October 31, 2002 Information Security Advisory

Microsoft Windows 2000 vulnerability. This bulletin was issued yesterday; 
System administrators running Microsoft Windows 2000 should read this. 

Impact of vulnerability: Trojan Horse program execution 

Maximum Severity Rating: Moderate 

Recommendation: Administrators should consider changing access permissions
on the Windows 2000 system root directory. 

Technical description: 


On Windows 2000, the default permissions provide the Everyone group with Full
access (Everyone:F) on the system root folder (typically, C:\). In most cases,
the system root is not in the search path. However, under certain conditions – 
for instance, during logon or when applications are invoked directly from the 
Windows desktop via Start | Run – it can be. 

This situation gives rise to a scenario that could enable an attacker to mount
a Trojan horse attack against other users of the same system, by creating a 
program in the system root with the same name as some commonly used program, 
then waiting for another user to subsequently log onto the system and invoke 
the program. The Trojan horse program would execute with the user’s own 
privileges, thereby enabling it to take any action that the user could take. 

The simplest attack scenario would be one in which the attacker knew that a 
particular system program was invoked by a logon script. In that case, the 
attacker could create a Trojan horse with the same name as the system program, 
which would then be executed by the logon script the next time someone logged 
onto the system. Other scenarios almost certainly would require significantly 
greater user interaction – for instance, convincing a user to start a particular 
program via Start | Run – and would necessitate the use of social engineering. 

The systems primarily at risk from this vulnerability would be workstations that 
are shared between multiple users, and local terminal server sessions. Other 
systems would be at significantly less risk: 


Workstations that are not shared between users would be at no risk, because the 
attacker would require the ability to log onto the system in order to place the 
Trojan horse. 
Servers would be at no risk, if standard best practices have been followed that 
advocate only allowing trusted users to log onto them. 
Remote Terminal server sessions would be at little risk, because each user’s 
environment is isolated. That is, the system root is never the current folder – 
instead, the user’s Documents and Settings folder is, but the permissions on 
this folder would not enable an attacker to place a Trojan horse there. 
Mitigating factors: 

An attacker would require the ability to log onto the system interactively in 
order to place the Trojan horse program. It could not be placed remotely 
As discussed above, dedicated workstations, servers and remote terminal server 
sessions would be at less risk (or, in some cases, none at all) from the vulnerability. 
Severity Rating:  Internet Servers Intranet Servers Client Systems 
Windows 2000 Low Low Moderate 
The above assessment is based on the types of systems affected by the vulnerability, 
their typical deployment patterns, and the effect that exploiting the vulnerability 
would have on them. 

Vulnerability identifier: CAN-2002-1184 

Tested Versions:
Microsoft tested Windows NT, Windows 2000, and Windows XP to assess whether they 
are affected by this vulnerability.


Patch availability

Download locations for this patch This vulnerability requires an administrative 
procedure rather than a patch. 

Why isn’t there a patch for this vulnerability?

A patch simply isn’t practical for this case. The right permissions vary from 
organization to organization and can depend on factors like the specific applications 
in use. Any set of permissions set by a patch would almost certainly need to be adjusted 
in many cases, and as a result we believe the most effective way to remediate the problem 
is for system administrators to set the permissions that are right for their organizations. 

What’s a good baseline set of permissions?

The default permissions for Windows XP can serve as a guide for a set of permissions which 
have been thoroughly designed and tested. The default permissions for the root directory 
on the system drive for Windows XP are: 

Administrators: Full (This Folder, Subfolder and Files) 
Creators Owners: Full (Subfolders and Files) 
System: Full (This Folder, Subfolder and Files) 
Everyone: Read and Execute (This Folder Only) 


Acknowledgments
Microsoft thanks Jason Miller of Security Focus (http://www.securityfocus.com) 
for reporting this issue to us and working with us to protect customers. 

Revisions: 

V1.0 (October 30, 2002): Bulletin Created. 


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Scan your computer for viruses every week and don't forget to back up your files! 

Fredo
http://www.workingarts.com/infosecarchives