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Comment by jnorthrop | Hacker News Reader

Anyone know how this works? How would a plain .txt file load a dll? In any case this looks like it would be difficult to execute since the text file has to be in the same directory as the dll.

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A bit of digging around suggests it's a DLL preloading attack which somehow convinces the target apps to load the local directory instance of the dll, rather than the system version.

http://www.crn.com/news/security/226900204/microsoft-warns-u...

has a bit of detail, but not specifically about this attack.

I guess it's conceptually similar to doing something like

    export PATH=.:$PATH; cat foo.txt

where 'cat' is an executable file in the current dir.

The actual linux equivalent would probably involve $LD_LIBRARY_PATH ($DYLD_LIBRARY_PATH on OSX, not sure about other unices).

This is called DLL Hijacking and it's a class of flaws that gained prominence on Windows earlier this year. Although I would say that Mandiant discussed the issue first, it was only after HDM practically described the WebDAV and other remote exploit vectors that people started taking it seriously.

https://community.rapid7.com/community/infosec/blog/2010/08/...

http://blogs.msdn.com/b/david_leblanc/archive/2008/02/20/dll...

http://www.n00bz.net/blog/2010/9/15/dll-hijacking-with-metas...

I wonder if Notepad, Wordpad, and maybe Word all call some library (or all have copy/pasted code) that, for whatever reason, looks for a particular DLL in the working folder and executes it?

Not sure how the network drive part would fit into that hypothesis, though.

Windows suffers some famous dain bramage wrt to DLL loading paths: http://unhandledexpression.com/2010/08/23/fixing-the-dll-loa... Basically, even when "safe" DLL loading is turned on, the search path still includes the current directory. I'm guessing that Notepad and friends all look for some (legacy?) DLL that isn't always installed, so if an attacker puts a malicious DLL by its same name in the same path as the file being opened, it'll be loaded and executed. The part about network drives is probably just an assumption about trust boundaries: Microsoft assumes that attacker can't ordinarily put a malicious file in the same local folder that you're opening a text file in, but on a network drive anything goes.

This is all just speculation. I don't code for Windows, and I don't know anything more about this vulnerability than what's stated in the advisory.

The article doesn't mention it, but does anyone know if a requirement to reproduce the vulnerability is that the files be opened in Notepad, Wordpad, or Word? What if you used a third-party editor?

I'm guessing it's actually a bug with Explorer (that is, the actual opening action) than any particular editor, since RTF files are also affected - and doubtlessly many other file types.

It's not Explorer, it's a bug with the Win32 API that loads DLLs. Any program that uses Win32 to load libraries is affected.

Not true, see here:

http://msdn.microsoft.com/en-us/library/ms682586%28v=vs.85%2...

The vulnerability, as I understand it (I did a little research by examining one of the vulnerable applications), since we don't have any actual PoCs to examine:

notepad.exe, like many of the stock Windows apps, uses a bunch of system libraries. One of those system libraries loads shdocvw.dll (an Internet Explorer related component). shdocvw.dll has a delay-load dependency on a library called 'ieshims.dll'. On my computer, when I start notepad.exe, shdocvw.dll tries to load ieshims.dll, and fails, but continues normally.

This means that since ieshims.dll is not found in the app directory or the windows system directory, the search for it will continue all the way into the current working directory, which would make it possible for an exploit to put a payload in an 'ieshims.dll' stored in the current working directory (next to the .txt file), and it would then be loaded.

Assuming my research is correct, this looks like a security vulnerability introduced by one of the Internet Explorer developers, probably for some sort of compatibility purpose. They ignored the fact that IE is used as a component in many system applications, and basically added a vulnerability to every app using shdocvw (there are a lot of them).

It's a generic vulnerability; any program that loads DLLs while the current directory is untrusted has the same problem.

It's a malicious dll, not a malicious txt. It could be any txt file, as long as the dll is in the right place. So it must be something to do with the search path the program uses to look for dlls.

Doesn't sound too difficult if you're sending someone an archive with both files in it.

This only works on remote file systems, so you just have to send someone a link to a text file. There would be no indication that a DLL even exists on the target FS.

This apparently works on local directories also. However, either on remote or local, you have to set the working directory before making the "open" action.

Therefore I am not sure a direct link would do the trick (can you change the working dir using an URL and then point to the file ? I m not sure)

Anyway, the DLL does not have to be visible for the trick to work.

> this looks like it would be difficult to execute since the text file has to be in the same directory as the dll. Seems like a great way to compromise your boss' computer since so many businesses use Windows and networked file systems.

Unless you have an exceptionally paranoid workplace, a much easier way to compromise your boss's computer is just to walk over to it and pop in a livecd after he's gone home for the day.

While I generally agree (even though it's locked doors all the way up for me and there are even easier ways I could compromise machines), it's foolish to disregard remote code execution vulnerabilities just because physical access is not secured.

Oh, I certainly agree. But I only agree because my threat model includes adversaries other than malicious coworkers; particularly, it includes innocent coworkers with malware-infected workstations.

By the way, ten minutes I wrote the grandparent to this comment, a coworker IMed me and asked me to break into his workstation :-). He was working remotely, trying to SSH in, his system had gotten wedged due to what turned out to be disk errors, and he needed me to rescue it.

Yeah, it has to be "an untrusted remote file system location."

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