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Date: Mon, 10 Feb 2025 18:36:46 -0800
To: Splitline Huang <splitline@devco.re>
Cc: cygwin@cygwin.com
Subject: Re: Potential Argument Injection Issue in Cygwin's Command Line
 Handling
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From: Kaz Kylheku via Cygwin <cygwin@cygwin.com>
Reply-To: Kaz Kylheku <kaz@kylheku.com>
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On 2025-02-03 22:15, Splitline Huang via Cygwin wrote:
> Hello Cygwin team,
> 
> I am splitline from DEVCORE research team. I recently have observed an
> inconsistency
> in how Cygwin handles command-line parsing compared to Microsoft’s
> implementation.

Hi,

I maintain a small fork of the Cygwin DLL called Cygnal.

https://www.kylheku.com/cygnal/

The purpose of Cygnal is to leverage Cygwin as a run-time for "native"
Windows applications. 

It gives us a beautfiful development workflow: you simply write
your program under Cygwin as a Cygwin program, compiling with the Cygwin
GCC suite. You can test your program and make sure it functions
as a Cygwin application.

Then you can slide the cygwin1.dll under it, which brings back certain
Windows-like behaviors. Test it that way and ship it to Windows
users who don't know anything about POSIX or Cygwin.
(And also ship it as Cygwin program to Cygwin people too!)

In the Cygnal project, I /do/ care about interoperability with Windows
programs. I'd like a Cygnal program to accept arguments the same way
as something that calls CommandLineToArgvW, or the same way as a
Visual C++ program that begins with wmain.

I don't want Cygnal programs to be susceptible to the alleged argument
injection when invoked by non-Cygwin applications that are following
the Microsoft-recommended command line convention.

Therefore, if you produce a good patch for this issue, I will
likely merge it in the Cygnal project, even if Cygwin doesn't want it.

The patch has to address both generation of arguments when invoking
processes, as well as parsing.

Quite recently, Cygwin developer Corina Vinschen pointed me (via
a post in this mailing list) to where the commanda rgument parsing
logic takes place. It's under the function dll_crt0_1 in the file
dcrt0.cc. Here, I think:

https://cygwin.com/cgit/newlib-cygwin/tree/winsup/cygwin/dcrt0.cc#n800

This calls something called build_argv, in the same file.

In the Cygnal project, I don't care for having argv[0] massaged
for POSIX conventions, either.

Somewhere under the exec family of functions there is code for
the reverse: encoding arguments into a Windows sommand line.

Cheers ...

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