Design Decisions: Implicit Command Resolving

This is the first article of my Design Decisions series.

There was a very interesting thing related to command resolving in Microsoft Windows. This can be explained using an example. Let's say a user wants to start the command c:\Program Files\emacs-25\bin\epdfinfo.exe /h. What is the executable and what are the command line arguments (or parameters) here? In theory, we have three possibilities:

1. Look out for c:\Program.exe, c:\Program.com, or c:\Program.bat and interpret Files\emacs-25\bin\epdfinfo.exe as the first command line parameter and /h as the second parameter.
2. Start c:\Program Files\emacs-25\bin\epdfinfo.exe and use /h as a command line parameter.
3. Look out for c:\Program Files\emacs-25\bin\epdfinfo.exe /h.exe, c:\Program Files\emacs-25\bin\epdfinfo.exe /h.com, or c:\Program Files\emacs-25\bin\epdfinfo.exe /h.bar.

To be honest, I don't know if the third option was ever possible. In contrast, the first two options were actually possible. So how did Microsoft knew what executable to start and what parameter to use?

They did not. Microsoft's solution was astonishing: They simply tried.

Microsoft Windows was guessing, what executable to start. That's right: for many years, Microsoft Windows had a trial-and-error method to parse command line executions. For a technical savvy person, this is a really weird one.

Microsoft just tried, if there is an executable like c:\Program.exe. When there was no exe, com, or bat file, they continued parsing and tried the next possible candidate which is c:\Program Files\emacs-25\bin\epdfinfo.exe in our example above.

This had consequences: any c:\Program.exe, c:\Program.com, or c:\Program.bat could turn a Windows computer useless. When somebody was able to place such a file on a Windows computer, it got executed any time the trial-and-error algorithm was applied.

For bad guys, this was also a formidable wrapper-method to control the computer. Such an executable was able to track all program invocations and could decide to really execute the command in question or to choose a different one according to malicious rules.

Please note that back-then, people were constantly working as local Administrator. There was no UAC around which allows temporary doing things as Administrator. For installing programs, you really had to log-out as user and log-in as Administrator. After installing, you had to log-out as Administrator and log-in as user again. Of course, this resulted in almost averybody working as Administrator. And when you are Administrator, every tool has the ability to place a c:\Program.exe file.

And if the technical reasons above are not enough to refrain from creating such an awful algorithm: it's simply poor design beyond understanding.

I can not confirm the wrong behavior with XP Pro 2002 SP3 any more. More modern Windows versions corrected this issue as well. Nowadays, a command containing spaces has to be put into double quotes.

However, the trial-and-error algorithm was the default and standard behavior on Windows operating systems for many years. It was not well known among normal users and it was not only an annoyance but also a harmful security threat to all Windows users.

You can find out more on this issue by visiting this source from 2007 that refers to this page from 2005. The localization of the program folder (a very bad idea in itself) and the guessing issue are discussed on this page from 2005. People who are able to read Japanese might enjoy this article from 2012 which explains the negative impacts as well.

So how did other operating systems deal with this issue? All other operating systems require the user to escape spaces in command lines that are not a component separator with a backslash. Unfortunately, Microsoft is using the backslash character as the path separator. This had other negative impacts and it will be discussed in another blog article.

In my opinion, we should learn from this example:

• Never guess, make everything explicit.
• Learn from already established systems and look how they solve the same issue. You can do better but you should not do worse.
• When something feels wrong, it is wrong.