New Exchange RCE vulnerability actively exploited

Exchange admins now have another exploit to deal with despite still reeling from a number of high profile attacks this year including ProxyLogon and ProxyShell. A new high severity Remote Code Execution (RCE) exploit for on-premise Exchange Servers has been published and is being actively exploited in the wild.

The exploit being tracked as CVE-2021-42321, has a CVSSv3 score of 8.8, placing it firmly into the serious category. Thankfully, the issue was patched by Microsoft in this month’s updates, but earlier this week the proof of concept (PoC) exploit code was published by security researcher Janggggg, meaning there will likely be an increase in attacks. Janggggg garnered some attention when he released a PoCs for the Exchange ProxyShell exploit in August.

This new deserialization exploit targets Exchange Server 2019 and Exchange Server 2016 including those in Exchange Hybrid mode and requires authentication. Previously, Exchange 2013 was thought to be affected but Microsoft updated its guidance on the 17th of November to state it was not.

Exchange Online customers are already protected and are not required to take any further action. Currently, the only known remediation is to apply the November Security Updates for Exchange which were released by Microsoft on 9th November as part of Patch Tuesday.

Microsoft has published some guidance on how updates might be completed, as it may be necessary to update to one of the supported Cumulative Updates (CUs) before applying the security update.

To identify servers that need updating, the Exchange Server Health Checker script can be run against your environment. Microsoft also offers the Exchange Update Wizard to help ensure that all necessary updates are installed.

Some users running in Exchange Hybrid mode had reported an issue after installing the update and receiving a “Something went wrong” error. The temporary workaround is to navigate to https://outlook.office.com/owa directly without URL redirects.

The following PowerShell query was also circulated to identify if your server has been targeted:

As Microsoft Exchange remains a popular target for exploitation, the guidance is to immediately apply the latest security patches to protect your environment.

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