Top 10 Malware May 2021

In May 2021, the MS-ISAC observed Quasar’s return to the Top 10. The Top 10 Malware variants comprise 75% of the total malware activity in May 2021, decreasing 1% from April 2021. Shlayer is likely to continue its prevalence in the Top 10 Malware for the coming quarter as more organizations move back to an in-person model. A recent patch from Apple addresses the zero-day vulnerability Shlayer used, which is likely affecting its campaign as we saw a significant decrease from the previous month.


MS-ISAC Malware Notifications May 2021


Top 10 Malware May 2021



In May 2021, malvertisement accounted for the greatest number of alerts. Malvertisement continues as the top initial infection vector due to Shlayer activity. Activity levels for dropped and malspam increased, while activity for malvertisement and multiple decreased.

Shlayer activity dropped from the previous month likely due to the recent patch from Apple affecting its campaign. The patch fixes a zero-day vulnerability exploited by Shlayer to bypass Apple’s Gatekeeper, File Quarantine, and Notarization security checks. This patch will likely affect Shlayer’s ability to download second-stage malicious payloads, as well as its ability to download its own payload. That said, it might not necessarily stop its initial infection vector as a whole. It is likely that malvertisement will remain the primary infection vector as the Shlayer campaign pans out.


TOP 10 Malware Initial Infection Vectors May 2021


Dropped – Malware, delivered by other malware already on the system, an exploit kit, infected third-party software, or manually by a cyber threat actor. Gh0st is the only malware dropped.

Multiple – Malware that currently favors at least two vectors. Currently, CoinMiner, CryptoWall, and ZeuS are the malware utilizing multiple vectors. ZeuS is dropped by other malware, but it is also delivered via malvertisement.

Malspam – Unsolicited emails, which either direct users to malicious web sites or trick users into downloading or opening malware. The Top 10 Malware using this technique are Agent Tesla, Hancitor, NanoCore, and Quasar.

Malvertisement – Malware introduced through malicious advertisements. Currently, Shlayer and CopperStealer are the only Top 10 Malware using this technique.

Top 10 Malware and IOCs

Below are the Top 10 Malware ranked in order of prevalence. The respective indicators of compromise (IOCs) are provided to aid in detecting and preventing infections from these Top 10 Malware variants. Note: The associated URIs are aligned with malware’s respective domain(s) or IP(s) and increase the likelihood of maliciousness when found together. The URIs alone are not inherently malicious.

1. Shlayer

Shlayer is a downloader and dropper for MacOS malware. It is primarily distributed through malicious websites, hijacked domains, and malvertizing posing as a fake Adobe Flash updater.
All Shlayer domains follow the same pattern . Below are several examples of domains Shlayer uses.


  • api[.]interfacecache[.]com
  • api[.]scalableunit[.]com
  • api[.]typicalconfig[.]com
  • api[.]standartanalog[.]com
  • api[.]fieldenumerator[.]com
  • api[.]practicalsprint[.]com
  • api[.]searchwebsvc[.]com
  • api[.]connectedtask[.]com
  • api[.]navigationbuffer[.]com
  • api[.]windowtask[.]com

2. CoinMiner

CoinMiner is a cryptocurrency miner that uses Windows Management Instrumentation (WMI) and EternalBlue to spread across a network. CoinMiner uses the WMI Standard Event Consumer scripting to execute scripts for persistence. CoinMiner spreads through malspam or is dropped by other malware.

3. NanoCore

NanoCore is a RAT spread via malspam as a malicious Excel XLS spreadsheet. As a RAT, NanoCore can accept commands to download and execute files, visit websites, and add registry keys for persistence.

4. CryptoWall

CryptoWall is a ransomware commonly distributed through malspam with malicious ZIP attachments, Java vulnerabilities, and malicious advertisements. Upon successful infection, CryptoWall will scan the system for drive letters, network shares, and removable drives. CryptoWall runs on both 32-bit and 64-bit systems.

5. Gh0st

Gh0st is a RAT used to control infected endpoints. Gh0st is dropped by other malware to create a backdoor into a device that allows an attacker to fully control the infected device.

6. Quasar

Quasar is an open-source remote administration tool on the Windows platform. It is used as a RAT to create backdoors.

7. ZeuS

ZeuS is a modular banking trojan which uses keystroke logging to compromise victim credentials when the user visits a banking website. Since the release of the ZeuS source code in 2011, many other malware variants have adopted parts of its codebase, which means that events classified as ZeuS may actually be other malware using parts of the ZeuS code.


  • 176.121.14[.]151


  • /gate.php

8. Agent Tesla

Agent Tesla is a RAT that exfiltrates credentials, logs keystrokes, and captures screenshots from an infected computer.

9. CopperStealer

CopperStealer is an infostealer that infects victims via malvertisement. It has multiple capabilities including anti-analysis, account credential theft (specifically service providers like Facebook, Instagram Twitter, Google, Amazon, and PayPal), data exfiltration, and the ability to drop other malware, such as Smokeloader.

10. Hancitor

Hancitor, also known as Chanitor or Tordal, is a downloader that spreads through malspam containing malicious Microsoft Office documents, links, and attachments. This malware has been known to download additional malware, such as Pony, Ursnif, and Vawtrak.


  • Cationfrob[.]ru
  • Conaboory[.]ru
  • Lpertion[.]com
  • Orialoussin[.]ru
  • Plogesuct[.]com
  • Scientemplud[.]ru
  • Watiounds[.]ru


  • /4/forum.php

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