Dharma Ransomware Uses AV Tool to Distract from Malicious Activities

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by Raphael Centeno

The Dharma ransomware has been around since 2016, but it has continued to target and successfully victimize users and organizations around the world. One high profile attack happened in November 2018 when the ransomware infected a hospital in Texas, encrypting many of their stored records; luckily the hospital was able to recover from the attack without paying the ransom. Trend Micro recently found new samples of Dharma ransomware using a new technique: using software installation as a distraction to help hide malicious activities.

Dharma ransomware actors abuse AV tool

New samples of Dharma ransomware show that it is still being distributed via spam mail. Typical of spam, the message pressures users into downloading a file. If a user clicks on the download link, they will be prompted for a password (provided in the email message) before getting the file.

Figure 1. Dharma ransomware infection chain

The downloaded file is a self-extracting archive named Defender.exe, which drops the malicious file taskhost.exe as well as the installer of an old version of ESET AV Remover renamed as Defender_nt32_enu.exe. Trend Micro identifies taskhost.exe as a file connected to the Dharma ransomware (detected as RANSOM.WIN32.DHARMA.THDAAAI)

Figure 2. Spam mail for Dharma ransomware
Figure 3. Running the self-extracting archive (Defender.exe)

The ransomware uses this old ESET AV Remover installer, which appears unmodified based on initial scanning, to divert attention as it encrypts files on the victim’s device. When the self-extracting archive runs, Dharma starts encrypting files in the background and the ESET AV Remover installation begins. The user will see the ESET GUI onscreen, a distraction from Dharma’s malicious activities.

Figure 4. Software installation distracts from the ransomware’s activities
Figure 5. Software installation runs on a different instance than malware

The AV Remover is a working tool that goes through the familiar installation routine if it is executed. However, the ransomware will still encrypt files even if the installation is not started. The malware runs on a different instance than the software installation, so their behavior is not related.

The tool is legitimate software bundled with the malware, so user interaction is necessary to fully install it. The ransomware will run even if the tool installation is not triggered, and the tool can be installed even if the ransomware does not run. The installation process seems included just to trick users into thinking no malicious activity is going on.

Figure 6. The ESET installer file also has a valid digital signature, so this also helps it stay under the radar.

Cybercriminals have a history of abusing authentic tools, and this recent Dharma tactic of using an installer as a diversion or screen of legitimacy is simply another method they are experimenting with. This new version is designed to trick users and allow the ransomware to stealthily operate in the background. As malware authors continue to adopt layered evasion tactics and malicious techniques, users also have to adopt stronger and smarter security solutions to protect their assets.

ESET was informed of this research before publishing and issued this response:

The article describes the well-known practice for malware to be bundled with legitimate application(s). In the specific case Trend Micro is documenting, an official and unmodified ESET AV Remover was used. However, any other application could be used this way. The main reason is to distract the user, this application is used as a decoy application. ESET threat detection engineers have seen several cases of ransomware packed in self-extract package together with some clean files or hack/keygen/crack recently. So this is nothing new.
In the specific case described by Trend Micro, the ransomware is executed right after our remover application, but the remover has a dialogue and waits for user interaction, so there is no chance to remove any AV solution before the ransomware is fully executed.

How to defend against ransomware

There has been a growing awareness about ransomware as well as improved solutions for organizations and users, which contributes to ransomware’s continuing decline. However, as proven by the new samples of Dharma, many malicious actors are still trying to upgrade old threats and use new techniques. Ransomware remains a costly and versatile threat; earlier this month a ransomware family was spotted targeting vulnerable Samba servers. This particular ransomware first emerged as a threat targeting victim’s network-attached storage device before it evolved to target other devices.

Users and organizations should prepare for Dharma and similar threats by adopting good cybersecurity hygiene. Some best practices to follow include:

Indicators of Compromise

File NameSHA256Detection
taskhost.exe1         703b57adaf02eef74097e5de9d0bbd06fc2c29ea7f92c90d54a0b9a01172babeRansom.Win32.DHARMA.THDAAAI
Defender_nt32_enu.exe10d7e4d980ae644438ee17c1ea61ac076983ec3efb3cc9d3b588d2d92e52d7c83normal ESET AV remover     


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