A massive campaign spreading the Dofoil Trojan has been detected by Microsoft. The campaign has already seen almost half a million PCs infected with the malware in just 12 hours. The Dofoil Trojan is otherwise known as Smoke Loader – a downloader that has been active for several years.

The Dofoil Trojan is a small application which once installed on a PC is capable of downloading other forms of malware. The Dofoil Trojan has been used in various campaigns since at least 2011 to install malware, with the latest campaign used to install cryptocurrency mining malware.

More than 400,000 Dofoil Trojan Infections Detected in Just 12 Hours

The alarm was raised on March 6 when Windows Defender detected around 80,000 instances of the Trojan on PCs with the number rising rapidly to well over 400,000 in the following 12 hours.  Several variants of the Dofoil Trojan were being used in the campaign which was mostly targeting devices in Russia, Ukraine, and Turkey.

The cryptocurrency mining malware is being used to mine Electroneum coins on infected devices, although the malware can mine various different cryptocurrencies.

Detecting the malware can be difficult as it uses process hollowing to create a new instance of a legitimate Windows process for malicious purposes. In this case the malware is disguised as a Windows binary file to avoid detection – wuauclt.exe. Explorer.exe is used to create a copy of the malware in the Roaming AppData folder which is renamed ditereah.exe. The Windows registry is also altered to ensure persistence, modifying an existing entry to point to the malware copy. The malware communicates with its C2 server and is also capable of installing further malware variants onto an infected device.

While Microsoft was able to detect infections, what is not known at this stage is how the malware was installed on so many devices in such as short space of time. While the malware could potentially have been distributed by spam email, another means of distribution is suspected. Microsoft notes that in several cases the malware is believed to have been spread via torrent files, which are used in P2P file sharing, often to obtain pirated movies, music, and software.

Microsoft has only reported on the number of infections it has detected via Windows Defender. The company does not have visibility into devices that do not have the anti-malware software installed. The total number of infections is therefore likely to be far greater. The 400,000+ infections are likely to be just the tip of the iceberg.

Microsoft notes that its efforts to disrupt the operation did not just stop devices from mining cryptocurrencies. Infection with the Dofoil Trojan allows the attackers to download any number of additional malicious payloads including more dangerous malware variants and ransomware.

More than 50,000 Websites Discovered to Host Cryptocurrency Mining Malware

According to a recent Bad Packets report, more than 50,000 websites have been infected with cryptocurrency mining malware which runs via browsers whenever visitors are on a compromised website. The JavaScript-based malware uses the CPUs of visitors’ computers to generate cryptocurrencies.

While many of these sites have been hijacked and loaded with the malware without the knowledge of the site owner, many sites have the JavaScript added deliberately by site owners as a further method of making money from the websites.

These sites do not result in infection with malware. Typically, the only problems experienced by website visitors is a slowing down of their computers. However, in some cases, the malware has been configured to take full advantage of visitors’ computers and some hardware damage has been caused as a result.

Since it is difficult to determine which sites have been infected or are using cryptocurrency miners, the solution for users is to use a browser extension such as minerBlock to prevent the scripts from running. Users of the Opera browser need do nothing as the browser already blocks cryptocurrency mining scripts from running.