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Tuesday, August 20, 2019

Finding Hidden Treasure on Owned Boxes: Post-Exploitation Enumeration with wmiServSessEnum


TLDR: We can use WMI queries to enumerate accounts configured to run any service on a box (even non-started / disabled), as well as perform live session enumeration.  Info on running the tool is in the bottom section.


Background


On a recent engagement I had gotten local admin privileges on ~20 boxes, and after querying active sessions on them got me nothing interesting I was ready to look for other potential escalation paths.  I ran secretsdump against several of the systems to grab local account hashes, and found that in the process of running it, I had also obtained plaintext credentials for a domain account that was not mentioned in any of the session enumeration information I had pulled.  This got me thinking about how this was possible, as well as how I could more reliably hunt for similar configurations on other systems I could remotely execute code on.

First, to explain what was going on – the NetWkstaUserEnum WINAPI function used by a majority of session enumeration tools is great at what it does, but only pulls data for active sessions on the remote system (interactive, service, and batch logins).  However, if a service is configured on the system but is currently not running, it will not be listed as a current session when enumerating the system.  This makes sense, as a non-running service has no processes associated with it.  After further investigation of the systems in question, I validated this is indeed what happened, as each of the systems was configured with a stopped service that would run using non-default credentials.

I’ve included an example below showing this in practice on a lab system using the GetNetLoggedOnUsers() functionality of @Cobbr_io’s SharpSploit, which uses the NetWkstaUserEnum WINAPI function to query sessions on a remote system, and a test service I configured (TestService) to run as the local ‘admin’ user on the box.  It shows that when the service is not running, the admin user is not enumerated (as expected):



For a bit more context on why this matters to us at all, we have to take a look at how credentials for service accounts are cached by Windows.  When a service is configured with a set of credentials to run as, the OS needs to store them so they don’t have to be re-entered every reboot / every time the service is ran.  Windows stores these service account credentials within the HKEY_LOCAL_MACHINE\Security registry hive, in an encrypted storage space known as LSA Secrets.  However, the passwords themselves, although encrypted, are stored as plaintext values (opposed to NTLM hashes).  Items stored in this space are only readable by NT_Authority/SYSTEM by default, but users with administrative rights on the system can create a backup of the registry hive that can subsequently be accessed and decrypted to extract the data contained within.  As the screengrab below shows, the credentials are sitting in LSA secrets, ready to be used whenever next needed.


And if we dump the contents of LSA secrets, we see we can actually retrieve the plaintext password for the account configured to run the service:


So at this point I was a bit stumped, how could I quickly and reliably enumerate accounts configured to run services on a relatively large number of remote systems?  It’s not really a best practice to start randomly secretsdumping boxes, and even if you threw opsec concerns to the wind it would still take a relatively long amount of time if you wanted to dump anything more than just a few systems.  With that in mind, I wanted something that would ideally be agentless, and could be ran in a multi-threaded process to increase collection speed against multiple systems. I settled on writing something that would check these boxes in c#, primarily as that’s what I’ve been doing the majority of my development in lately.



Building the Tool



Note: This section doesn’t have anything critical on the functionality or usage of the tool, but instead outlines the development process and roadblocks I ran into as I built it.  If this doesn’t interest you, I recommend scrolling through to the next section.

When I first sat down to write this tool, I thought WMI would be a good candidate to use for collection as I had some knowledge of the Win32_Service class and figured it would be pretty easy to pull the needed information from the remote system.  As I prepared to start coding I checked out similar projects that implemented WMI connectivity in .net applications.  From an offensive tooling standpoint, I didn’t find too much outside of tools designed to facilitate code execution, and overwhelmingly they appeared to use the older System.Management namespace to build their WMI objects. In my reading of Microsoft docs, I found that the newer Microsoft.Management.Infrastructure namespace was recommended to use to access WMI.


As I began to build out the functionality of the tool and started exploring other WMI classes I figured it would make sense to extend the tool’s functionality to also include the optional enumeration of sessions on the system via WMI, similar to the sessionEnum functionality seen earlier through SharpSploit.  To explore various WMI classes I used WMI Explorer (https://github.com/vinaypamnani/wmie2/releases) which provides a super helpful interface that allows you to browse WMI classes and get information on specific properties & methods.


Through this I found the  Win32_LoggedOnUser WMI class.  At first it seemed like this would be exactly what was needed for enumerating active sessions, and my initial tests worked great: I log in with user1, user1 shows up when I query the class, I log in with user2, user1 & user2 now show up when I query the class.  The issue came when I logged off with user2 and queried the class again; user2 still showed up as having a session on the system.  I tried giving it a few minutes, thinking that the session was temporarily caching on log-off, but still user2 appeared to be logged in when querying the class.  This led me to a bunch of googling and the unfortunate conclusion that the Win32_LoggedOnUser class tracks ALL login sessions since last reboot, including ‘stale’ connections, or those that are no longer exist.  This isn’t great for us, as these stale sessions do not retain cached credentials in memory by default, potentially leading to a plethora of false positives based on old logins.  There are definitely operational uses for this information, ex. looking for a system where there have been administrative logins at some point since last reboot – likely within the past month – and targeting them for long-term surveillance or persistence with the theory being that an admin may log in there again; however those uses are outside of the scope of this tool.

The array of session objects returned when querying the Win32_LoggedOnUser class have two properties: an antecedent, and a dependent.  The antecedent is the value that contains the ‘human-readable’ information regarding a specific session – the hostname, domain, etc.  The dependent contains a ‘loginID’ value, a unique int corresponding to the specific instance of an account logging into the system.  If a single user logs in & out multiple times prior to a reboot, each instance will receive a unique loginID and thus be tracked independently by the Win32_LoggedOnUser class. 


There wasn’t a whole lot I could do directly with the LoggedOnUser class to filter to only live sessions, but through a bit more exploration of WMI classes I landed on the Win32_SessionProcess class.  Similarly to LoggedOnUser, this class also only returns an antecedent and a dependent.  However, the antecedent and dependent values returned for objects of the SessionProcess class are different, with the antecedent containing the LoginID tied to each active process on the system and the dependent containing a handle to each of these processes.  Although by themselves there isn’t much that can be done with these values, the LoginID returned by SessionProcess can be cross-referenced against the LoginIDs associated with LoggedOnUser objects, giving a listing of actual logins (those that have at least one running process associated with their loginID).


Once this connection had been made, it was fairly straightforward to get session enumeration functionality up and running.  From there, everything was pretty much in its final state as far as functionality goes.  Things were looking good until I started using Wireshark to watch execution across the wire in real-time.  When enumerating sessions using the NetWkstaUserEnum WINAPI function, approximately 15 packets were sent over the wire.  When running session enumeration over WMI, that number was up to ~200 packets.  Quite a bit larger, but makes sense when considering that the session has to be set up and multiple requests have to be made (although if anyone can further update the queries to shave this number further I would be happy to include).  However, when I ran service enumeration, packet counts shot up to a monstrous ~1700 per host.  This was just simply too high for my liking, and I could imagine network congestion, downed boxes, etc. if this was ran over too many hosts in parallel.




The breakthrough in getting the amount of traffic sent over the wire down was the realization that the WQL query sent to retrieve objects was processed server-side.  WMI connectivity using the Microsoft.Management.Infrastructure namespace involves creating a CimSession to a remote host, which in turn is queried using a WQL (WMI-Query-Language) query.  This is a SQL-like statement that can be used to retrieve data based on certain criteria.  I had (mistakenly) assumed that filters applied to these queries (ex. select * from Win32_Service where startname like ‘%admin%’) would be applied to data after it was returned to our system; or in other words all the data would be pulled back across the wire, and then filtered using the given rules prior to displaying.  Luckily, I found this not to be the case, and the entire query is sent to the remote host where it is processed on their system.  From there, only results that match the given filter are sent back over the wire to our system.  Almost all services can be filtered out, as we’re not interested in those running under ‘default’ accounts such as SYSTEM, LOCAL SERVICE, and NETWORK SERVICE. With these new filters applied, traffic for service is down to a much more manageable ~170 packets per host (varies with # services identified).


One other interesting point that became apparent as I analyzed traffic from both WMI and API-based enumeration methods, this method uses solely RPC connections, whereas API methods use SMB to remotely pull information.  There are definitely improvements that can be made to this as well, API methods would likely be faster and may potentially be even lighter from a network traffic perspective (depending on what filtering can be done prior to returning service information), and the current queries could likely be further refined to likewise reduce traffic further. Overall though, with this last hurdle overcome, I figured the tool was in a decent enough place to release. 


wmiServSessEnum Usage


Like other tools that use WMI to connect to other systems, admin rights are required on the remote system.

An IP/ comma-separated list of IPs is required to be entered in on the command line when executing the tool, or a reference to a file on the local system containing one IP per line to target.  I looked into incorporating CIDR notation into the tool, but ultimately decided against it, so as of now only specific IP addresses are supported.  Ideally this shouldn’t be a huge deal as the addresses that are being tested are ones that you already have valid credentials for, meaning initial network enumeration has already occurred.  

By default the tool will use whatever credentials you’re currently running your session as, but also accepts username+domain+plaintext passwords (use a domain of ‘.’ For a local user).

WmiServSessEnum can be ran in several different modes:
·        sessions  – similar to other user enumeration methods, will return a list of active sessions on the remote system
·        services – returns a list (if any) of non-default accounts configured to run services on the remote system
·        all (default) – runs both

flags should be inputted in the format of –u=UserName etc.

When everything works you should get something back that looks like this when running against a remote system: