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CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, CVE-2021-27065: Four Zero-Day Vulnerabilities in Microsoft Exchange Server Exploited in the Wild

Four zero-day vulnerabilities in Microsoft Exchange servers have been used in chained attacks in the wild.

Update March 8, 2021: The Identifying Affected Systems section has been updated with information about the availability of additional plugins as well as a link to our blog post that details them.


On March 2, Microsoft published out-of-band advisories to address four zero-day vulnerabilities in Microsoft Exchange Server that have been exploited in the wild.

CVE Vulnerability Type CVSSv3
CVE-2021-26855 Server-Side Request Forgery (SSRF) 9.1
CVE-2021-26857 Insecure Deserialization 7.8
CVE-2021-26858 Arbitrary File Write 7.8
CVE-2021-27065 Arbitrary File Write 7.8

In a blog post, Microsoft attributes the exploitation of these flaws to a state-sponsored group it calls HAFNIUM. The group has historically targeted U.S.-based institutions, which include “infectious disease researchers, law firms, higher education institutions, defense contractors, policy think tanks, and NGOs,” according to the Microsoft blog. Researchers at Volexity also published a blog post about this attack, referring to it as Operation Exchange Marauder.

The vulnerabilities affect the on-premises version of Microsoft Exchange Server. Microsoft Exchange Online is not affected by these vulnerabilities.


CVE-2021-26855 is a SSRF vulnerability in Microsoft Exchange Server. An unauthenticated, remote attacker could exploit this flaw by sending a specially crafted HTTP request to a vulnerable Exchange Server. In order to exploit this flaw, Microsoft says the vulnerable Exchange Server would need to be able to accept untrusted connections over port 443. Successful exploitation of this flaw would allow the attacker to authenticate to the Exchange Server.

Volexity, one of three groups credited with discovering CVE-2021-26855, explained in its blog post that it observed an attacker leverage this vulnerability to “steal the full contents of several user mailboxes.” All that is required for an attacker to exploit the flaw is to know the IP address or fully qualified domain name (FQDN) of an Exchange Server and the email account they wish to target.

CVE-2021-26857 is an insecure deserialization vulnerability in Microsoft Exchange. Specifically, the flaw resides in the Exchange Unified Messaging Service, which enables voice mail functionality in addition to other features. To exploit this flaw, an attacker would need to be authenticated to the vulnerable Exchange Server with administrator privileges or exploit another vulnerability first. Successful exploitation would grant the attacker arbitrary code execution privileges as SYSTEM.

CVE-2021-26858 and CVE-2021-27065 are both arbitrary file write vulnerabilities in Microsoft Exchange. These flaws are post-authentication, meaning an attacker would first need to authenticate to the vulnerable Exchange Server before they could exploit these vulnerabilities. This could be achieved by exploiting CVE-2021-26855 or by possessing stolen administrator credentials. Once authenticated, an attacker could arbitrarily write to any paths on the vulnerable server.

Microsoft’s blog says its researchers observed the HAFNIUM threat actors exploiting these flaws to deploy web shells onto targeted systems in order to steal credentials and mailbox data. The attackers reportedly were also able to obtain the offline address book (OAB) for Exchange. Possessing this information would be useful for a determined threat actor performing further reconnaissance activity on their target.

Intrusions detected going back to at least January 2021

Despite the initial disclosure on March 2, Steven Adair, president of Volexity, says his team has worked on “several intrusions since January” involving these vulnerabilities.

The Volexity blog post includes a video demo showing the successful exfiltration of individual emails associated with a targeted user without authentication. This was achieved by sending an HTTP POST request using an XML SOAP payload to the vulnerable Exchange Server’s Web Services API.

Other threat actors are reportedly leveraging these flaws in the wild

According to a Twitter thread from ESET research, “several cyber-espionage groups” — whose targets not only include the United States, but other countries including Germany, France, Kazakhstan, and more — have actively exploited the SSRF vulnerability (CVE-2021-26855).

Microsoft also addressed three unrelated Exchange Server vulnerabilities

In addition to the four zero-day vulnerabilities, Microsoft also patched three unrelated remote code execution (RCE) vulnerabilities in Microsoft Exchange Server that were disclosed to them by security researcher Steven Seeley.

CVE-2021-26412 9.1
CVE-2021-26854 6.6
CVE-2021-27078 9.1

Proof of concept

At the time this blog post was published, there were no proofs-of-concept available for any of the four zero-day vulnerabilities disclosed by Microsoft.


Microsoft released out-of-band patches for Microsoft Exchange Server on March 2 that address all four vulnerabilities exploited in the wild as well as the three unrelated vulnerabilities.

Exchange Server Version Knowledgebase Article
2010 Service Pack 3 KB5000978
2013 Cumulative Update 23 KB5000871
2016 Cumulative Update 18 KB5000871
2016 Cumulative Update 19 KB5000871
2019 Cumulative Update 7 KB5000871
2019 Cumulative Update 8 KB5000871

Both Microsoft and Volexity have shared indicators of compromise (IOCs) for the attacks in their respective blog posts.

There are some mitigations organizations can apply until patching is feasible, such as restricting untrusted connections to Exchange Server. However, Tenable strongly encourages all organizations that deploy Exchange Server on-premises to apply these patches as soon as possible. We expect that once a working proof-of-concept becomes available, attackers will begin leveraging these flaws indiscriminately.

Identifying affected systems

A list of Tenable plugins to identify these vulnerabilities will appear here as they’re released.

In addition to the version check plugins, we have released a direct check plugin and an IOC plugin. For more details on these plugins as well as guidance on how Tenable can help you identify compromised systems, please visit our latest blog post.

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