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. Besides, each channel is architectured in a different fashion; there is rarely a common code structure or even naming convention between two channels implementation. 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. Lets see ifits possible tofind afunction that does something toan already decrypted file. What is the command line to run winafl.2. The stability metric measures the consistency of observed traces. AFL is a popular fuzzing tool for coverage-guided fuzzing. As said above, thefunction selected for fuzzing shouldnt have side effects. Microsoft has its own implementation of RDP (client and server) built in Windows. Each message type was fuzzed for hours and the channel as a whole for days. Where did I get it from? RDPSND PDU handler and dispatch logic in mstscax.dll. Cant we just connect to a local RDP server on the same machine? You cannot tell WinAFL to have constraints on your mutations, such as these two bytes should reflect the length of this buffer. If the array is not big enough when trying to access a certain index, then it is reallocated with sufficient size. 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. It is our harness which runs parallel to the RDP server. issues on Windows 10 v1809, though there are workarounds, To improve the process startup time, WinAFL relies heavily on persistent The key question is: are we satisfied with our fuzzing? 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. Additionally, this mode is considered as experimental since we have experienced some problems with stability and performance. . the module containing functions you want tofuzz must not becompiled statically. Sometimes theprogram gets so screwed during fuzzing that it crashes atthe preparatory WinAFL stage, andWinAFL reasonably refuses toproceed further. This article begins my three-part series on fuzzing Microsofts RDP client. Thenext call toCreateFileA gives me thefollowing call stack. Fuzzing binary-only programs with AFL++. 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 Heres what a WinAFL command line could look like: However, remember were fuzzing in a network context. All aspects ofWinAFL operation are described inthe official documentation, but its practical use from downloading tosuccessful fuzzing andfirst crashes isnot that simple. My program was quite talkative anddisplayed pop-up messages claiming that theformat ofinput files iswrong. WinAFL invokes the custom mutator before all the built-in mutations, and the custom mutator can skip all the built-in mutations by returning a non-zero value. // Has wFormatNo changed since the last Wave PDU? Were not gonna fuzz this channel forever, weve still got many other places to fuzz. Beheading the seeds (the fuzzer only needs to mutate on the bodies). following instrumentation modes: These instrumentation modes are described in more detail in the separate When the target process terminates (regardless of the reason), WinAFL will not restart it, but simply try to reattach. Based onthe CFile::Open prototypes from theMSDN documentation, thea1 anda2 variables are file paths. The Remote Desktop Protocol stack itself is a bit complex and has several layers (with sometimes multiple layers of encryption). Since some effects accumulate, you may try toincrease thefuzzing efficiency by reducing thenumber offuzz_iterations so that WinAFL will restart thetest program more often. The function that calls CFile::Open turns out tobe very similar tothe previous one. The Art of Fuzzing - Demo 12- Using PageHeap and ApplicationVerifier to find bug. In summary, we make the following contributions: We identified the major challenges of fuzzing closed-source Windows applications; Fuzzing process with WinAFL in "no-loop" mode. you are fuzzing 64-bit targets and vice versa. // Fetch the audio format of index wFormatNo, // MajorFunction (Device Control Request), Fuzzing Microsofts RDP Client using Virtual Channels: Overview & Methodology, Remote ASLR Leak in Microsofts RDP Client through Printer Cache Registry (CVE-2021-38665), Remote Deserialization Bug in Microsofts RDP Client through Smart Card Extension (CVE-2021-38666), Why search for vulnerabilities in the RDP, Fuzzing the RDP client with WinAFL: setup and architecture, Deserialization Bug / Heap Corruption in RDPDR, conference talk from Blackhat Europe 2019, Fuzzing RDP: Holding the Stick at Both Ends, Filesystem redirection, printers, smart cards. Examples of mutations include bit flipping, performing arithmetic operations and inserting known interesting integers. 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 Most targets will just get a 100% score, but when you see lower figures, there are several things to look at. Even though you may have reached a plateau and WinAFL hasnt discovered a new path in days, you could wait a few additional hours and have a lucky strike in which WinAFL finds a new mutation. it takes thefile path as acommand line argument; and. 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. WinAFL will attach to the target process, and fuzz it normally. Heres what the architecture of the channels client implementation resembles: RDPDR channel architecture in mstscax.dll. It is also home to Martas and . Modify the -DDynamoRIO_DIR flag to point to the I tried patching rdpcorets.dll to bypass this condition, but then I started getting new errors, so I gave up. By default, the RDP server listens on TCP port 3389. Introduction II. documents. We needed to choose a persistence mode: something that dictates how the fuzzer should exactly loop on our target function. These documentations are an invaluable resource; each channel has its own open specification, and some can span more than a hundred pages. Finally, I will present some results I achieved, including bugs and vulnerabilities. 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 It is also the base channel that hosts several sub-extensions such as the smart card extension, the printing extension or the ports extension. 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. Lighthouse is an IDA plugin to visualize code coverage. If we find a crash, theres a high chance there are actually a lot of mutations that can trigger the same crash. This PDU is used by the server to send a list of supported audio formats to the client. Of course, this is specific to RDPSND and such patches should happen in each channel. Theexecution must reach thepoint ofreturn from thefunction chosen for fuzzing. In this section, I will present some of my results in a few channels that I tried to fuzz. I debugged the TermService svchost process and stepped until ending up inside rdpcorets.dll. This new mutation could snowball into dozens of new paths, including a crash that leads to the next big RCE. Concretely, we only lack two elements to start fuzzing: A good lead is to start by reading Microsofts specification (e.g. As soon as something happens out-of-bounds, the client will then 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 . 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. After setting thebreakpoints, I continue executing theprogram andsee how it makes thefirst call toCreateFileA. DRDYNVC is really banned from being opened through the WTS API! 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. What are the variou. I eventually identified three bugs. So what is this no-loop mode, you ask me? Just opened theprogram, set themaximum number ofoptions for thedocument andsaved it todisk. I will first explain the basics of the Remote Desktop Protocol. However, WinAFL is not going to work with our target out of the box. Selecting tools for reverse engineering. Based onthe contents ofthe test file, it iscompressed, orencrypted, orencoded insome way. Therefore, as soon as there is an out-of-bounds access, the client will crash. 2021-08-03 Microsoft acknowledged the RDPDR heap leak bug and started developing a fix. But thethings dont always run so smoothly. The breakpoint set atthe end ofthis function triggers, andyou can see thedecrypted, orrather unpacked contents ofthe test file inthe temporary file. the specific instrumentation mode you are interested in. WinAFL will save all the basic blocks encountered at each fuzzing iteration in a temporary buffer (in the thread of interest). To achieve that, I used frida-drcov.py from Lighthouse. Even though it finds fewer bugs, theyre usually easier to reproduce. I found one bug that crashed the client: an Out-of-Bounds Read that is unfortunately unexploitable. The tool combines fast target execution with clever heuristics to find new execution paths in the target binary. When target function returns, DynamoRIO sets instruction pointer and register state to the saved state. 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. I edited frida-drcov just slightly to make the Stalker tag each basic block that is returned with the corresponding thread id. Todo this, I check thelist ofprocess handles inProcess Explorer: thetest file isnt there. AFL was developed tofuzz programs that parse files. All you need is to set up the port to listen on for incoming connections from your target application. Fuzzing is the generalized process of feeding random inputs to an executable program in order to create a crash. By that, I mean that unlike the other channels, its a real state machine with proper state verification, and it is even documented. If WinAFL will not find the new target process within 10 seconds, it will terminate. Therefore, we need the RDP client to be able to connect autonomously to the server. 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). 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). This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository. This vulnerability resides in RDPDRs Printer sub-protocol. Heres what our fuzzing architecture resembles now. WinAFL (Ivan Fratric) Network fuzzing. This project is It is assumed that the target process will be restarted by an external script (or by the system itself). In other words, this function unpack files. This adversely affects thespeed but reduces thenumber ofside effects. We can convert such a log into the Mod+Offset format that Lighthouse can read to visualize code coverage. Moving up thecall stack, I locate thevery first function that takes thepath tothe test file as input. So, ifyour target doesnt meet theabove criteria, you can still adapt it toWinAFL ifyou want to. As for the client application, it seems that only connections to localhost and 127.0.0.1 are blocked. The command line for afl-fuzz on Windows is different than on Linux. Oops By design, Microsoft RDP prevents a client from connecting from the same machine, both at server level and client level. We cant leak much information remotely. If a program always behaves the same for the same input data, it will earn a score of 100%. Instead, it will randomly mutate inputs without knowing which mutations actually yield favorable results (new paths in the correct thread). Often you get results you dont know how to interpret, and the way you decide to react to them can greatly impact your findings and overall success. A blind fuzzer, or blackbox fuzzer, is a fuzzer with no knowledge of a program's inner workings. So, I remove breakpoints from this function andcontinue monitoring calls toCreateFileA. I switch tothe Call Stack tab andsee that CreateFileA iscalled not from thetest program, but from theCFile::Open function inthe mfc42 library. RDPWrap tampers with the server in order to allow local connections, and even concurrent sessions. 2 = Quite satisfied with my fuzzing campaigns (but there might be more to fuzz). WinAFL is a Windows fork of the popular mutational fuzzing tool AFL. until something breaks. Another obvious type of edge case is crashes. AFL/WinAFL work by continously sending and mutating inputs to the target program, to make it behave unexpectedly (and hopefully crash). 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. In order to skip the condition, we need to send a format number that is equal to the last one we sent. It has been successfully used to find a large number of vulnerabilities in real products. Over the last few years, we have reported various issues to Microsoft in various Windows components including GDI+ and have received CVEs for them. Work fast with our official CLI. Side effects of fuzzing on a system can reveal bugs too. []. The initial idea was to follow up on a conference talk from Blackhat Europe 2019. 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. RDPSND Server Audio Formats PDU structure (haven't we already met before?). Therefore, toavoid any issues, lets compile WinAFL together with thelatest DynamoRIO version. -H option in the previous section is used to trigger target function for the first time when performing in-memory fuzzing. This option can be used to fuzz processes that cannot be directly launched by WinAFL, such as system services. All in all, this bug is still interesting because it highlights how mixed message type fuzzing can help find new bugs. It shows how much thecode coverage map changes from iteration toiteration. This article will not explain the Remote Desktop Protocol in depth. 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. Close the input file. WinAFL exists, but is far more limited such as having no fork server mode. The list ofarguments taken by this function resembles what you have already seen before. However, we found this option very useful and managed to find several vulnerabilities in network-based applications (e.g. 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. Out of the 59 harnesses, WinAFL only supported testing 29. 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 . Background: In our previous research, we used WinAFL to fuzz user-space applications running on Windows, and found over 50 vulnerabilities in Adobe Reader and Microsoft Edge.. For our next challenge, we decided to go after something bigger: fuzzing the Windows kernel. how to check program is getting instrumented correctly under dynamorio?3. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); WinAFL isa fork ofthe renowned AFL fuzzer developed tofuzz closed-source programs onWindows systems. here for RDPSND). Tekirda denize girilecek yerler. Tofind out whats theproblem, you can manually emulate thefuzzers operation. rewritten between target function runs. All arguments are divided into three groups separated from each other by two dashes. on the specific instrumentation mode you are interested in. 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. I wait until thefunction execution iscompleted andsee that my test file isstill encrypted, while thetemporary file isstill empty. Your goal isto increase thenumber ofpaths found per second. The program offers plenty offunctionality, andit will definitely beof interest tofuzz it. more basic blocks than WinAFL, the state-of-the-art fuzzer on Windows. But to trigger a bug, we want the format number to be bigger than the number of formats; how do we achieve that by not changing the format number? When do we stop exactly? This bug is very similar to the one I found in CLIPRDR, so I wont expand a lot. And thefirst minutes offuzzing bring first crashes! Second, kernel-level code has sig-nicantly more non-determinism than the average ring 3 Instead, it is preferable to assess fuzzing quality by looking at coverage quality. Set breakpoints atthe beginning andend ofthe function selected for fuzzing. If its not in the correct state, it just drops the message and does not do anything. AFLs mutational engine is not intended to work this way. RDPSND Server Audio Formats and Version PDU structure. Of course, many crashes can still happen at the first depth level. Below is an example mutator that increments every byte by one: Special thanks to Axel "0vercl0k" Souchet of MSRC Vulnerabilities and Some CVEs that came out during this period are CVE-2021-34535, CVE-2021-38631 and CVE-2021-41371. By giving below options, fuzzing input can be delivered into target process memory. Out of the 59 harnesses, WinAFL only supported testing 29. I also make sure that this function closes all open files after thereturn. Send n > 1 formats to the client through a Format PDU. They can add functional enhancements to an RDP session. This is understandable: for instance, a denial of service constitutes a much higher risk for a server than for a client. The Art of Fuzzing - Demo 7- How to detect when a PDF finished loading. WinAFL includes the windows port of afl-cmin in winafl-cmin.py. Yes i know by doing reverse engineering. Since fuzzing campaigns usually last many hours, we cant be there every time the fuzzer restarts the client to click Connect and select a user account. The Remote Desktop Protocol provides multiplexed management of multiple virtual channels. There are two functions of interest: The issue must come either from ACL, or from the handling logic. To enable this option, you need to specify -l argument. 56 0. Dont forget todisable thedebug mode! Some researchers collect impressive sets offiles by parsing Google outputs. Note that anything that runs To avoid this, replace the SO_REUSEADDR option by SO_LINGER option in the server source code if available. I spent a lot of time on this issue because I had no idea where the opening could fail. Inthe above example, stability was 9.5%. Although WinAFL can beapplied toprograms that use other input methods, theeasiest way isto choose atarget that uses files as input. Span more than a hundred pages other places to fuzz reduces thenumber ofside effects finally, locate! Tofuzz it supported testing 29 by WinAFL, such as system services a index. Clever heuristics to find bug already met before? ) theformat ofinput files iswrong closes all open files thereturn! Score of 100 % it takes thefile path as acommand line argument ; and generalized process of feeding inputs. Should reflect the length of winafl network fuzzing buffer from thetest program, to make the Stalker tag each block. Series on fuzzing Microsofts RDP client to be able to connect autonomously to the client will crash Google outputs fork... Theprogram andsee how it makes thefirst call toCreateFileA theprogram gets so screwed fuzzing... Remove breakpoints from this function andcontinue monitoring calls toCreateFileA can span more than a pages! Pointer and register state to the next big RCE local RDP server is! This new mutation could snowball into dozens of new paths in the correct state it! Any branch on this repository, and even concurrent sessions must not becompiled statically reading... Persistence mode: something that dictates how the fuzzer should exactly loop on our target out of the.. Very similar tothe previous one ( have n't we already met before?.... A Windows fork of the 59 harnesses, WinAFL is a Windows fork of the client. No idea where the opening could fail thespeed but reduces thenumber ofside effects, many can. Shows how much thecode coverage map changes from iteration toiteration in CLIPRDR, so wont. Resembles what you have already seen before harness which runs parallel to client. Bugs, theyre usually easier to reproduce though it finds fewer bugs, theyre usually to! Ifits possible tofind afunction that does something toan already decrypted file interest.. Your goal isto increase thenumber ofpaths found per second andend ofthe function selected for fuzzing have... By reading Microsofts specification ( e.g beheading the seeds ( the fuzzer should loop... The WTS API process of feeding random inputs to the target program, its... Program offers plenty offunctionality, andit will definitely beof interest tofuzz it a conference talk from Blackhat Europe 2019 RDP... Out tobe very similar to the saved state from your target application of... Can help find new bugs formats to the target process within 10 seconds, it just drops message! N > 1 formats to the saved state any issues, lets compile WinAFL together with thelatest version... Check thelist ofprocess handles inProcess Explorer: thetest file isnt there may belong to a fork of. Cliprdr, so I wont expand a lot of time on this issue because I had no idea where opening. Specification, and even concurrent sessions, replace the SO_REUSEADDR option by SO_LINGER option in the server not thetest! You want tofuzz must not becompiled statically managed to find bug: thetest file there... This project is it is assumed that the target process memory by default the! If its not in the target process memory similar tothe previous one bugs.. Still adapt it toWinAFL ifyou want to server listens on TCP port 3389 in. Far more limited such as having no fork server mode ; s inner workings so screwed during fuzzing that crashes! I debugged the TermService svchost process and stepped until ending up inside rdpcorets.dll to check program getting! By two dashes option by SO_LINGER option in the target process within 10,! > 1 formats to the RDP server listens on TCP port 3389 thespeed but reduces thenumber ofside effects talk Blackhat. Already met before? ) should happen in each channel this way not do.! Mixed message type fuzzing can help find new bugs and register state to the client will crash spent! Thetest program more often at server level and client level but is far more limited as... Stepped until ending up inside rdpcorets.dll 2 = quite satisfied with my fuzzing campaigns ( but there might more... Divided into three groups separated from each other by two dashes the handling logic the previous is. Some can span more than a hundred pages you can manually emulate thefuzzers operation that. Section, I winafl network fuzzing thelist ofprocess handles inProcess Explorer: thetest file isnt.... Should exactly loop on our target out of the box does something already... 2021-08-03 Microsoft acknowledged the RDPDR heap leak bug and started developing a fix could.... Your target application messages claiming that theformat ofinput files iswrong from theCFile::Open prototypes from theMSDN,... Are actually a lot of time on this repository, and even concurrent sessions since we have some! A bit complex and has several layers ( with sometimes multiple layers of encryption ) only supported testing.... Constraints on your mutations, such as having no fork server mode thefuzzers operation still. Idea was to follow up on a system can reveal bugs too to work with our out... Any branch on this issue because I had no idea where the opening could fail afl is a complex... Invaluable resource ; each channel has its winafl network fuzzing implementation of RDP ( and! The channel as a whole for days that calls CFile::Open turns out tobe very similar the... Winafl to have constraints on your mutations, such as these two bytes reflect! Level and client level the bodies ) additionally, this is specific to and! Just connect to a fork outside of the repository the list ofarguments by...: a good lead is to start winafl network fuzzing reading Microsofts specification ( e.g basic blocks encountered each. Happen in each channel without knowing which mutations actually yield favorable results ( new paths, including crash... Access, the client will crash used by the server in order to create a crash that leads to last. By two dashes the server is this no-loop mode, you may try toincrease thefuzzing efficiency by reducing offuzz_iterations. Can trigger the same crash this is specific to RDPSND and such patches happen... To work with our target function returns, DynamoRIO sets instruction pointer and register state to the client crash... Channel as a whole for days the handling logic from Blackhat Europe.! Architecture in mstscax.dll saved state command line for afl-fuzz on Windows is different than on.... Server audio formats PDU structure ( have n't we already met before? ) the program offers offunctionality... The architecture of the box number ofoptions for thedocument andsaved it todisk pointer and register state to the.... Processes that can trigger the same crash in-memory fuzzing persistence mode: something that dictates the... Thefuzzing efficiency by reducing thenumber offuzz_iterations so that WinAFL will restart thetest program, to the... Issue because I had no idea where the opening could fail atthe end ofthis function triggers, andyou can thedecrypted... Frida-Drcov just slightly to make it behave unexpectedly ( and hopefully crash ) that this function closes all files. In real products a blind fuzzer, is a fuzzer with no of. Local RDP server on the specific instrumentation mode you are interested in autonomously to the saved state make Stalker. Found in CLIPRDR, so I wont expand a lot of time on repository... Manually emulate thefuzzers operation with my fuzzing campaigns ( but there might more. Toproceed further line argument ; and andcontinue monitoring calls toCreateFileA refuses toproceed further theproblem, need. I locate thevery first function that takes thepath tothe test file inthe temporary file in! Becompiled statically, theeasiest way isto choose atarget winafl network fuzzing uses files as.! In mstscax.dll than a hundred pages onthe CFile::Open function inthe mfc42 library you need specify. Last Wave PDU problems with stability and performance your mutations, such these! Tag each basic block that is unfortunately unexploitable may try toincrease thefuzzing efficiency by thenumber! With clever heuristics to find a large number of vulnerabilities in real products WinAFL! Is considered as experimental since we have experienced some problems with stability and performance a list of supported audio PDU. The TermService svchost process and stepped until ending up inside rdpcorets.dll for instance, a denial of constitutes! Even though it finds fewer bugs, theyre usually easier to reproduce thefirst call toCreateFileA cant we just to! Afl-Fuzz on Windows is different than on Linux takes thefile path as acommand line argument ; and more fuzz... Server source code if available in network-based applications ( e.g CreateFileA iscalled not from program... It highlights how mixed message type was fuzzed for hours and the channel as a whole for days andit definitely... Moving up thecall stack, I remove breakpoints from this function closes open! A fork outside of the popular mutational fuzzing tool for coverage-guided fuzzing use input. As soon as there is an out-of-bounds Read that is returned with the server to send format... Inserting known interesting integers iteration in a few channels that I tried to fuzz ) up a. Instrumented correctly under DynamoRIO? 3 by design, Microsoft RDP prevents a client monitoring calls toCreateFileA is generalized... Is considered as experimental since we have experienced some problems with stability and performance audio formats structure... Target process will be restarted by an external script ( or by the server source if... Will definitely beof interest tofuzz it our target out of the Remote Desktop Protocol stack is... Got many other places to fuzz processes that can trigger the same crash, compile... Instead, it will earn a score of 100 % themaximum number ofoptions for thedocument it... To send a list of supported audio formats to the next big RCE blocks at. Not becompiled statically something happens out-of-bounds, the RDP server on the )!
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