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Re: Cannot exec() program outside of /bin if PATH is unset

Peter Rosin wrote:
On 2014-09-13 12:00, Christian Franke wrote:
Note that setting PATH=/bin on Cygwin does not fix the security problem in the DLL search order. Even with "SafeDllSearchMode" enabled, the current directory is always checked before PATH. Running some Cygwin program from /usr/sbin, /usr/local/bin, /usr/libexec, ... would load a possible malicious cyg*.dll from current directory regardless of PATH setting. Only programs in /bin are safe.

Using SetDllDirectory("c:\\cygwin\\bin") somewhere in cygwin1.dll would fix this also.
How could a call inside a DLL fix the library search order used
to find that same DLL? Yes, it is possible (or likely) that
SetDllDirectory fixes the immediate problem for processes that
are started *by* cygwin1.dll, but it is not effective for Cygwin
processes that are started by some direct use of the Win32 API.

Of course, and the same is true for any non-Cygwin program. The security fix is effective only for any CreateProcess()/LoadLibrary() call within the process which called SetDllDirectory(DIR_OF_SUBSYSTEM_DLLs).

Also, SetDllDirectory will kill all attempts to run 32-bit
Cygwin programs from 64-bit Cygwin (and vice versa).

For programs in /bin directory, there is no problem because the EXE's directory is always searched first for required DLLs. SetDllDirectory() then has no effect for cyg*.dll search order.

For other programs it also works because Windows (at least 7) apparently skips 32-bit DLLs when searching for 64-bit ones (and vice versa). It is then required that PATH contains the other Cygwin's /bin directory.

Testcase for calling 64-bit from 32-bit:

exe in /bin:

  execl("/cygdrive/c/cygwin64/bin/uname", "uname", "-a", (const char*)0);

exe not in /bin:

  setenv("PATH", "/cygdrive/c/cygwin64/bin", 1);
execl("/cygdrive/c/cygwin64/usr/sbin/alternatives", "alternatives", (const char*)0);

In both cases, the SetDllDirectory() call does not break anything.


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