Microsofts RDP client. 2021-07-30 Microsoft assessed the CLIPRDR malloc DoS bug as low-severity and closed the case. Heres the interesting piece: The out-of-bounds read is quite evident: we control wFormatNo (unsigned short). If guessing wont work, another possibility is to capture code coverage at the moment we send a PDU over the target virtual channel. This is a critical fact we must take into account for when we are fuzzing later! the target binary. 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). As a drawback, DynamoRIO will add some overhead, but execution speed will still be decent. Out of the 59 harnesses, WinAFL only supported testing 29. This is understandable: for instance, a denial of service constitutes a much higher risk for a server than for a 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. Please 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. However, DynamoRIO does not have such a feature, and we cant do it through procdump or MiniDumpWriteDump either because the client is already a debuggee of DynamoRIO (drrun). We did gather earlier a little list of channels that looked like fruitful targets. Our target will be a test DLL vulnerable with a stack-overflow vulnerability. I tried logging debug strings from winsta!WinStationVirtualOpenEx with DebugView++. WTSVirtualChannelOpenEx(WTS_CURRENT_SESSION. Then, if the iteration produced a new path, afl-fuzz will save the log into a file. until something breaks. I found one bug that crashed the client: an Out-of-Bounds Read that is unfortunately unexploitable. Well, Im not sure myself it is not documented (at least at the time I am writing this article). Maybe this will lead me to new findings, and even a reproducible bug.. For this purpose, it uses three techniques: Lets focus onthe classical first variant since its theeasiest andmost straightforward one. The tool combines 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. Modify the -DDynamoRIO_DIR flag to point to the 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. It is opened by default. The DLL should export the following two functions: We have implemented two sample DLLs for network-based applications fuzzing that you can customize for your own purposes. In-memory fuzzing implementation not only restores register context, but also writes fuzzing input at the process memory pointing PDU buffer. Then I restart theprogram andsee that thetwo arguments are thepaths tomy test file anda temporary file. I also got two CVEs in FreeRDP. In practice, this . Then I select thekernelbase.dll library onthe Symbols tab andset breakpoints atexports ofthe CreateFileA andCreateFileW functions. In this first installment, I set up a methodology for fuzzing Virtual Channels using WinAFL and share some of my findings. Virtual Channels (or just channels) are an abstraction layer in the Remote Desktop Protocol used to generically transport data. By giving below options, fuzzing input can be delivered into target process memory. Besides, each channel is architectured in a different fashion; there is rarely a common code structure or even naming convention between two channels implementation. 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. 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. When the target process terminates (regardless of the reason), WinAFL will not restart it, but simply try to reattach. 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. RDPSND PDU handler and dispatch logic in mstscax.dll. 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. WinAFL includes the windows port of afl-cmin in winafl-cmin.py. This vulnerability resides in RDPDRs Smart Card sub-protocol. AFL++, libfuzzer and others are great if you have the source code, and it allows for very fast and coverage guided fuzzing. The virtual machines RAM would very quickly fill up, until at some point having to start filling up swap. However, manually sending the malicious PDU again does not do anything we are unable to reproduce the bug. fast target execution with clever heuristics to find new execution paths in No luck. Your goal isto increase thenumber ofpaths found per second. Heres what a WinAFL command line could look like: However, remember were fuzzing in a network context. It is opened by default. When theprogram execution reaches theend ofthe function, edit thearguments, align thestack, change theRIP/EIP tothe beginning ofthe function, etc. The no-loop mode lets the program loop by its own, just like in-app persistence. Using Android to keep tabs on your girlfriend. Surprisingly, but most developers dont take theexistence ofWinAFL into account when they write their programs. This video contain:1. It would be painfully slow, especially with the RDP client, which can sometimes take 10 or 20 seconds to connect. For RDPSND, we can get something like this. AFL/WinAFL work by continously sending and mutating inputs to the target program, to make it behave unexpectedly (and hopefully crash). There is no guarantee whatsoever you will be able to reproduce the crash with this mutation only. By fuzzing these 59 harnesses, WINNIE successfully found 61 bugs from 32 binaries. -H option in the previous section is used to trigger target function for the first time when performing in-memory fuzzing. Instead, it will randomly mutate inputs without knowing which mutations actually yield favorable results (new paths in the correct thread). To improve the process startup time, WinAFL relies heavily on persistent Now lets do some fuzzing! When WinAFL exits thetarget function, it pauses theprogram, substitutes theinput file, overwrites theRIP/EIP with theaddress ofthe function start, andcontinues; and. WinAFL supports loading a custom mutator from a third-party DLL. 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. Since the seeds include the header, the fuzzer will also mutate it, including the msgType field. This strategy is what youd get by fuzzing the channel naively . In summary, we make the following contributions: We identied the major challenges of fuzzing closed-source Windows applications; But thethings dont always run so smoothly. It is our harness which runs parallel to the RDP server. This requires patching winsta.dll to activate g_bDebugSpew: With some help, we eventually managed to identify the endpoint of the RPC call, in termsrv.dll. From this bug, we learned a golden rule of fuzzing: that it is not only about crashes. III. 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. Also, you can use In App Persistence mode described above if your application runs the target function in a loop by its own. 2021-07-22 Sent vulnerability reports to Microsoft Security Response Center. Thus, my exploit sends the malicious payloads with smaller 128 MB increments to adapt to the amount of RAM on the victims system. What is more, the four aforementioned SVCs (as well as a few DVCs) being opened by default makes them an even more interesting target risk-wise. Additionally, this mode is considered as experimental since we have experienced some problems with stability and performance. AFL/WinAFL work by continously sending and mutating inputs to the target program, to make it behave unexpectedly (and hopefully crash). 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. 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. Something very valuable would be having a call stack dump on crashes. Type the following commands. 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. 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. Lets say we fuzzed a channel for a whole week-end. The objective was to go even further, by coming up with a general methodology for attacking Virtual Channels in RDP, and fuzz more of Microsofts RDP client with WinAFL. see googleprojectzero/winafl#145. We have to be extra careful with patches though, because they can modify the clients behavior. Strings or magic numbers from the specification can also help. If its not in the correct state, it just drops the message and does not do anything. Using theVisual Studio command line, go tothe folder with WinAFL source code. Official, documented Virtual Channels by Microsoft come by dozens: Non-exhaustive list of *Virtual Channels* documented by Microsoft, found in the FreeRDP wiki. instrumentation, forkserver etc.). 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. Luke, I am your fuzzer. Fuzzing is gambling. In summary, we make the following contributions: We identified the major challenges of fuzzing closed-source Windows applications; Youll get tons of the same crashes in a row, which can heavily slow down fuzzing for certain periods of time. It allows to copy several types of data (text, image, files) from server to client and from client to server. It is assumed that the target process will be restarted by an external script (or by the system itself). How tofuzz theLinux kernel, synthesize valid JPEG files without any additional information, Herpaderping and Ghosting. below command to see the options and usage examples: WinAFL supports third party DLLs that can be used to define custom test-cases processing (e.g. the specific instrumentation mode you are interested in. Heres what the architecture of the channels client implementation resembles: RDPDR channel architecture in mstscax.dll. When restoring register context, we patched WinAFL pre-fuzz handler to write fuzzing input at the memory pointed by 3rd argument register, and set 2nd argument register to length of fuzzing input. WinAFL will save all the basic blocks encountered at each fuzzing iteration in a temporary buffer (in the thread of interest). Its easy to lack motivation to have the right attitude at the right time towards a certain type of result, and actually getting stuff done (investigating, confirming/rejecting hypotheses, etc.). In this case, just reverse to understand the root cause, analyze risk, and maybe grow the crash into a bigger vulnerability. To compile the32-bit version, execute thefollowing commands: In my case, these commands look as follows: After thecompilation, thefolder
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