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Story of the Cutwail/Pushdo hidden C&C server


This is a loose sequel to the Cutwail botnet analysis blogpost published on the In this blogpost I will primarily focus on the downloaded PE executable itself (SHA256: 5F8FCC9C56BF959041B28E97BFB5DB9659B20A6E6076CFBA8CB2D591184C9164) and the network traffic that it generates. I will also reveal a hidden C&C server.

But first let’s quickly go through the things it does at the beginning:
– It registers an exception handler that will only start the process again using CreateProcess().
– It performs a check whether it has admin privileges.
– It checks or creates a mutex named “xoxkycomvoly” (hardcoded identifier used on multiple occasions).
– It checks or creates couple of registry entries under HKCU\Software\Microsoft\Windows\CurrentVersion.
– It checks if the process image filename is “xoxkycomvoly.exe” (it restarts for the first time).
– It nests into the system by creating autorun entry in registry under HKCU\Software\Microsoft\Windows\CurrentVersion\Run.
– It copies itself to the user’s profile directory named as “xoxkycomvoly.exe”.
Then on the first time an exception occurs and the sample is restarted from the user’s profile location named as “xoxkycomvoly.exe”.
Initial startup activities

Initial startup activities

After these initial steps, the sample starts communicating heavily over the network.

The sample contains number of hardcoded hostnames in a plain text form, making a little bit of an impression that it is a simple program, but those are rather only decoys. Let’s go chronologicaly and see what it does exactly.

The first group of hardcoded hostnames that takes place is the following list of 6 SMTP servers:

These SMTP servers are only used to try outgoing TCP connection on port 25, to see if it is filtered in any way (to see if it’s gonna be able to send spam).

Then there is another list of plain text hostnames, which look somewhat suspiciously:

Almost all of these hostnames listen on port 25, giving a premature idea that they may be the SMTP relays through which it sends spam, but the sample doesn’t really do anything much with these, they are only supposed to distract us from the real business!

The interesting thing that comes next is that the sample also contains several enxored data blobs, each one enxored with different xor key, and these blobs contain some very interesting things, hidden from the plain sight.

The first data blob that gets dexored is this list of hostnames:

1st group of hidden hostnames

1st group of hidden hostnames

These dexored hostnames look like a good candidates for C&C servers to me. Indeed as the next step the sample is trying to connect to one of these hostnames on port 443 (HTTPS) possibly to obtain a command (Cutwail/Pushdo uses custom binary executable modules). Usage of the HTTPS protocol prevents the actual content to be inspectable in the network capture records, but through reverse engineering we can see that it requests GET /. If the server responds with a reasonable reply (at least 1024 bytes long), it proceeds to check whether the response contains the familiar HTML mark (which is also enxored btw):
<img src=”data:image/jpeg;base64

If that’s indeed found in the server response, the data after the mark (obviously not a jpeg) are decrypted with some interesting decryption routines and we get a binary executable (Cutwail/Pushdo module). Then a new svchost.exe process is started in a suspended state and the binary is injected into it and executed.
On the other hand, if the obtained server response does not contain the special HTML mark, no other C&C server from this list is tried. It is simply happy with a web server test page, having no orders from the C&C server. Seems a bit fishy!

Search for special <img decoding=

Search for special “<img src” mark

Anyway, next enxored blob that comes to the program flow turns out to be this large list of domains:

2nd group of hidden hostnames

2nd group of hidden hostnames

Pretty impressive list of 200 hostnames, actually only 176 unique:,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

The next thing is that it starts a pair of 8 threads. One pair will connect randomly to these hosts on port 80 and send bogus HTTP requests (like those infamous ptrxcz_ POST requests) and the other pair will connect randomly to those hosts on port 25 and just send some random data to it, not even SMTP traffic, not even waiting for the server banner. This goes on in an endless loop, in the background, 16 noisy threads sending bogus traffic, apparently trying to mask some activity, which is about to start soon.
Start 16 threads generating a bogus traffic noise

Start 16 threads generating a bogus traffic noise

Bogus HTTP 1

Bogus HTTP 2Bogus HTTP traffic examples

Bogus SMTP 1

Bogus SMTP 2Bogus SMTP traffic examples

OT: Then the sample collects some informations about our client machine such as disk, network adapter info, operating system version etc, encrypts it and sends it to over HTTP in the background:

Reporting client machine info to

Reporting client machine info to

Now there is an interesting code block in the program, where it iterates through all those 200 dexored hostnames (victims of bogus traffic). It calculates some hash value from each one and then compares it to some strange hardcoded value, which only now started to make sense. The comparison condition triggers when comparing “” hostname, which happens to match the hash. This is the special C&C server, whose identity is primarily being masked. It’s not only hidden in the traffic noise from those 16 annoying threads in the background, but also being supposedly one of the victims.

Iterate through all the hosts from 2nd group and find the special one

Iterate through all the hosts from 2nd group and find the special one

This special C&C server is then used to obtain commands (modules) from in the final stage of the malware, where it enters an endless loop and contacts the special C&C server for orders, modules to load. If it is unable to obtain an order from it, then it resorts to generate those characteristic KZ domain names with the DGAs and try to obtain an order from them. Personally I have seen orders to come only from the special C&C “”. When an order comes and everything goes fine, the sample sleeps for 12 hours, otherwise it sleeps for 2 minutes to repeat the loop and get some command.

Communication with the C&C server:
Obtained 4 modules from this one

Obtained 4 modules from this one

Cutwail/Pushdo module decrypted in the memory:
Looks like a typical executable file

Looks like a typical executable file

The modules then perform actual spamming and other malicious activities.

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