CMSTAR Downloader Malware

Two new variants of the CMSTAR downloader malware have been discovered containing two new previously unseen backdoors named, BYEBY and PYLOT. These have recently been used in phishing campaigns.

The malware is delivered via phishing emails containing a number of decoy documents and folders including JPEG images. If the attachments are opened the malicious CMSTAR payload is executed including the PYLOT and BYEBYE backdoors.

Affected Platforms:

Microsoft Windows – all versions



  • A robust program of education and awareness training is delivered to users to ensure they don’t open attachments or follow links within unsolicited emails.
  • All operating systems, antivirus and other security products are kept up to date.
  • All day to day computer activities such as email and internet are performed using non-administrative accounts.
  • Strong password policies are in place and password reuse is discouraged.
  • Network, proxy and firewall logs should be monitored for suspicious activity.

Duncan Newell

Duncan is a technology professional with over 20 years experience of working in various IT roles. He has a interest in cyber security, and has a wide range of other skills in radio, electronics and telecommunications.

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