Patch Time: Microsoft Issues Patches for Exchange Server Zero-Day Threats

Patch Time: Microsoft Issues Patches for Exchange Server Zero-Day Threats

Microsoft recently issued out-of-band security updates for zero-day vulnerabilities affecting Microsoft Exchange Server.

Out-of-band security updates refer to security updates released outside the normal release time. Zero-day vulnerabilities, meanwhile, refer to software security vulnerabilities that are exploited before updates become available.

Microsoft Exchange Server is Microsoft's email server solution that’s available both on-premise and online. This email server solution runs exclusively on Windows Server operating systems. Exchange servers are primarily used by organizations. As such these Exchange servers are high-value targets.

In the blog post "HAFNIUM targeting Exchange Servers with 0-day exploits," Microsoft Threat Intelligence Center, Microsoft 365 Defender Threat Intelligence Team, and Microsoft 365 Security said that on-premises Exchange servers are affected by the zero-day vulnerabilities, while Exchange online isn’t affected. On-premises Exchange servers that are specifically affected are Exchange Server 2010, Exchange Server 2013, Exchange Server 2016, and Exchange Server 2019.

Microsoft Exchange Server Zero-Day Vulnerabilities

According to Microsoft Threat Intelligence Center, Microsoft 365 Defender Threat Intelligence Team, and Microsoft 365 Security, the threat actor, collectively called “Hafnium,” used the following zero-day vulnerabilities in on-premises Exchange servers to initially access their victims’ networks:

CVE-2021-26855: A server-side request forgery (SSRF) vulnerability in on-premises Exchange servers that allowed the attacker to send arbitrary HTTP requests and authenticate as the Exchange server.

CVE-2021-26857: This vulnerability allowed the attacker to run code as SYSTEM on the Exchange server. This vulnerability, however, needs administrator permission or another vulnerability to exploit.

CVE-2021-26858: After exploiting CVE-2021-26855 or by compromising a legitimate admin’s credentials, this vulnerability allowed the attacker to write a file to any path on the server.

CVE-2021-27065: Similar to CVE-2021-26858, after exploiting CVE-2021-26855 or by compromising a legitimate admin’s credentials, this vulnerability allowed the attacker to write a file to any path on the server. 

“After exploiting these vulnerabilities to gain initial access, HAFNIUM operators deployed web shells on the compromised server,” Microsoft Threat Intelligence Center, Microsoft 365 Defender Threat Intelligence Team, and Microsoft 365 Security said. “Web shells potentially allow attackers to steal data and perform additional malicious actions that lead to further compromise.”

By exploiting these 4 zero-day vulnerabilities, the attacker was able to bypass authentication, including two-factor authentication. These vulnerabilities allowed the attacker to access email accounts that are of interest within the targeted organizations and allowed the attacker to remotely execute code on vulnerable Microsoft Exchange servers.

Researchers from Volexity and Dubex were credited by Microsoft for reporting about the zero-day vulnerabilities in on-premises Exchange servers. Security researchers at Volexity and Dubex reported that the zero-day exploits in on-premises Exchange servers started as early as January 2021.

Security researchers at Volexity reported that in January 2021 they detected anomalous activity from two of its customers’ Microsoft Exchange servers. The Volexity security researchers said they identified a large amount of data being sent to IP addresses it believed weren’t tied to legitimate users.

Dubex security researchers said they observed the zero-day exploit in a set of Exchange servers in Denmark. They particularly observed CVE-2021-26857, an insecure deserialization vulnerability in the Unified Messaging service. “Through analysis of the systems, Dubex Incident Response Team determined that feeding the UM [Unified Messaging] Server with a sufficiently malformed voicemail file caused it to spawn a UMWorkerProcess that deserialised the voicemail and executed contents,” Dubex security researchers said.

In the blog post "New nation-state cyberattacks," Tom Burt, Corporate Vice President, Customer Security & Trust at Microsoft, said that Hafnium, the group behind the zero-day exploits in on-premises Exchange servers, operates from China. Hafnium, he said, targets research organizations, law firms, higher education institutions, defense contractors, policy think tanks, and NGOs.

Burt added Hafnium attacked by following these three steps: “First, it would gain access to an Exchange Server either with stolen passwords or by using the previously undiscovered vulnerabilities to disguise itself as someone who should have access. Second, it would create what’s called a web shell to control the compromised server remotely. Third, it would use that remote access – run from the U.S.-based private servers – to steal data from an organization’s network.”

Preventive and Mitigating Measures

According to Microsoft, zero-day vulnerabilities in on-premises Exchange servers, which include CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065, are all part of an attack chain. The initial attack, Microsoft said, requires the ability to make an untrusted connection to Exchange server port 443.

To prevent the initial attack, Microsoft recommends protection against untrusted connection to Exchange server port 443. To separate the Exchange server from external access,

Microsoft recommends setting up a virtual private network (VPN).

Microsoft, however, noted that protection against untrusted connection to Exchange server port 443 and setting up a VPN only serve as protection against the initial portion of the attack. The company warned that other portions of the attack chain can be triggered if an attacker already has access or can convince an administrator to open a malicious file.

If the latest security updates can't be immediately deployed, it’s recommended to restrict external access to OWA URL, restrict external access to Exchange Admin Center (EAC), and disconnect vulnerable Exchange servers from the internet until the latest security updates can be applied. 

Microsoft recommends the following best practices to better defend on-premises Exchange servers:

• Deploy the latest security updates, especially for Microsoft Exchange Server, as soon as they become available.
• Use security solutions such as firewall protection, multi-factor authentication (MFA), and VPN.
• Regularly review privileged groups such as Administrators, Remote Desktop Users, and Enterprise Admins. Attackers typically add their own accounts to these groups to establish a foothold on a server. Review these privileged groups for suspicious additions.
• Restrict access by avoiding the use of domain-wide, admin-level service accounts, and by enforcing strong randomized, just-in-time local administrator passwords.