Prevent the impact of a Linux worm by updating Exim (CVE-2019-10149)

This week, MSRC confirmed the presence of an active Linux worm leveraging a critical Remote Code Execution (RCE) vulnerability, CVE-2019-10149, in Linux Exim email servers running Exim version 4.87 to 4.91. Microsoft Azure infrastructure and Services are not affected; only customer’s Linux IaaS instances running a vulnerable version of Exim are affected. Azure customers running VMs with Exim 4.92 are not affected by this vulnerability.

Azure has controls in place to help limit the spread of this worm from work we’ve already done to combat SPAM, but customers using the vulnerable software would still be susceptible to infection.

Customers using Azure virtual machines (VMs) are responsible for updating the operating systems running on their VMs. As this vulnerability is being actively exploited by worm activity, MSRC urges customers to observe Azure security best practices and patterns and to patch or restrict network access to VMs running the affected versions of Exim.

There is a partial mitigation for affected systems that can filter or block network traffic via Network Security Groups (NSGs). The affected systems can mitigate Internet-based ‘wormable’ malware or advanced malware threats that could exploit the vulnerability. However, affected systems are still vulnerable to Remote Code Execution (RCE) exploitation if the attacker’s IP Address is permitted through Network Security Groups.

It is for these reasons that we strongly advise that all affected systems – irrespective of whether NSGs are filtering traffic or not – should be updated as soon as possible.

Resources:

Links to Azure Network Security Group Documentation
Links to Update Management Solutions using Azure Automation
Links to Azure Security Best Practices and Patterns

JR Aquino Manager, Azure Incident Response Microsoft Security Response Center (MSRC)

updated 18 June 2019 to clarify “Microsoft Azure infrastructure and Services are not affected; only customer’s Linux IaaS instances running a vulnerable version of Exim are affected.”