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APT28 Exploits Microsoft Outlook with NotDoor Backdoor Malware

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📰 2 unique sources, 2 articles

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APT28, a Russian state-sponsored threat group, has been using a new backdoor malware called NotDoor to target Microsoft Outlook. NotDoor leverages Outlook as a covert communication, data exfiltration, and malware delivery channel. The malware is deployed via a legitimate signed binary, Microsoft's OneDrive.exe, which is vulnerable to DLL sideloading. The backdoor is triggered by specific strings in incoming emails, allowing attackers to execute commands, exfiltrate data, and upload files. NotDoor illustrates APT28's continued evolution in bypassing established defense mechanisms. The malware has been observed targeting multiple companies from different sectors in NATO member countries. NotDoor is designed as an obfuscated Visual Basic for Applications (VBA) project for Outlook that makes use of the Application.MAPILogonComplete and Application.NewMailEx events to run the payload every time Outlook is started or a new email arrives. The malware supports four different commands: cmd, cmdno, dwn, and upl. Files exfiltrated by the malware are saved in the folder, encoded using the malware's custom encryption, sent via email, and then deleted from the system. The attacks are notable for the abuse of Microsoft Dev Tunnels (devtunnels.ms) as C2 domains for added stealth. Attack chains entail the use of bogus Cloudflare Workers domains to distribute a Visual Basic Script like PteroLNK, which can propagate the infection to other machines by copying itself to connected USB drives, as well as download additional payloads.

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  1. 03.09.2025 23:49 📰 2 articles · ⏱ 12d ago

    APT28 deploys NotDoor backdoor to target Microsoft Outlook

    APT28 has been using a new backdoor malware called NotDoor to exploit Microsoft Outlook. The malware is deployed via a legitimate signed binary, Microsoft's OneDrive.exe, which is vulnerable to DLL sideloading. NotDoor uses PowerShell commands encoded in Base64 and features obfuscated code to evade detection. The backdoor is triggered by specific strings in incoming emails, allowing attackers to execute commands and exfiltrate data. The malware is designed as an obfuscated Visual Basic for Applications (VBA) project for Outlook that makes use of the Application.MAPILogonComplete and Application.NewMailEx events to run the payload every time Outlook is started or a new email arrives. NotDoor creates a folder at the path %TEMP%\Temp if it does not exist, using it as a staging folder to store TXT files created during the course of the operation and exfiltrate them to a Proton Mail address. The malware supports four different commands: cmd, cmdno, dwn, and upl. Files exfiltrated by the malware are saved in the folder, encoded using the malware's custom encryption, sent via email, and then deleted from the system. The attacks are notable for the abuse of Microsoft Dev Tunnels (devtunnels.ms) as C2 domains for added stealth. Attack chains entail the use of bogus Cloudflare Workers domains to distribute a Visual Basic Script like PteroLNK, which can propagate the infection to other machines by copying itself to connected USB drives, as well as download additional payloads.

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