EEYE: Windows Workstation Service Remote Buffer Overflow

["Derek Soeder" <dsoeder () eeye com>]

Windows Workstation Service Remote Buffer Overflow

Release Date:
November 11, 2003

Date Reported:
September 15, 2003

Severity:
High (Remote Code Execution)

Systems Affected:
Windows 2000
Windows XP

Description:
eEye Digital Security has discovered a remote buffer overflow in the Windows Workstation Service (WKSSVC.DLL). An 
unauthenticated attacker could exploit this vulnerability to execute arbitrary code with system-level privileges on 
Windows 2000 and Windows XP machines. The susceptible Workstation functionality is accessible via the WKSSVC named pipe 
(TCP ports 139 and 445).

This buffer overflow bug is within network management functions provided by the DCE/RPC service. These functions 
provide the ability to manage user accounts and network resources locally and remotely. Some network management 
functions generate a debug log file in the "debug" subdirectory located in the Windows directory.

A logging function implemented in WKSSVC.DLL is called to write entries to the log file.  In this function, the 
vsprintf() routine is used to create a log entry.  The string arguments for this logging function are supplied as 
parameters to vsprintf() without any bounds checking, so if we can pass a long string argument to the logging function, 
then a buffer overflow will occur.

We found some RPC functions which will accept a long string as a parameter, and will attempt to write it to the debug 
log file.  If we specify a long string as a parameter to these RPC functions, a stack-based buffer overflow will happen 
in the Workstation service on the remote system. Attackers who successfully leverage this vulnerability will be 
executing code under the SYSTEM context of the remote host.

Technical Description:
The buffer overflow bug is in a logging function which generates a string for the log file using vsprintf().  The name 
of the log file is "NetSetup.LOG", and it is located in the Windows "debug" directory.

This logging routine is called from some functions which handle commands for the Workstation service, such as 
"NetValidateName", "NetJoinDomain", etc.  In the case of NetValidateName(), the "computer name" specified as the second 
argument is eventually recorded in the log file.

For example, if we use NetValidateName() API as follows:

    NetValidateName(L"\\\\192.168.0.100","AAAAAAAA",NULL,NULL,0);

then we can confirm the following log entry on the remote host "192.168.0.100":

    08/13 13:01:01 NetpValidateName: checking to see if '' is valid as type 0 name
    08/13 13:01:01 NetpValidateName: '' is not a valid NetBIOS \\AAAAAAAA name: 0x57

If we specify a long string as the second argument to the NetValidateName() API, a buffer overflow happens on the 
specified host if the debug file is writeable.

Generally, the "debug" subdirectory in the Windows directory is not writeable by everyone if the drive is formatted as 
NTFS, which means that we cannot append to the log using a null session.  The WsImpersonateClient() API is called 
before opening the log file, and if the connected client does not have the privilege to write to the log file, then 
CreateFile() will fail, and the vulnerable call to vsprintf() is not performed.  So, in this case, we can exploit FAT32 
systems (which do not support ACLs on directories), or systems where the "%SYSTEMROOT%\debug" directory is writeable by 
everyone.

However, there are some extended RPC functions implemented in Windows XP which open the logfile before calling 
WsImpersonateClient().  They are undocumented RPC functions, but we can observe them in the function table in 
WKSSVC.DLL.  The RPC numbers for these extended commands start at 0x1B; for example, function 0x1B invokes the 
NetpManageComputers() API internally, which does not call WsImpersonateClient() before opening the log file.

The usage of NetpManageComputers() is not published; however, we found the prototype definition of the 
NetAddAlternateComputerName() API in "LMJoin.h", which calls NetpManageComputers() internally.  This API is exported 
from NETAPI32.DLL. This API is also undocumented.  We can generate the packet to execute this RPC function (number 
0x1B) using the API as follows:

    NetAddAlternateComputerName(L"\\\\192.168.0.200",long_unicode_string,NULL,NULL,0);

We do not need special privileges to write the second argument into the log file on the remote host.  If we specify a 
long Unicode string as the second argument ("AlternateName"), the remote system specified in the first argument will 
crash due to a buffer overflow.  The Unicode string "long_unicode_string" will be translated into an ASCII string 
before the logging function is called.

Protection:
Retina Network Security Scanner has been updated to identify this vulnerability.

Vendor Status:
Microsoft has released a patch for these vulnerabilities.  The patch is available at:
http://www.microsoft.com/technet/security/bulletin/MS03-049.asp

Credit:
Yuji Ukai

Greetings:
All AD200X attendees, speakers, volunteers, and members.

Related Links:
Retina Network Security Scanner - Free 15 Day Trial
http://www.eeye.com/html/Products/Retina/index.html

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