Silent Watcher: Dissecting Cmimai Stealer's VBS Payload

SUMMARY :

A VBS-based infostealer called Cmimai Stealer has emerged, targeting Windows systems since June 2025. It collects system information, browser metadata, and screenshots, exfiltrating data via Discord webhooks. The malware uses PowerShell scripts for browser data collection and screen capture, running in a persistent loop every hour. It leverages WMI for system information gathering and employs JSON formatting for data exfiltration. While lacking advanced features like encrypted communication or credential theft, Cmimai Stealer serves as both an infostealer and a reconnaissance tool. Defensive considerations include monitoring high-risk process combinations, watching for specific PowerShell scripts and image files, and detecting Discord traffic with a unique User-Agent.

OPENCTI LABELS :

powershell,infostealer,windows,exfiltration,discord,vbs,wmi,browser-data,screenshot,cmimai stealer


AI COMMENTARY :

1. Introduction: Silent Watcher: Dissecting Cmimai Stealer’s VBS Payload unveils a VBS-based infostealer that first emerged in June 2025 and has since targeted Windows endpoints. This malware has drawn attention in the threat intel community for its reliance on simple scripting languages rather than complex native code, allowing threat actors to deploy it with minimal dependencies. By leveraging common Windows features, Cmimai Stealer operates under the radar while collecting valuable system details and user data for exfiltration.

2. Technical Architecture: The stealer is built on a VBS payload that executes in a persistent hourly loop. It spawns PowerShell scripts to harvest browser-data such as cookie stores, history logs, and form data. For system intelligence, it uses Windows Management Instrumentation (WMI) queries to extract processor, memory, and network interface information in JSON format. The malware then captures screenshots via PowerShell-driven .NET calls and encodes the images before staging them for upload.

3. Data Collection Workflows: Cmimai Stealer’s primary focus lies in gathering system metadata and user artifacts. Browser-data extraction runs at each cycle, targeting Chrome, Firefox, and Edge profiles. WMI routines gather OS version, hardware identifiers, and running processes. A custom PowerShell module handles screenshot capture by invoking the Windows.Graphics.Capture API, ensuring that each image reflects the current user session context for effective reconnaissance.

4. Exfiltration Mechanism: Instead of generic HTTP or FTP channels, the malware uses Discord webhooks to send JSON-formatted packets containing system details and Base64-encoded screenshots. The threat actors leverage a unique User-Agent string in Discord API calls, enabling them to track infected hosts. This approach simplifies exfiltration since many organizations allow traffic to known social media and communication platforms by default.

5. Limitations and Trade-Offs: While Cmimai Stealer bypasses advanced detection by using scripts and well-known protocols, it lacks encrypted communications and direct credential theft modules. Its absence of native code diminishes its footprint on disk but also limits the complexity of its payloads. As an infostealer without built-in credential harvesters, it relies on subsequent exploitation stages to leverage its reconnaissance findings.

6. Reconnaissance Utility: Beyond pure data theft, this stealer functions as a reconnaissance tool that maps enterprise environments. By collecting hardware, software, and network data at regular intervals, it enables threat actors to gauge defense postures, identify high-value targets, and plan lateral movement. The hourly persistence loop ensures that any changes in the environment are promptly reported to the adversary’s Discord channel.

7. Defensive Considerations: Security teams should monitor for atypical PowerShell script invocations that download or execute VBS files, especially paired with image file writes in temporary directories. Alerting on WMI queries issued by non-administrative processes can surface suspicious system-info gathering. Network monitoring should include inspection of outbound Discord traffic for the unique User-Agent used by Cmimai Stealer’s webhook calls. Implementing application allow-listing and script block logging in Windows can further disrupt its operation.

8. Conclusion: Cmimai Stealer stands out in the threat landscape by combining VBS and PowerShell to execute infostealer and reconnaissance tasks on Windows systems. Its use of Discord for exfiltration highlights the evolving tactics of adversaries leveraging benign services. By understanding its capabilities—browser-data harvesting, screenshot capture, WMI queries, and JSON exfiltration—defenders can tune detection and response strategies to counter this stealthy threat.


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Silent Watcher: Dissecting Cmimai Stealer's VBS Payload