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Malware Hidden in Encrypted Traffic Surges Amid Pandemic

Zscaler says attacks involving the use of SSL/TLS encryption jumped 260% in the first nine months of 2020 compared to the same period last year.

The fact that attackers hide malware inside encrypted traffic is not news any longer. What might be surprising, however, is just how much the practice has recently spiked due in part to the shift to remote work in the COVID-19 pandemic.

Researchers from Zscaler analyzed attack data gathered from customers of the company's cloud security platform between January and September this year. The security vendor discovered a 260% increase over 2019 in the number of encrypted attacks that it handles per month.

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In the first nine months of this year, Zscaler's cloud blocked an average of 773 million attacks per month where malware was hidden in encrypted traffic. Last year, that number was around 283 million. In the first nine months of this year, Zscaler blocked some 6.6 billion encrypted attacks. 

Ransomware attacks delivered via SSL/TLS channels soared 500% between March and September, with a plurality of the attacks (40.5%) targeted at telecommunication and technology companies.

Healthcare organizations were targeted more so than entities in other verticals and accounted for 1.6 billion, or over 25%, of all SSL-based attacks Zscaler blocked this year. Finance and insurance companies clocked in next with 1.2 billion or 18% of attacks blocked, and manufacturing organizations were the third-most targeted, with some 1.1 billion attacks directed against them.

Deepen Desai, CISO and vice president of security research at Zscaler, says the trend shows why security groups need to be wary about encrypted traffic traversing their networks.

While many organizations routinely encrypt traffic as part of their security best practices, fewer are inspecting it for threats, he says.

"Most people assume that encrypted traffic means safe traffic, but that is unfortunately not the case," Desai says. "That false sense of security can create risk when organizations allow encrypted traffic to go uninspected."

Zscaler's analysis showed an in increase in SSL/TLS based attacks delivered through trusted cloud storage services such as Google, AWS, Dropbox, and OneDrive. In many of these attacks, cybercriminals hosted malicious content on these services and then lured users to the content via phishing emails and other social engineering methods. Some 30% of all SSL/TLS-based attacks that Zscaler blocked in the first nine months of 2020 — double the number from last year — involved the use of such trusted services.

"Cloud storage services have seen a marked increase as a popular means of attack," Desai says. "Organizations often consider these services [as] trusted, allowing them to pass by uninspected, and offering criminals a way through security checks."

Attackers have been using encrypted traffic to hide malware for some time. But the large-scale pandemic-related shift to remote work and the resulting increase in the use of cloud-based collaboration this year appears to have accelerated the practice, according to Zscaler.

The trend has posed a big challenge for organizations. As more traffic gets encrypted, it has become mission-critical for organizations to be able to inspect it, he says. However, legacy security tools often lack the performance and capacity to inspect encrypted traffic at scale.

"This is one of the biggest challenges — in addition to privacy concerns — that prevents most organizations from inspecting all encrypted traffic," he says.

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