The US Cybersecurity and Infrastructure Security Agency (CISA) and National Security Agency (NSA) have issued advice on how to harden on-premises Microsoft Exchange Server instances.
The advisory comes amidst a surge in attacks against Exchange Server in recent months, according to Nick Andersen, executive assistant director for the Cybersecurity Division (CSD) at CISA.
“With the threat to Exchange servers remaining persistent, enforcing a prevention posture and adhering to these best practices is crucial for safeguarding our critical communication systems," Andersen said.
"This guidance empowers organizations to proactively mitigate threats, protect enterprise assets, and ensure the resilience of their operations.”
Anderson added that CISA recommends organizations also “evaluate the use of cloud-based email services” rather than “managing the complexities” of hosting their own.
Shoring up Microsoft Exchange Server
So what does the advisory recommend? First and foremost, organisations should restrict administrative access, implement multi-factor authentication, enforce strict transport security configurations, and adopt zero trust security model principles.
On top of this, as certain Exchange Server versions have recently become end-of-life (EOL), they strongly encourage organizations to take proactive steps to mitigate risks and prevent malicious activity.
Attackers frequently target these servers, which usually receive less monitoring and security updates than cloud-based alternatives.
Organizations should decommission any remaining end-of-life on-prem or hybrid Exchange servers after transitioning to cloud-based Microsoft 365, the duo said.
Stay up to date with patching
The most effective defense against exploitation, according to the NSA, is ensuring that all Exchange servers are running the latest version and Cumulative Update (CU).
Organizations that are running an unsupported version of Exchange should migrate to Exchange Server SE or an alternative supported email server software or service.
Elsewhere, ensuring that Emergency Mitigation Service remains enabled is vital, and enterprises are also advised to apply and maintain the Exchange Server baseline, Windows security baselines, and applicable mail client security baselines.
As far as administrative access is concerned, this should remain limited, according to CISA, while authentication and encryption should be hardened by configuring Transport Layer Security and Extended Protection.
Similarly, instead of NT LAN Manager (NTLM), Kerberos and Server Message Block (SMB) should be configured; Modern Authentication and multi-factor authentication should be used, along with certificate-based signing of PowerShell serialization, HTTP Strict Transport Security (HSTS), and Download Domains.
Exchange server flaws are heavily targeted
The publication of the guide follows a warning from CISA in August about a high-severity post-authentication vulnerability in Exchange Server.
Tracked as CVE-2025-53786, the vulnerability allowed an attacker to move laterally from on-premises Exchange to the M365 cloud environment.
CISA said that while exploitation of this flaw would require highly specific circumstances, it still represents a serious concern.
"Although exploitation of this vulnerability is only possible after an attacker establishes administrative access on the on-premises Exchange server, CISA is deeply concerned at the ease with which a threat actor could escalate privileges and gain significant control of a victim’s M365 Exchange Online environment," it said.
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