A remote code execution vulnerability exists in Microsoft Exchange software when the software fails to properly handle objects in memory, aka 'Microsoft Exchange Memory Corruption Vulnerability'.

Microsoft disclosed four actively exploited zero-day vulnerabilities being used to attack on-premises versions of Microsoft Exchange Server. The vulnerabilities identified are CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065, all of which affect Microsoft Exchange Server. Exchange Online is not affected. In the attacks observed, the threat actor used these vulnerabilities to access on-premises Exchange servers which enabled access to email accounts, and allowed installation of additional malware to facilitate long-term access to victim environments. Microsoft Threat Intelligence Center (MSTIC) attributes this campaign with high confidence to HAFNIUM, a group assessed to be state-sponsored and operating out of China, based on observed victimology, tactics and procedures.

ProxyShell is an exploit chain targeting on-premise installations of Microsoft Exchange Server. It was demonstrated by Orange Tsai at Pwn2Own in April 2021 and is comprised of three CVEs that, when chained, allow a remote unauthenticated attacker to execute arbitrary code on vulnerable targets. The three CVEs are CVE-2021-34473, CVE-2021-34523, and CVE-2021-31207. Details are available in Orange Tsai's [Black Hat USA 2020 talk](https://i.blackhat.com/USA21/Wednesday-Handouts/us-21-ProxyLogon-Is-Just-The-Tip-Of-The-Iceberg-A-New-Attack-Surface-On-Microsoft-Exchange-Server.pdf) and follow-on [blog series](https://blog.orange.tw/2021/08/proxylogon-a-new-attack-surface-on-ms-exchange-part-1.html). ProxyShell is being broadly exploited in the wild as of August 12, 2021.

Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Exchange Server Spoofing Vulnerability

Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Exchange Server Information Disclosure Vulnerability

Microsoft Exchange Server Remote Code Execution Vulnerability