Windows 2000 Default Permissions Could Allow Trojan Horse Program New

Due to the default permissions used on the Windows 2000 system root folder it could enable an attacker to mount a Trojan horse attack against other users of the same system.

Issue

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.

Affected Products

• Microsoft Windows 2000

Solution

Set permissions to remove vulnerability. 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)

Further Details

Source: Microsoft Corporation

Reference: Microsoft Corporation

Updated: October 30, 2002

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