SMB Exploited: WannaCry Use of “EternalBlue”

Server Message Block (SMB) is the transport protocol used by Windows machines for a wide variety of purposes such as file sharing, printer sharing, and access to remote Windows services. SMB operates over TCP ports 139 and 445. In April 2017, Shadow Brokers released an SMB vulnerability named “EternalBlue,” which was part of the Microsoft security bulletin MS17-010.

The recent WannaCry ransomware takes advantage of this vulnerability to compromise Windows machines, load malware, and propagate to other machines in a network. The attack uses SMB version 1 and TCP port 445 to propagate.


SMB provides support for what are known as SMB Transactions. Using SMB Transactions enables atomic read and write to be performed between an SMB client and server. If the message request is greater than the SMB MaxBufferSize, the remaining messages are sent as Secondary Trans2 requests. This vulnerability affects the srv2.sys kernel driver and is triggered by malformed Secondary Trans2 requests.

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