*** A3 Security Consulting: CR@K Vulnerability Research ***


Title : MS IIS out of process privilege elevation vulnerability(A3CR@K-Vul-2002-06-002)
Reporter : li0n (li0n@a3sc.co.kr)
Affected software : IIS 4.0, 5.0, 5.1
Risk : High
Local/Remote : Local
Category : Windows - IIS - Privilege elevation
Date : June, 25, 2002

Advisory URL : http://www.li0n.pe.kr/eng/advisory/ms/iis_impersonation.txt



[1] Summary

An impersonation vulnerability of dllhost.exe allows a local user to gain the SYSTEM privilege.

This vulnerability arises from the fact that the process of dllhost.exe harbors an impersonation
token of SYSTEM account while processing user's request. 
Because a process of dllhost.exe is executed with  IWAM_machinename privilege, an attacker can
manipulate the process's memory space and steal the SYSTEM privilege when the process has the
impersonation token of SYSTEM account.  In other words, an attacker can execute arbitrary codes
with SYSTEM privilege through this impersonation.



[2] Affected software

Internet Information Services 4.0
Internet Information Services 5.0
Internet Information Services 5.1



[3] Patch

http://www.microsoft.com/technet/treeview/default.asp?url=/technet/security/bulletin/MS02-062.asp



[4] Testing environments

Microsoft Windows 2000 advanced server with
  Service pack2
  Security rollup1
  Cumulative Patch for Internet Information Services (Q319733)



[5] Required Knowledge

Architecture of Microsoft IIS
Impersonation



[6] Technical Details

Microsoft IIS provides two protection modes, in-process mode and out-of-process mode.
If IIS is set to in-process mode, .asp request and ISAPI dlls are executed within
inetinfo.exe process that is running with SYSTEM privilege.
So, an attacker can obtain SYSTEM privilege easily with some effort to control the process
to serve his purpose.

On the other hand, if IIS is set to out-of-process mode, .asp request and ISAPI dlls are
executed within dllhost.exe process which is running with IWAM_machinename account.
In this case, an attacker can only obtain IWAM_machinename account even if he or she is 
successful with the attack. For reference, the default setting for IIS4 is in-process and 
the default setting for IIS5 is out-of-process.

dllhost.exe process calls ole32!CoImpersonateClient function to process user's request.
If CoImpersonateClient function returns successfully, the process becomes to harbor an
impersonation token of SYSTEM account for a moment.  Later on, dllhost.exe process finishes up
its impersonated SYSTEM privilege by calling RevertToSelf function to destroy SYSTEM privilege
and returns to IWAM_machinename privilege.

Because a process of dllhost.exe is executed with IWAM_machinename privilege, an attacker can 
manipulate the process's memory space and steal the SYSTEM privilege when the process has the
impersonation token of SYSTEM account.

For an example, an attacker can make dllhost.exe process call MyCoImpersonateClient function
instead of ole32!CoImpersonateClient function using API hooking technique.  An attacker can forge
a MyCoImpersonateClient function and he or she can execute malicious code using SYSTEM privilege.

Another method to execute malicious code is to attach the dllhost.exe process to a debugger and
modify dllhost.exe process's memory contents.

An attacker can create and assign an arbitrary account to the local administrator group
exploiting this vulnerability and execute programs with administrator privilege.



[7] Exploit Code

Not included in this advisory on purpose



[8] Informations

** Vendor status **
Patch Released (MS02-062)


** Credit **
Discovery and exploitation Research : li0n@a3sc.co.kr
Advisor : CR@K TEAM


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