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Sophos PureMessage for UNIX Documentation. Sophos Anti-Virus is the virus protection software recommended by MIT. This software detects and cleans up viruses, Trojans, worms, spyware, adware and other potentially unwanted applications. The Sophos Diagnostic Utility is included in the Windows package downloaded by Sophos Update Manager (SUM) for Endpoint Security and Control. This version of the Sophos Diagnostic Utility does not use an installer, there will be no Start menu links created and will be updated by Sophos AutoUpdate.

Transport Layer Security has been one of the greatest contributors to the privacy and security of Internet communications over the past decade. The TLS cryptographic protocol is used to secure an ever-increasing portion of the Internet’s web, messaging and application data traffic. The secure HTTP (HTTPS) web protocol, StartTLS email protocol, Tor anonymizing network, and virtual private networks such as those based on the OpenVPN protocol all leverage TLS to encrypt and encapsulate their contents—protecting them from being observed or modified in transit.

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Over the past decade, and particularly in the wake of revelations about mass Internet surveillance, the use of TLS has grown to cover a majority of Internet communications. According to browser data from Google, the use of HTTPS has grown from just over 40 percent of all web page visits in 2014 to 98 percent in March of 2021.

It should come as no surprise, then, that malware operators have also been adopting TLS for essentially the same reasons: to prevent defenders from detecting and stopping deployment of malware and theft of data. We’ve seen dramatic growth over the past year in malware using TLS to conceal its communications. In 2020, 23 percent of malware we detected communicating with a remote system over the Internet were using TLS; today, it is nearly 46 percent.

There’s also a significant fraction of TLS communications that use an Internet Protocol port other than 443—such as malware using a Tor or SOCKS proxy over a non-standard port number. We queried against certificate transparency logs with the host names associated with malware Internet communications on ports other than 443, 80, and 8080, and found that 49 percent of the hosts had TLS certificates associated with them that were issued by a Certificate Authority (CA). A small fraction of the others manually checked used self-signed certificates.

But a large portion of the growth in overall TLS use by malware can be linked in part to the increased use of legitimate web and cloud services protected by TLS—such as Discord, Pastebin, Github and Google’s cloud services—as repositories for malware components, as destinations for stolen data, and even to send commands to botnets and other malware. It is also linked to the increased use of Tor and other TLS-based network proxies to encapsulate malicious communications between malware and the actors deploying them.

Google’s cloud services were the destination for nine percent of malware TLS requests, with India’s BSNL close behind. During the month of March 2021, we saw a rise in the use of Cloudflare-hosted malware—largely because of a spike in the use of Discord’s content delivery network, which is based on Cloudflare, which by itself accounted for 4 percent of the detected TLS malware that month. We reported over 9,700 malware related links to Discord; many were Discord-specific, targeting the theft of user credentials, while others were delivery packages for other information stealers and trojans.

In aggregate, nearly half of all malware TLS communications went to servers in the United States and India.

We’ve seen an increase in the use of TLS use in ransomware attacks over the past year, especially in manually-deployed ransomware—in part because of attackers’ use of modular offensive tools that leverage HTTPS. But the vast majority of what we detect day-to-day in malicious TLS traffic is from initial-compromise malware: loaders, droppers and document-based installers reaching back to secured web pages to retrieve their installation packages.

To gain insight into how usage of TLS in malware has changed, we took a deep dive into our detection telemetry to both measure how much TLS is used by malware, identify the most common malware that leverage TLS, and how those malware make use of TLS-encrypted communications. Based on our detection telemetry, we found that while TLS still makes up an average of just over two percent of the overall traffic Sophos classifies as “malware callhome” over a three-month period, 56 percent of the unique C2 servers (identified by DNS host names) that communicated with malware used HTTPS and TLS. And of that, nearly a quarter is with infrastructure residing in Google’s cloud environment.

Surprise packages

Malware communications typically fall into three categories: downloading additional malware, exfiltration of stolen data, and retrieval or sending of instructions to trigger specific functions (command and control). All these types of communications can take advantage of TLS encryption to evade detection by defenders. But the majority of TLS traffic we found tied to malware was of the first kind: droppers, loaders and other malware downloading additional malware to the system they infected, using TLS to evade basic payload inspection.

It doesn’t take much sophistication to leverage TLS in a malware dropper, because TLS-enabled infrastructure to deliver malware or code snippets is freely available. Frequently, droppers and loaders use legitimate websites and cloud services with built-in TLS support to further disguise the traffic. For example, this traffic from a Bladabindi RAT dropper shows it attempting to retrieve its payload from a Pastebin page. (The page no longer exists.)

We’ve seen numerous cases of malware behaving this way in our research. The PowerShell-based dropper for LockBit ransomware was observed retrieving additional script from a Google Docs spreadsheet via TLS, as well as from another website. And a dropper for AgentTesla (discussed later in this report) also has been observed accessing Pastebin over TLS to retrieve chunks of code. While Google and Pastebin often quickly shut down malware-hosting documents and sites on its platform, many of these C2 sources are abandoned after a single spam campaign, and the attackers simply create new ones for their next attack.

Sometimes malware uses multiple services this way in a single attack. For example, one of the numerous malware droppers we found in Discord’s content delivery network dropped another stage also hosted on Discord, which in turn attempted to load an executable directly from GitHub. (The GitHub code had already been removed as malicious; we disclosed the initial stages of the malware attack to Discord, along with numerous other malware, who removed them.)

Malware download traffic actually makes up the majority of the TLS-based C2 traffic we observed. In February 2021, for instance, droppers made up over 90 percent of the TLS C2 traffic—a figure that closely matches the static C2 detection telemetry data associated with similar malware month-to-month from January through March of 2021.

Covert channels

Malware operators can use TLS to obfuscate command and control traffic. By sending HTTPS requests or connecting over a TLS-based proxy service, the malware can create a reverse shell, allowing commands to be passed to the malware, or for the malware to retrieve blocks of script or required keys needed for specific functions. Command and control servers can be a remote dedicated web server, or they can be based on one or more documents in legitimate cloud services. For example, the Lampion Portuguese banking trojan used a Google Docs text document as the source for a key required to unlock some of its code—and deleting the document acted as a kill-switch. By leveraging Google Docs, the actors behind Lampion were able to conceal controlling communications to the malware and evade reputation-based detection by using a trusted host.

The same sort of connection can be used by malware to exfiltrate sensitive information—transmitting user credentials, passwords, cookies, and other collected data back to the malware’s operator. To conceal data theft , malware can encapsulate it in a TLS-based HTTPS POST, or export it via a TLS connection to a cloud service API, such as Telegram or Discord “bot” APIs.

SystemBC

One example of how attackers use TLS maliciously is SystemBC, a multifaceted malicious communications tool used in a number of recent ransomware attacks. The first samples of SystemBC, spotted over a year ago, acted primarily as a network proxy, creating what amounted to a virtual private network connection for attackers based on SOCKS5 remote proxy connection encrypted with TLS—providing concealed communications for other malware. But the malware has continued to evolve, and more recent samples of SystemBC are more full-featured remote access trojans (RATs) that provide a persistent backdoor for attackers once deployed. The most recent version of SystemBC can issue Windows commands, as well as deliver and run scripts, malicious executables, and dynamic link libraries (DLLs)—in addition to its role as a network proxy.

SystemBC is not entirely stealthy, however. There’s a lot of non-TLS, non-Tor traffic generated by SystemBC—symptomatic of the incremental addition of features seen in many long-lived malware. The sample we recently analyzed has a TCP “heartbeat” that connects over port 49630 to a host hard-coded into the SystemBC RAT itself.

The first TLS connection is an HTTPS request to a proxy for IPify, an API that can be used to obtain the public IP address of the infected system. But this request is sent not on port 443, the standard HTTPS port—instead, it’s sent on port 49271. This non-standard port usage is the beginning of a pattern.

SystemBC then attempts to obtain data about the current Tor network consensus, connecting to hard-coded IP addresses with an HTTP GET request, but via ports 49272 and 49273. SystemBC uses the connections to download information about the current Tor network configuration.

Next, SystemBC establishes a TLS connection to a Tor gateway picked from the Tor network data. Again, it uses another non-standard port: 49274. And it builds the Tor circuit to the destination of its Tor tunnel using directory data collected via port 49275 via another HTTP request. There, the progression of sequential ports ends, and in the sample we analyzed it tries to fetch another malware executable via an open HTTP request over the standard port.

The file retrieved by this sample, henos.exe, is another backdoor that connects over TLS on the standard port (443) to a website that returns links to Telegram channels—a sign that the actor behind this SystemBC instance is evolving tactics. SystemBC is likely to continue to evolve as well, as its developers address the mixed use of HTTP and TLS and the somewhat predictable non-standard ports that allow SystemBC to be easily fingerprinted.

AgentTesla

Like SystemBC, AgentTesla—an information stealer that can also function in some cases as a RAT—has evolved over its long history. Active for more than seven years, AgentTesla has recently been updated with an option to use the Tor anonymizing network to conceal traffic with TLS.

We’ve also seen TLS used in one of AgentTesla’s most recent downloaders, as the developers have used legitimate web services to store chunks of malware encoded in base64 format on Pastebin and a lookalike service called Hastebin. The first stage downloader further tries to evade detection by patching Windows’ Anti-Malware Software Interface (AMSI) to prevent in-memory scanning of the downloaded code chunks as they’re joined and decoded.

The Tor addition to AgentTesla itself can be used to conceal communications over HTTP. There is also another optional C2 protocols in AgentTesla that that could be TLS protected—the Telegram Bot API, which uses an HTTPS server for receiving messages. However, the AgentTesla developer didn’t implement HTTPS communications in the malware (at least for now)—it fails to execute a TLS handshake. Telegram accepts unencrypted HTTP messages sent to its bot API.

Dridex

Dridex is yet another long-lived malware family that has seen substantial recent evolution. Primarily a banking Trojan, Dridex was first spotted in 2011, but it has evolved substantially. It can load new functionality through downloaded modules, in a fashion similar to the Trickbot Trojan. Dridex modules may be downloaded together in an initial compromise of the affected system, or retrieved later by the main loader module. Each module is responsible for performing specific functions: stealing credentials, exfiltrating browser cookie data or security certificates, logging keystrokes, or taking screenshots.

Dridex’s loader has been updated to conceal communications, encapsulating them with TLS. It uses HTTPS on port 443 both to download additional modules from and exfiltrate collected data to the C2 server. Exfiltrated data can additionally be encrypted with RC4 to further conceal and secure it. Dridex also has a resilient infrastructure of command and control (C2) servers, allowing installed malware to fail over to a backup if its original C2 server goes down.

These updates have made Dridex a continuing threat, and Dridex loaders are among the most common families of malware detected using TLS—overshadowed only by the next group of threats in our TLS rogues’ gallery: off-the-shelf “offensive security” tools repurposed by cybercriminals.

Metasploit and Cobalt Strike

Offensive security tools have long been used by malicious actors as well as security professionals. These commercial and open-source tools, including the modular Cobalt Strike and Metasploit toolkits, were built for penetration testing and “red team” security evaluations—but they’ve been embraced by ransomware groups for their flexibility.

Over the last year, we’ve seen a surge in the use of tools derived from offensive security platforms in manually-deployed ransomware attacks, used by attackers to execute scripts, gather information about other systems on the network, extract additional credentials, and spread ransomware and other malware.

Taken together, Cobalt Strike beacons and Metasploit “Meterpreter” derivatives made up over 1 percent of all detected malware using TLS—a significant number in comparison to other major malware families.

And all the rest

Potentially unwanted applications (PUAs), particularly on the macOS platform, also leverage TLS, often through browser extensions that connect surreptitiously to C2 servers to exfiltrate information and inject content into other web pages. We’ve seen the Bundlore use TLS to conceal malicious scripts and inject advertisements and other content into web pages, undetected. Overall, we found over 89 percent of macOS threats with C2 communications used TLS to call home or retrieve additional harmful code.

There are many other privacy and security threats lurking in TLS traffic beyond malware and PUAs. Phishing campaigns increasingly rely on websites with TLS certificates—either registered to a deceptive domain name or provided by a cloud service provider. Google Forms phishing attacks may seem easy to spot, but users trained to “look for the lock” alongside web addresses in their browser may casually type in their personally identifying data and credentials.

Traffic analysis

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All of this adds up to a more than 100 percent increase in TLS-based malware communications since 2020. And that’s a conservative estimate, as it’s based solely on what we could identify through telemetry analysis and host data.

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As we’ve noted, some use TLS over non-standard IP ports, making a completely accurate assessment of TLS usage impossible without deeper packet analysis of their communications. So the statistics sited in this report do not reflect the full range of TLS-based malicious communications—and organizations should not rely on the port numbers related to communications alone to identify potential malicious traffic. TLS can be implemented over any assignable IP port, and after the initial handshake it looks like any other TCP application traffic.

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Even so, the most concerning trend we’ve noted is the use of commercial cloud and web services as part of malware deployment, command and control. Malware authors’ abuse of legitimate communication platforms gives them the benefit of encrypted communications provided by Google Docs, Discord, Telegram, Pastebin and others—and, in some cases, they also benefit from the “safe” reputation of those platforms.

We also see the use of off-the-shelf offensive security tools and other ready-made tools and application programming interfaces that make using TLS-based communications more accessible continuing to grow. The same services and technologies that have made obtaining TLS certificates and configuring HTTPS websites vastly simpler for small organizations and individuals have also made it easier for malicious actors to blend in with legitimate Internet traffic, and have dramatically reduced the work needed to frequently shift or replicate C2 infrastructure.

All of these factors make defending against malware attacks that much more difficult. Without a defense in depth, organizations may be increasingly less likely to detect threats on the wire before they have been deployed by attackers.

SophosLabs would like to acknowledge Suriya Natarajan, Anand Aijan, Michael Wood, Sivagnanam Gn, Markel Picado and Andrew Brandt for their contributions to this report.

The branch office of one

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Prior to the pandemic, there was already a shift in networking underway, with an increasing percentage of the workforce beginning to work from home – at least part-time. This trend has dramatically accelerated over the last year, with the vast majority of organizations either mandating their employees work from home, or strongly encouraging it.

This has transformed many organizations almost overnight into a highly-distributed model with hundreds, if not thousands, of one-person branch offices. The “branch office of one” has become the new normal for many organizations.

This massive shift has created a similarly massive challenge for many IT organizations, who have been scrambling to implement VPN access for their remote workers. As just one example, utilization of our Sophos Connect VPN client with XG Firewall has shot up over 10x to more than 1.4 Million active clients in recent months.

And while VPN technology has been a savior and has served us well, it was never really designed for this new world. VPN can be difficult to deploy and enroll new staff, it can be challenging for end-users to use and creates unnecessary friction, and it does not provide the kind of granular security that most organizations require.

Gartner’s recent report, Solving the Challenges of Modern Remote Access, also highlights the challenges with VPN: licensing, efficiency, relevancy, and suitability for the task.

Protecting your data

If it wasn’t enough that IT organizations are grappling with this massive shift in remote working, the whole industry has come under siege by bad actors and hackers attempting to take advantage of the current situation with increasing attacks on corporate systems and data. The latest Sophos 2021 Threat Report provides an excellent look at how cybercriminals have upped their game.

With a massive collection of branch offices of one and an ever-increasing need for tighter security that is transparent and frictionless, what are the options?

We’re actively working to get Sophos ZTNA, or zero trust network access, into your hands as fast as possible. To help overcome some of the challenges you’re facing with remote workers, it provides a simpler, better, more secure solution to connect your users to important applications and data.

Zero trust network access

ZTNA is founded on the principle of zero trust and is all about verifying the user. It typically leverages multi-factor authentication to prevent stolen credentials from being a source of compromise, then validates the health and compliance of the device to ensure it’s enrolled, up to date, and properly protected. ZTNA then uses that information to make policy-based decisions to determine access and privilege to important networked applications.

Benefits of ZTNA compared to remote access VPN

While remote access VPN continues to serve us well, ZTNA offers a number of added benefits that make it a much more compelling solution:

  • More granular control: ZTNA allows more granular control over who can access certain applications and data, minimizing lateral movement and removing implied trust. VPN is all-or-nothing: once on the network, VPN generally offers access to everything.
  • Better security: ZTNA includes device and health status in access policies to further enhance security. VPN does not consider device status, which can put application data at risk to a compromised or non-compliant device.
  • Easier to enroll staff: ZTNA is much easier to roll out and is better when it comes to enrolling new employees. VPN involves more challenging and difficult setup and deployment.
  • Transparent to users: ZTNA offers “just works” transparency to users with frictionless connection management. VPN can be difficult and prone to initiating support calls.

Overall, ZTNA offers a welcome solution to connecting the branch office of one.

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Sophos ZTNA

Sophos ZTNA is a brand new cloud-delivered, cloud-managed product to easily and transparently secure your important business applications with granular controls.

Sophos ZNTA consists of three components:

  • Sophos Central provides the ultimate cloud management and reporting solution for all your Sophos products, including Sophos ZTNA. Sophos ZTNA is fully cloud-enabled, with Sophos Central providing easy deployment, granular policy management, and insightful reporting from the cloud.
  • Sophos ZTNA Gateway will be available as a virtual appliance for a variety of platforms to secure networked applications on-premise or in the public cloud. AWS and VMware ESXi support will be available initially, closely followed by support for Azure, Hyper-V, Nutanix, and others.
  • Sophos ZTNA Client provides transparent and frictionless connectivity to controlled applications for end users based on identity and device health. It is super easy to deploy from Sophos Central, with an option to deploy alongside Intercept X with just one click or instead work standalone with any desktop AV client. It will initially support MacOS and Windows, and later Linux and mobile device platforms as well.

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Coming soon

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The early access program (EAP) for the initial version of our ZTNA solution will kick off in the next couple of weeks, so stay tuned for additional news. I hope you will all join us in test-driving Sophos ZTNA to make it the best product it can be for launch!