Nynaeve

Last time, I described some of the details behind impersonation (including a very brief overview of some of the dangers of using it improperly, and how to use impersonation safely via Security Quality of Service). I also mentioned that it might be possible to go from a compromised LocalService / NetworkService to full LocalSystem access in some circumstances. This article expands upon that concept, and ties together just what impersonation means for low-privileged services and the clients that talk to them.

As I mentioned before, impersonation is not really broken by design as it might appear at first glance, and it is in fact possible to use it correctly via setting up a correct SQOS when making calls out to impersonation-enabled IPC servers. Many things do correctly use impersonation, in fact, just to be clear about that. (Then again, many things also use strcpy correctly (e.g. after an explicit length check). It’s just the ones that don’t which get all the bad press…)

That being said, as with strcpy, it can be easy to misuse impersonation, often to dangerous consequences. As they say, the devil is often in the details. The fact is that there exist a great many things out there which simply don’t use impersonation correctly. Whether this is just due to how they are designed or ignorance of the sensitive nature of allowing an untrusted server to impersonate ones security concept is debatable (though in many cases I would tend towards the latter), but for now, many programs just plain get impersonation wrong.

Microsoft is certainly not ignorant to the matter (for example, David’s post describes how Office wrappers CreateFile to ensure that it never gets tricked into allowing impersonation, because the wrapper by default doesn’t permit remote servers to fully impersonate the caller via the proper use of SQOS). However, the defaults remain permissive at least as far as APIs that connect to impersonation-enabled servers (e.g. named pipes, RPC, LPC), and by default allow full impersonation by the server unless the client program explicitly specifies a different SQOS. Even the best people make mistakes, and Microsoft is certainly no exception to this rule – programmers are, after all, only human.

In retrospect, if the security system were first being designed in today’s day and age instead of back in the days of NT 3.1, I’m sure the designers would have chosen a less security sensitive default, but the reality is that the default can probably never change due to massive application compatibility issues.

Back to the issue of LocalService / NetworkService and svchost, however. Many of these low privileged services have “careless” clients that connect to impersonation-enabled IPC servers with high privileges, even in Windows Server 2008. Moreover, due to the fact that LocalService / NetworkService isolation is from a security standpoint all but paper thin prior to Windows Server 2003 (and better, though only in the non-shared-process, that is, non-svchost case in Vista and Windows server 2008), the sort of additive attack surface problem I described in the previous article comes into play. To give a basic example, try attaching to the svchost that runs the “LocalServiceNetworkRestricted” service group, including Eventlog, Dhcp (the DHCP client), and several other services (in Windows Server 2008) and setting the following breakpoint (be sure to disable HTTP symbol server access beforehand or you’ll deadlock the debugger and have to reboot – another reason why I dislike svchost services in general):

bp RPCRT4!RpcImpersonateClient "kv ; gu ; !token ; g"

Then, wait for an event log message to be written to the system event log (a fairly regular occurance, though if you want you can use msg.exe to send a TS messge from any account if you don’t want to wait, which will result in the message being logged to the system event log immediately courtsey of the hard error logging facility). You’ll see something like this:

Call Site
RPCRT4!RpcImpersonateClient
wevtsvc!EvtCheckAccess+0x68
wevtsvc!LegacyAccessCheck+0x15e
wevtsvc!ElfrReportEventW+0x2b2
RPCRT4!Invoke+0x65
[…]

TS Session ID: 0x1
User: S-1-5-18 (LocalSystem)
Groups:
00 S-1-5-32-544 (Administrators)
Attributes – Default Enabled Owner
01 S-1-1-0
Attributes – Mandatory Default Enabled
02 S-1-5-11
Attributes – Mandatory Default Enabled
03 S-1-16-16384 (System Integrity)
Attributes – GroupIntegrity GroupIntegrityEnabled
Primary Group: S-1-5-18
Privs:
00 0x000000002 SeCreateTokenPrivilege Attributes –
[…]
Auth ID: 0:3e7 (SYSTEM_LUID)
Impersonation Level: Impersonation
TokenType: Impersonation

Again, checking winnt.h, it is immediately obvious that the eventlog service (which runs as LocalService) gets RPC requests from LocalSystem at System integrity level, with the caller enabling full impersonation and transferring all of its far-reaching privileges, many of them alone enough to completely compromise the system. Now, this and of itself might not be so bad, if not for the fact that a bunch of other “non-privileged” services share the same effective security context as eventlog (thanks to the magic of svchost), such as the DHCP client, significant parts of the Windows Audio subsystem (in Vista or in Srv08 if you enable audio), and various other services (such as the Security Center / Peer Networking Identity Manager / Peer Name Resolution Protocol services, at least in Vista).

What does all of this really mean? Well, several of those above services are network facing (the DHCP client certainly is), and they all likely expose some sort of user-facing IPC interface as well. Due to the fact that they all share the same security context as eventlog, a compromise in any one of those services could trivially be escalated to LocalSystem by an attacker who is the least bit clever. And that is how a hypothetical vulnerability in a non-privileged network-facing service like the DHCP client might get blown up into a LocalSystem compromise, thanks to svchost.

(Actually, even the DHCP client alone seems to expose its own RPC interface that is periodically connected to by LocalSystem processes who allow impersonation, so in the case of an (again hypothetical) vulnerability DHCP client service, one wouldn’t even need to go to the trouble of attacking eventlog as the current service would be enough.)

The point of this series is not to point fingers at the DHCP / Windows Audio / Eventlog (or other) services, however, but rather to point out that many of the so-called “low privileged” services are not actually as low privileged as one might think in the current implementation. Much of the fault here actually lies with whatever programs connect to these low-privileged services with full impersonation enabled than the actual services themselves, in fact, but the end result is that many of these services are not nearly as privilege-isolated as we would prefer to believe due to the fact that they are called irresponsibly (e.g. somebody forgot to fill out a SECURITY_QUALITY_OF_SERVICE and accepted the defaults, which amounts to completely trusting the other end of the IPC call).

The problem is even more common when it comes to third party software. I would put forth that at least some of this is a documentation / knowledge transfer problem. For example, how many times have you seen SECURITY_QUALITY_OF_SERVICE mentioned in the MSDN documentation? The CreateFile documentation mentions SQOS-related attributes (impersonation restriction flags) in passing, with only a hint of the trouble you’re setting yourself up for by accepting the defaults. This is especially insidious with CreateFile, as unlike other impersonation-enabled APIs, it’s comparatively very easy to sneak a “bad” filename that points to a dangerous, custom impersonation-enabled named pipe server anywhere a user-specified file is opened and then written to (other impersonation attack approaches typically require that an existing, “well-known” address / name for an IPC server to be compromised as opposed to the luxury of being able to set up a completely new IPC server with a unique name that a program might be tricked into connecting to).

The take-home for this series is then to watch out when you’re connecting to a remote service that allows impersonation. Unless absolutely necessary you should specify the minimum level of impersonation (e.g. SecurityIdentification) instead of granting full access (e.g. SecurityImpersonation, the default). And if you must allow the remote service to use full impersonation, be sure that you aren’t creating a “privilege inversion” where you are transferring high privileges to an otherwise low-privileged, network-facing service.

Oh, and just to be clear, this doesn’t mean that eventlog (or the other services mentioned) are full of security holes outside of the box. You’ll note that I explicitly used a hypothetical vulnerability in the DHCP client service for my example attack scenario. The impersonation misuse does, however, mean that much of the work that has been done in terms of isolating services into their own compartmentalized security contexts isn’t exactly the bullet proof wall one would hope for most LocalService / NetworkService processes (and especially those sharing the same address space). Ironically, though, from certain respects it is the callers of these services that share part or all of the blame (depending on whether the service really requires the ability to fully impersonate its clients like that or not).

As a result, at least from an absolute security perspective, I would consider eventlog / DHCP / AudioSrv (and friends) just as “LocalSystem” in Windows Server 2008 as they were back in Windows 2000, because the reality is that if any of those services are compromised, in today’s (and tommorow’s, with respect to Windows Server 2008, at least judging from the Beta 3 timeframe) implementation, the attacker can elevate themselves to LocalSystem if they are sufficiently clever. That’s not to say that all the work that’s been done since Windows 2000 is wasted, but rather that we’re hardly “all of the way there yet”.

This entry was posted on Wednesday, August 8th, 2007 at 7:00 am and is filed under NT Internals, Security, Windows. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.