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The thread is passed the offset to as the code to run. If I jump down to the section I have labeled in green in the image above, I’ll see the code that handles connections and the call to create a thread to handle the connection:
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I’ll also notice just after the args are processed, it prints a message with the server version and a warning that this software is vulnerable: If I scroll through the code itself, looking at the functions that are being called, I’ll see I basically got that right. Without even looking at the assembly, I can make some guesses as to what’s going on. I see the overall flow of this function looks like this:
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So it’s likely going to create a socket, bind it to a post, listen, wait for connections, handle those connections by forking (in Windows that’s starting a thread). From the TopĪlternatively, when IDA loads, it starts me at main. I’ll scroll up to the top of the function to see it’s been given the name by IDA. In this case, it’s only once, so I’ll hit OK to go there in the code: IDA is also showing me there’s an cross reference (code that references 0x004077F3) in I click on aGetCoffee (it’ll highlight yellow) and then hit “x”, I’ll get a pop up with a list of places in the code this string is referenced: When I double click on it, I’m taken to where that string is in memory, with several other strings right around it, and I can see this string sits at 0x004077F3: I’ll pick GET /coffee since that’s the path that showed me the BigheadWebSrv in the first place.
That’s where my input with interact with the server, so I want to start there. Those are clearly strings for parsing the HTTP request that’s incoming. About half way down, this block will jump out at me: StringsĪs soon as I open IDA, I’ll hit Shift-F12 to bring up the Strings window. There are a few ways to find the function that IDA labels as I’ll show three simple ways that I did it. If I find an overflow in the handling of command line arguments, that doesn’t help me at this point, as I can’t control that. I want to find the function that handles web requests, since that the way that I can interact with the server.
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In this case, where I have a known vulnerable binary and I’m fairly confident there’s a bug in it (the note in the final git commit beyond just it’s being here on a HackTheBox host), I like practicing my reverse engineering, so before I even set up a Windows VM, I’m going to fire up the free version of IDA on Kali and see if I can find an overflow. This is a fine strategy, especially if your goal is to move fast through binaries that may or may not have overflows in them. This is exactly what they teach in day 3 of Sans SEC660. The most common and probably fastest way to find a vulnerability in a program like this would be fuzz it until you find a crash, and then continue to send input and look at the resulting crashes in something like Immunity Debugger. I got to learn a new technique, Egg Hunter, which is a small amount of code that will look for a marker I drop into memory earlier and run the shellcode after it. The primary factor that takes this above something like a basic jmp esp is the space I have to write to is small. On top of that, it has plugin support and lets you configure lots of parameters.As my buffer overflow experience on Windows targets is relatively limited (only the basic vulnserver jmp esp type exploit previously), BigHeadWebSrv was probably the most complicate exploit chain I’ve written for a Windows target. It provides you with a wide selection of relevant functions and displays it in a well-organized interface. In a nutshell, 圆4dbg is a handy, complex debugging tool. Optionally, you can use 圆4dbg.exe to register a shell extension and add shortcuts to your desktop.
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Installation of the app, you only need to download a snapshot and extract it in a location your user has write access to. With this, you will have an easier time using the app. The windows may look a bit outdated but all its functions are well-organized in menus or displayed directly on the menu screen. X64dbg comes with an extensive and comprehensive interface. By default, it features a single plugin, Scyla. If you are looking for more functions, the app allows plugins, broadening your range of possibilities. Also, you can choose between signed or unsigned calculation types. For example, you can set the application to break on TLS callbacks, DLL load, or DLL entries. There is also a Setting window that allows you to configure various parameters regarding engine, expectations, or events.