A flaw in the way Microsoft's Windows 2000 and NT 4.0 server operating systems authenticate users across domains could allow somebody with administrator privileges to extend that power to other domains, Microsoft warned yesterday.
Although the flaw is "extremely difficult" to exploit, the worst case scenario associated with it is "serious", Microsoft said in a security bulletin. Administrators are encouraged to consider installing a software fix, if physical and personnel security considerations indicate sufficient risk, the software maker said.
The flaw lies in the trust relationships between network domains in Windows 2000 and NT 4.0 environments. The trusting domain does not verify that the trusted domain is actually authoritative for all the SIDs (security identifiers) in the authorisation data, allowing an attacker to increase his or her access level, Microsoft said.
Trust relationships are commonly used in Windows 2000 and NT 4.0 environments. They are created between network domains to allow users in one domain to access resources in another domain without having to authenticate themselves. When a user in a trusted domain requests access to a resource in a trusting domain, the trusted domain supplies authorisation data in the form of a list of SIDs, that indicate a user's identity and group membership.
To exploit the flaw, an attacker has to add SIDs to the authorisation data of the trusted domain. To do this, the attacker needs administrator privileges on the trusted domain, as well as the technical knowledge to modify low-level operating system functions and data structures, as Windows NT 4.0 and 2000 don't offer a tool to add desired SIDs, Microsoft said.
The trusting domain is "too trusting" when judging the SIDs in the authorisation data, Microsoft said. The attacker, an administrator in the trusted domain with, for example, mere read-only rights in the trusting domain, could add a SID that identifies him or her as an administrator of the trusting domain. This would give full access over the computers in that domain.
The vulnerability can be eliminated with a new SID Filtering tool, Microsoft said. The fix sets the system up to inspect authorisation data and remove SIDs that don't belong to the originating domain.
Installing SID Filtering could have side effects, Microsoft cautioned. Legitimate SIDs that do not come from the originating domain are also filtered. This could, for example, result in trouble for users that are migrating from NT 4.0 to 2000 and using a feature called SIDHistory. SIDHistory is used to ease migration before new access control lists can be created for resources, Microsoft said.
Microsoft provides more information on SID Filtering in a white paper on its TechNet Web site.