How least privilege is that service, anyway (or much ado about impersonation) – part 1

Previously, I discussed some of the pitfalls with LocalService and NetworkService, especially with respect to pre-Windows-Vista platforms (e.g. Windows Server 2003).

As I alluded to in that post, however, a case can be made that there still exists a less than ideal circumstance even in Vista / Srv08 with respect to how the least-privilege service initiative has actually paid out. Much of this is due to the compromise between performance and security that was made with packing many unrelated (or loosely related) services into a single svchost process. On Windows Server 2003 and prior platforms, the problem is somewhat more exacerbated as it is much easier for services running under different processes to directly interfere with eachother while running as LocalService / NetworkService, due to a loose setting of the owner field in the process object DACL for LocalService / NetworkService processes.

As it relates to Vista, and downlevel platforms as well, many of these “low privileged” services that run as LocalService or NetworkService are really highly privileged processes in disguise. For a moment, assume that this is actually a design requirement (which is arguable in many cases) and that some of these “low privileged” services are required to be given dangerous abilities (where “dangerous” is defined in this context as could be leveraged to take control of the system or otherwise elevate privileges). A problem then occurs when services that really don’t need to be given high privileges are mixed in the same security context (either the same process in Vista, or the same account (LocalService / NetworService) in downlevel systems. This is due to the obvious fact that mixing code with multiple privilege levels in a way such that code at a lower privilege level can interfere with code at a higher privilege level represents a clear break in the security model.

Now, some of you are by this point probably thinking “What’s he talking about? LocalService and NetworkService aren’t the same thing as LocalSystem.”, and to a certain extent, that is true, at least when you consider a trivial service on its own. There are many ways in which these “low privileged” services are actually more privileged than might meet the eye at first glance, however. Many of these “low privileged services” happen to work in a way such that although they don’t explicitly run with System/Administrator privileges, if the service “really wanted to”, it could get high privileges with the resources made available to it. Now, in my mind, when you are considering whether two things are equivalent from a privilege level, the proper thing to do is to consider the maximum privilege set a particular program could run with, which might not be the same as the privilege set of the account the process runs as.

To see what I mean, it’s necessary to understand an integral part of the Windows security model called impersonation. Impersonation allows one program to “borrow” the security context of another program, with that program’s consent, for purposes of performing a particular operation on behalf of that program. This is classically described as a higher privileged server that, when receiving a request from a client, obtains the privileges of the client and uses them to carry out the task in question. Thus, impersonation is often seen as a way to ensure that a high privileged server is not “tricked” into doing a dangerous operation on behalf of a client, because it “borrows” the client’s security context for the duration of the operation it is performing on behalf of that client. In other words, for the duration of that operation, the server’s privileges are effectively the same as the client’s, which often results in the server appearing to “drop privileges” temporarily.

Now, impersonation is an important part of practically all of the Windows IPC (inter-process communication) mechanisms, such as named pipes, RPC, LPC, and soforth. Aside from the use of impersonation to authoritatively identify the identity of a caller for purposes of access checks, many services use impersonation to “drop privileges” for an operation to the same level of security as a caller (such that if a “plain user” tries to make a call that does some operation requiring administrative privileges, even if the service was running with those administrative privileges originally, any resources accessed by the server during impersonation are treated as if the “plain user” accessed them. This prevents the “plain user” from successfully performing the dangerous / administrative task without the proper privileges being granted to it).

All this is well and fine, but there’s a catch: impersonation can also effectively elevate the privileges of a thread, not just lower them. Therein lies the rub with svchosts and LocalService / NetworkService accounts, since while these accounts may appear to be unprivileged at first glance, many of them operate RPC or LPC or named pipe servers that that privileged clients connect to on a regular basis. If one of these “low privileged” services is then compromised, although an attacker might not immediately be able to gain control of the system by elevating themself to LocalSystem privileges, with a bit of patience he or she can still reach the same effect. Specifically, the attacker need only take over the server part of the RPC / LPC / other IPC interface, wait for an incoming request, then impersonate the caller. If the caller happens to be highly privileged, then poof! All of a sudden the lowly, unprivileged LocalService / NetworkService just gained administrative access to the box.

Of course, the designers of the NT security system forsaw this problem a mile away, and built in features to the security system to prevent its abuse. Foremost, the caller gets to determine to what extent the server can impersonate it, via the use of a little-known attribute known as the Security Quality of Service (SQOS). Via the SQOS attribute (which can be set on any call that enables a remote server to impersonate one’s security context), a client can specify that a service can query its identity but not use that identity to perform access or privilege checks, as opposed to giving the server free reign over its identity once it connects. Secondly, in the post-Windows 2000 era, Microsoft restricted impersonation to accounts with a newly-created privilege, SeImpersonatePrivilege, which is by default only granted to LocalService / NetworkService / LocalSystem and administrative accounts.

So, impersonation isn’t really broken by design (and indeed David LeBlanc has an excellent article describing how, if used correctly, impersonation isn’t the giant security hole that you might think it is.

That being said, impersonation can still be very dangerous if misused (like many other aspects of the security system).

Coming up: A look at how all of this impersonation nonsense applies to LocalService / NetworkService (and svchost processes) in Windows (past and future versions).

3 Responses to “How least privilege is that service, anyway (or much ado about impersonation) – part 1”

  1. […] Nynaeve Adventures in Windows debugging and reverse engineering. « How least privilege is that service, anyway (or much ado about impersonation) – part 1 […]

  2. Good Point says:

    Well, MSDN is a little confusing as to whether passing the correct bits to CreateFile() closes this impersonation hole. The page on CreateFile links to this page:

    Which says:

    When the named pipe, RPC, or DDE connection is remote, the flags passed to CreateFile to set the impersonation level are ignored. In this case, the impersonation level of the client is determined by the impersonation levels enabled by the server, which is set by a flag on the server’s account in the directory service. For example, if the server is enabled for delegation, the client’s impersonation level will also be set to delegation even if the flags passed to CreateFile specify the identification impersonation level.

  3. […] over the entire box. Ken “Skywing” Johnson wrote about this in a couple of posts on his blog (1 and 2) on his excellent blog Nynaeve. David LeBlanc has a subtly different take on this issue (see […]