Malware loaders (also known as droppers or downloaders) are a popular commodity in the criminal underground. Their primary function is to successfully compromise a machine and deploy one or multiple additional payloads.

A good loader avoids detection and identifies victims as legitimate (i.e. not sandboxes) before pushing other malware. This part is quite critical as the value of a loader is directly tied to the satisfaction of its “customers”.

In this blog post, we describe a malvertising campaign with a loader that was new to us. The program is written in the Go language and uses an interesting technique to deploy its follow-up payload, the Rhadamanthys stealer.

PuTTY is a very popular SSH and Telnet client for Windows that has been used by IT admins for years. The threat actor bought an ad that claims to be the PuTTY homepage and appeared at the top of the Google search results page, right before the official website.

In this example, the ad looks suspicious simply because ad snippet shows a domain name (arnaudpairoto[.]com) that is completely unrelated. This is not always the case, and we continue to see many malicious ads that exactly match the impersonated brand.

Fake PuTTY site

The ad URL points to the attacker controlled domain where they can easily defeat security checks by showing a “legitimate” page to visitors that are not real victims. For example, a crawler, sandbox or scanner, will see this half finished blog:

Real victims coming from the US will be redirected to a fake site instead that looks and feels exactly like putty.org. One of the big differences though is the download link.

The malicious payload is downloaded via a 2 step redirection chain which is something we don’t always see.

puttyconnect[.]info/1.php
HTTP/1.1 302 Found
Location: astrosphere[.]world/onserver3.php

astrosphere[.]world/onserver3.php
HTTP/1.1 200 OK
Server: nginx/1.24.0
Content-Type: application/octet-stream
Content-Length: 13198274
Connection: keep-alive
Content-Description: File Transfer
Content-Disposition: attachment; filename="PuTTy.exe"

We believe the astrosphere[.]world server is performing some checks for proxies while also logging the victim’s IP address. This IP address will later be checked before downloading the secondary payload.

That PuTTy.exe is malware, a dropper written in the Go language (version 1.21.0).

Its author may have given it the name “Dropper 1.3“:

Follow-up payload

Upon executing the dropper, there is an IP check for the victim’s public IP address. This is likely done to only continue with users that have gone through the malicious ad and downloaded the malware from the fake site.

zodiacrealm[.]info/api.php?action=check_ip&ip=[IP Address]

If a match is found, the dropper proceeds to retrieve a follow-up payload from another server (192.121.16[.]228:22) as seen in the image below:

To get this data, we see it uses the SSHv2 (Secure Shell 2.0) protocol implemented via OpenSSH on a Ubuntu server. We can only think of using this protocol to make the malware download more covert.

That payload is Rhadamanthys which is executed by the parent process PuTTy.exe:

Malvertising / loader combo

We have seen different types of loaders via malvertising campaigns, including FakeBat which we profiled recently. Given how closely the loader is tied to the malvertising infrastructure it is quite likely that the same threat actor is controlling both. The service they offer to other criminals is one of malware delivery where they take care of the entire deployment process, from ad to loader to final payload.

We reported this campaign to Google. Malwarebytes and ThreatDown users are protected as we detect the fake PuTTY installer as Trojan.Script.GO.

ThreatDown users that have DNS Filtering can enable ad blocking in their console to prevent attacks that originate from malicious ads.

Indicators of Compromise

Decoy ad domain

arnaudpairoto[.]com

Fake site

puttyconnect[.]info

PuTTY

astrosphere[.]world
0caa772186814dbf84856293f102c7538980bcd31b70c1836be236e9fa05c48d

IP check

zodiacrealm[.]info

Rhadamanthys

192.121.16[.]228:22
bea1d58d168b267c27b1028b47bd6ad19e249630abb7c03cfffede8568749203