Hand to Hand Combat: Finding Responder.py on your network using Wire Data

I was listening to the Security Weekly podcast this week with Paul Asadoorian and John Strand and I heard them talking about a product called Mazerunner that can detect responder.py running on your network. I began to wonder if there was a way to detect responder.py using the ExtraHop platform. So I downloaded a Kali OVA file and started looking into what the traffic looked like on the wire to see if I could make some sense of it. What I found was that I was, in fact, able to consistently detect Responder.py traffic by looking at LLMNR responder packet itself coming from the Kali server.

What I observed:
Over the last four years, I have literally spent my time pulling metadata and PCAPs at the core router or via some sort of Span aggretation like Gigimon or Arista. I can honestly say, in the realm of protocols that I come across LLMNR (udp 5355) is not one I come across very often. It is possible this is due to the request normally being sent out to a broadcast address (rarely the focus of my work) but seeing a response from an RFC1918 address was somewhat interesting. What I mean is, I would start up responder on my Kali box then see the request to the broadcast address with the response sourcing from the Kali system with its own separate flow. An analogy would be like if you asked a crowd “Where is TypoShare?” and someone, wearing a trench-coat, fedora and sunglasses, said “I got ya’ TypoShare right hereā€¦.just log in”. On the wire, this is easy to pick out as it does not respond over the ephemeral port rather it creates a new flow so I am able to easily parse it out as the Kali server appears as the sender and the Windows domain workstation appears as the receiver. This alone does NOT make a malicious situation but I want to start the process of gathering internal threat intelligence by keeping track of the system that responded to the LLMNR request.

Below is an example of a user “fat fingering” a share and having the lookup answered by the malicious Kali host (or anyone who basically “git clone” s Responder.py)
Note below, you see the Kali system located at answer the LLMNR request and provide name resolution for a file server/share that does not exist. It is not entirely uncommon for a large enterprise for someone to have a typo in a CIFS share name.


Below we see that we have observed someone answering an LLMNR/NBNS request

The next step is to write the system answering LLMNR requests to our session table so that we can look them up later. (cip and sip are variables for Client and Server IP respectively)

ExtraHop IOC protocol threading:


So what exactly is happening here?
For this test, I wanted to check a number of IOCs around SQL (TDS), Web Proxying and WPAD MITM hijacking as well as Hash stealing. To research this with logs, you would need to take the time to interrogate several different source log files then try to mash them up. I am trying to do this within seconds and in-flight vs. after they have been written to the SIEM. So to do this, I use the ExtraHop “Session” table which is a memcache key value pair location where I will part the system answering LLMNR queries and see if it shows up again when I match additional IOCs around CIFS, HTTP and TDS. (If that didn’t make sense, feel free to email me).

So the next step is to check for any type of hijacking/Tunneling that could occur as a result of someone running responder.py on your network. First let’s have a look at HTTP Tunneling (WPAD Proxy hijacking).

Responder.py tries to hijack proxy sessions on browsers that are set to auto-discover proxy servers. In this case, I open my browser and the Kali system happily resolves WPAN for me and offers to be my proxy server for my web session, in addition to obtaining my hash it may also prompt me for creds because, hey, clear text is always better than a hash.

How do we catch this?
To catch this, we look at the HTTP headers (in real time and at, up to, 40 Gbps) for the host WPAD and if the result is true, we check the server IP against the IP address we put in the session table earlier, if that ALSO comes back true, then we flag it as a potential actionable event. You may actually have a WPAD server in your environment, but I am comfortable in saying that you likely don’t have a WPAD server that had previously been resolving LLMNR and/or NBNS name requests. That makes it suspicious and actionable, the goal here is to give CSIRT and the SOC BETTER data, not more data. Also, keep in mind, all of this is being interrogated in flight so #LookMomNoSIEM!!! We can also send the data to a Syslog/SIEM workflow which would likely be a welcome change to get pre-parsed actionable Intel sent to the SIEM vs. thousands of logs that must be sifted through to find actionable intelligence.

Likewise, with Tunneling we do something similar where we look for the Tunneling protocol consistent with proxy servers and if we see it, we check the server IP against our LLMNR host from earlier (again, “sip” is a variable for Server IP). As I said before, you may have a proxy server but likely not one that speaks LLMNR.

Below you see the results, here we are just logging them to the debug window but we offer a number of ways to deal with IOC discovery that include but are not limited to:

  • ServiceNow SEVONE ticket creation
  • Syslog to a SIEM based workflow or orchestration
  • REST API HTTP.post to a workflow or orchestration solution



SQL Server Credential Stealing:
Responder.py can also listen for SQL Server browser requests, if someone installs SQL Management Studio on their system then proceeds to browse the network for a SQL Server, the Kali system running responder will offer them a system. Most folks would be curious and likely click on it and when they did so, would hand over their hash, likely a developer account that has access to sensitive data, or they will try to offer it SQL credentials and then the Kali server will have their SQL based creds. The first time I saw this I was imagining the horror of someone literally stealing hashes and creds and developers frustratingly offering their creds to it. So to combat this, we will look for the SQL browsing protocol (udp:1434) and if we see it, as before, we will check the server IP against our LLMNR responder that we parked in memcache (Session Table) to see if it matches. The caveat here is that I am not sure if the SQL Browser does not actually use LLMNR natively, I can say that I don’t see it very often when my head is in the packets at customer sites but for my part, I have spent the last 5 years disabling this service on most SQL Servers for security reasons. At any rate, we can detect it with regularity and many best practices advise disabling the SQL Browser service anyway so if it shows up on your network with anything other than a broadcast address it could be an issue.

Below you see the trigger logic were we check for the presence of the SQL Browsing traffic then we check it against the IP we put in the session table.


If we see a match, here we are warning inside the debug window but in an implementation this would be sent to the SIEM, ServiceNow, an Email alert or some form of orchestration to initiate an incident response.


Watching CIFS to see if hashes are being stolen:
The last thing I want to show in this post is how to detect if someone tries to steal a hash using CIFS. (This can also be done using WPAD but I will cover that in a later post).

If an end user inadvertently types the wrong share name into a browser, either via explorer or the “net use” command the normal name resolution will fail causing the system to broadcast the request. The Responder.py system will answer this request via NBT-NS or LLMNR and offer the user to type credentials (see graphic below). What I noticed on the wire is that there is a CIFS message for “SMB2_SESSION_SETUP: STATUS_ACCESS_DENIED” even though there wasn’t an official request thus you see the access denied message in the CIFS dialog box for the share \\1233\.

Casting the web:
So in preparing the trigger for this, I simply look for the error string then, as we have seen previously, cross-checked the IP address against those that have responded to LLMNR requests. In the event of a match, we are writing it out to the debug window but depending on the customer we would run the normal Incident Response regimen.


It had been a long time since I had used any PEN testing tools. One thing I have to say, after 20 years of being a blue-teamer, is DAMN, the red team’s toys are SO MUCH cooler than ours!! This is not an attempt to one up Security weekly or Mazerunner, it was an interesting use case that I had not tried to tackle before with the platform and it made for an interesting weekend project. If you are curious, this took a few hours to write once I got all of the systems in place (AD lab, Kali system). While we will write all of the code that we include with our specific security bundles, this is an example of how we have an open platform that, if desired, you can engage in your own version of hand-to-hand combat with IOCs and bad actors. In this example, we have not had to set up any honey-pots or misdirection, there are products that already do that. We are simply looking at the existing traffic and parsing out behavior that is indicative of an IOC. One of the challenges of doing this with logs or machine data is that if an IOC uses more than one protocol than you likely have disparate logs/PCAPs for each protocol and researching each data source is not agile at all. Using the Session table, we are able to park specific characteristics to be referenced in real time (in Microseconds) and reused with other transactions threading them together to create a picture within seconds, this is the true power being handed back to the blue team using our platform. With ExtraHop’s application inspection triggers you have the ability to engage directly with the wire to root out IOCs like LLMNR responses, SQL Browsers, SMBv1, expired/weak certificates, etc. This gives you the same agility that your adversaries have allowing you to pivot your surveillance to what is relevant for your environment. Lastly, (or any other RFC1918 address) will NEVER be in your TAXII/STIXX feed or Threat Intelligence subscription. Leveraging a surveillance platform like ExtraHop will position you to be able to gather and build your own internal threat intelligence on your internal addresses. As most breaches being/source from the inside, the ability to create internal intelligence is critical.

Thanks for reading!



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