Decade-old malware strains top annual list of most pervasive business exploits

The Windows logo on a phone in front of a malware warning
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US and Australian cyber security authorities have jointly revealed the top malware strains that targeted organisations in 2021, with two of them in operation for longer than a decade.

The US’ CISA and Australia’s ACSC said the most pervasive strains included remote access trojans (RATs), banking trojans, information stealers, and ransomware.

Qakbot and Ursnif are two of the top strains that have been in operation for the longest. Both authorities said this is because they have been under active development, with operators consistently adding new capabilities and methods to evade detection.

Most strains in the list have been in operation for longer than five years and their respective codebases evolved over that time into various variations.

The most prolific of the bunch, the authorities said, were stealers of financial or personal information, and ransomware.

The top 11 malware strains of 2021

Swipe to scroll horizontally
Malware strainType of malwareActive sinceDelivery method
Agent TeslaInformation stealer, RAT2014Phishing (attachments)
AZORultInformation stealer2016Phishing, exploit kits, infected websites
FormbookInformation stealer2016Phishing (attachments)
UrsnifBanking trojan2007Phishing (attachments)
LokiBotTrojan, information stealer2015Phishing (attachments)
MOUSEISLANDMacro downloader2019Phishing (attachments)
NanoCoreRAT2013Phishing (attachments), cloud storage
QakbotMulti-use trojan2007Phishing (attachments, hyperlinks, embedded images)
RemcosRAT2016Phishing (attachments)
TrickbotTrojan2016Phishing (hyperlink)
GootLoaderMalware loader2020Compromised websites

Overview of 2021's most pervasive malware strains

Agent Tesla

Around since 2014, the powerful tool can be used to steal information from email clients, web browsers, and file transfer protocol (FTP) servers, as well as capture screenshots and video from a desktop environment.


An information stealer that can be found available on underground hacking forums, AZORult is under constant development, the authorities said, and its capabilities include stealing browser data, user credentials, and cryptocurrency information.


Formbook is a malware strain that's consistently changed, according to the latest threats published in the common vulnerabilities and exposures (CVS) list, aiming to infect systems that have been left unpatched to the latest threats.

It's capable of keylogging and capturing passwords, and has been used in a variety of attacks in the past year such as those specifically targeting corporate email inboxes.


The banking Trojan Ursnif has been around since 2007, tying with Qakbot as the longest-running malware strain on the list. It has evolved to adopt a persistence mechanism, meaning that it can live on a system after it has rebooted, and can also avoid sandboxes and virtual machines, the authorities said.


This Trojan is designed to various types of steal sensitive information, such as user credentials and those to access cryptocurrency wallets. In circulation since 2015, it had a notable variant in 2020 that disguised itself as a launcher for the popular video game Fortnite.


This is one that's likely to drop off the list next year now Microsoft has blocked VBA macros by default, but the macro downloader has been prolific since 2019 and is thought to be used in the initial stages of some ransomware attacks.


The RAT NanoCore can allow attackers to spy on victims through webcams while also doubling as a stealer of passwords and emails. It's one of the oldest strains on the list beginning operation in 2013.


Qakbot was originally a banking Trojan, but since its 2007 inception, its capabilities have evolved to include data exfiltration and the capacity to deliver other malicious payloads. It’s modular in nature, allowing attackers to tailor its capabilities to their needs.


A lexical blend that’s short for Remote Control and Surveillance, Remcos is presented as a legitimate penetration testing tool but has been abused by cyber attackers, much like Cobalt Strike and more recently Brute Ratel C4. It can steal personal data and login credentials, and was used heavily in COVID-19-themed phishing campaigns.


This Trojan is thought to be operated and maintained by a sophisticated threat group, and has been used in the past as the initial exploit to deploy Conti and Ryuk ransomware. It has also been used against healthcare organisations to steal data and disrupt services.


Around since 2020 and now a multi-payload malware platform, Gootloader has evolved in recent years from a simple malware loader, typically associated with GootKit malware. It often provides attackers with the initial access exploit, usually via search engine poisoning.

What mitigations can your business deploy?

The authorities recommend reviewing and implementing all the necessary mitigations to defend against these malware strains - the ones targeting businesses the most.

The full list of instructions can be found in the complete joint advisory issued by CISA and ACSC this week, but recommendations include updating software against known vulnerabilities, enforcing the use of multi-factor authentication (MFA) across the organisations, monitor use of remote desktop protocol and maintain offline backups of data.

Connor Jones

Connor Jones has been at the forefront of global cyber security news coverage for the past few years, breaking developments on major stories such as LockBit’s ransomware attack on Royal Mail International, and many others. He has also made sporadic appearances on the ITPro Podcast discussing topics from home desk setups all the way to hacking systems using prosthetic limbs. He has a master’s degree in Magazine Journalism from the University of Sheffield, and has previously written for the likes of Red Bull Esports and UNILAD tech during his career that started in 2015.