Microsoft urges Exchange admins to patch bug exploited in the wild

LASER_oneXM

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Microsoft warned admins today to immediately patch a high severity Exchange Server vulnerability that may allow authenticated attackers to execute code remotely on vulnerable servers.

The security flaw tracked as CVE-2021-42321 impacts Exchange Server 2016 and Exchange Server 2019, and it is caused by improper validation of cmdlet arguments according to Redmond's security advisory.

CVE-2021-42321 only affects on-premises Microsoft Exchange servers, including those used by customers in Exchange Hybrid mode (Exchange Online customers are protected against exploitation attempts and don't need to take any further action).

"We are aware of limited targeted attacks in the wild using one of the vulnerabilities (CVE-2021-42321), which is a post-authentication vulnerability in Exchange 2016 and 2019," Microsoft explained.

"Our recommendation is to install these updates immediately to protect your environment."

For a quick inventory of all Exchange servers in your environment behind on updates (CUs and SUs), you can use the latest version of the Exchange Server Health Checker script.
 

LASER_oneXM

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Feb 4, 2016
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Proof-of-concept exploit code has been released online over the weekend for an actively exploited high severity vulnerability impacting Microsoft Exchange servers.

The security bug tracked as CVE-2021-42321 impacts on-premises Exchange Server 2016 and Exchange Server 2019 (including those used by customers in Exchange Hybrid mode) and was patched by Microsoft during this month's Patch Tuesday.

Successful exploitation allows authenticated attackers to execute code remotely on vulnerable Exchange servers.

On Sunday, almost two weeks after the CVE-2021-42321 patch was issued, researcher Janggggg published a proof-of-concept exploit for the Exchange post-auth RCE bug. "This PoC just pop mspaint.exe on the target, can be use to recognize the signature pattern of a successful attack event," the researcher said.

Admins warned to patch immediately​

"We are aware of limited targeted attacks in the wild using one of the vulnerabilities (CVE-2021-42321), which is a post-authentication vulnerability in Exchange 2016 and 2019," Microsoft said.