“Customers using Azure virtual machines (VMs) are responsible for updating the operating systems running on their VMs,” said JR Aquino, manager for Azure Incident Response at Microsoft Security Response Center, in an advisory posted over the weekend. In an advisory, Microsoft said that Azure customers using the vulnerable software (Azure customers running virtual machines that use Exim version 4.87 to 4.91) are susceptible to the attack. The warning comes after a widespread worm campaign was disclosed on Friday, targeting a flaw in the Exim mail transport agent (MTA), which are Linux-based mail servers that receive, route and deliver email messages from local users and remote hosts. However, the issue also plagues Azure users: Linux virtual machines, which run Exim servers, can be created through the Azure portal (a browser-based user interface to create VMs and their associated resources).

Microsoft is warning customers that some Azure installations are vulnerable to a recently-disclosed critical Linux Exim mail server flaw that is under active attack.

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