Microsoft Releases Out-of-Band Security Updates for Exchange Server

Microsoft has issued several security updates for Microsoft Exchange Server to address vulnerabilities that have been used in limited targeted attacks.  Due to the critical nature of these vulnerabilities, they recommend that customers apply the updates to affected systems immediately to protect against these exploits and to prevent future abuse across the ecosystem.  

The vulnerabilities affect Microsoft Exchange Server. 

Exchange Online is not affected.  

The versions affected are: 

  • Microsoft Exchange Server 2013  
  • Microsoft Exchange Server 2016  
  • Microsoft Exchange Server 2019 

Microsoft Exchange Server 2010 is being updated for Defense in Depth purposes.

These vulnerabilities are used as part of an attack chain. The initial attack requires the ability to make an untrusted connection to Exchange server port 443. This can be protected against by restricting untrusted connections, or by setting up a VPN to separate the Exchange server from external access. Using this mitigation will only protect against the initial portion of the attack; other portions of the chain can be triggered if an attacker already has access or can convince an administrator to run a malicious file.

We recommend prioritizing installing updates on Exchange Servers that are externally facing. All affected Exchange Servers should ultimately be updated.

Further information and guidance  

Jason Davies

UK based technology professional, with an interest in computer security and telecoms.

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