CVE-2021-26855: Microsoft Exchange Server Remote Code Execution Vulnerability

CVE-2021-26855 is a remote code execution vulnerability in Microsoft Exchange Server. It is part of the ProxyLogon exploit chain and has been used by ransomware operators. Organizations running Exchange should treat this as a high-priority issue because successful abuse can give an attacker a foothold on the mail server and a path deeper into the environment.

Public detail on exact mechanics is limited beyond the CWE and CISA description; confirm all version, configuration, and patch specifics against the Microsoft vendor advisory before acting.

How it works

The weakness is classified as CWE-918 (Server-Side Request Forgery). In this class of flaw, the server can be tricked into making requests that the attacker controls or influences. When chained as part of ProxyLogon, the SSRF step helps an unauthenticated or low-privilege attacker reach internal Exchange components that should not be directly exposed, ultimately enabling remote code execution on the server.

An attacker typically sends crafted requests to the Exchange front-end so that the server itself performs actions or reaches endpoints on the attacker’s behalf. Because Exchange often holds privileged access to mailboxes, Active Directory, and internal networks, code execution on the server can lead to data theft, persistence, lateral movement, or deployment of ransomware. Exact request formats and exploit steps are not detailed here; treat any public proof-of-concept material with caution and validate behavior only in isolated lab conditions against the official advisory.

Am I affected? How to find it in your systems

Microsoft Exchange Server is commonly deployed on-premises or in hybrid configurations to provide corporate email, calendaring, and collaboration. It usually runs on Windows Server and is reachable via HTTPS on ports 443 (and sometimes 80) from internal clients and, if internet-facing, from the outside.

• Inventory every host that runs the Exchange Server role (Mailbox, Client Access, Edge, etc.). Use configuration-management databases, Microsoft’s own Exchange health/check scripts, or vulnerability scanners that fingerprint Exchange.
• Record the installed cumulative update / build number and compare it with the fixed builds listed in the Microsoft advisory for CVE-2021-26855. Do not rely on version guesses; confirm directly against the vendor list.
• Note whether the server is internet-facing, published through a reverse proxy or load balancer, or only internal. Internet exposure materially increases risk for this class of flaw.
• Review IIS and Exchange protocol logs (especially HTTP proxy and front-end request logs) for anomalous requests to Exchange endpoints, unusual authentication patterns, or sudden spikes in errors that coincide with known ProxyLogon activity windows. Endpoint detection and response (EDR) telemetry looking for unexpected child processes of Exchange or w3wp.exe can also surface post-exploitation.
• If you run hybrid Exchange, verify that on-premises servers still in scope have received the required updates; cloud-only Microsoft 365 tenants are outside the scope of this on-premises CVE.

How to remediate

The primary remediation is to apply the security updates Microsoft released for this vulnerability, following the vendor’s installation order and prerequisites exactly. CISA’s required action is to apply updates per vendor instructions.

• Download the correct update package only from official Microsoft channels and test in a representative non-production environment if possible, then deploy to production Exchange servers as quickly as change control allows.
• After patching, reboot if required and verify the new build number. Re-run Microsoft’s Exchange security or health checker scripts to confirm the CVE is no longer reported.
• As hardening for the broader SSRF / ProxyLogon class: restrict administrative access to Exchange management tools, enforce least-privilege service accounts, keep the underlying Windows Server and IIS components current, and disable or tightly control any legacy or unnecessary Exchange virtual directories or features that are not required for your mail flow.
• Ensure Exchange servers are covered by regular vulnerability scanning and that the scanner signatures include this CVE family.

If you can't patch immediately

If immediate patching is blocked by operational constraints, apply compensating controls while you schedule the update.

• Network segmentation: place Exchange servers in a restricted VLAN or security zone; allow only necessary client and SMTP traffic and block direct internet access to management interfaces where feasible.
• Virtual patching / WAF: if you terminate TLS at a web application firewall or reverse proxy, enable rules that detect or block known ProxyLogon-related request patterns. Tune carefully to avoid breaking legitimate Outlook or ActiveSync traffic; confirm rule efficacy against the vendor advisory guidance.
• Reduce the attack surface: temporarily disable any non-essential Exchange services or virtual directories that the advisory or Microsoft guidance indicates are involved in the chain, provided mail flow remains intact.
• Heightened monitoring: increase logging verbosity for IIS and Exchange, forward logs to a SIEM, and alert on anomalous URL patterns, unusual POST bodies, or new processes spawned by the Exchange worker processes. Retain logs long enough for later forensic review.
• Credential and privilege hygiene: rotate service-account passwords and review which accounts have Exchange admin rights so that any successful foothold yields less value.

These measures reduce risk but do not replace the vendor patch. Schedule the official update as the definitive fix.

If your data may have been exposed

This vulnerability has been actively exploited and is known to have been used by ransomware actors. If your Exchange servers were unpatched and reachable during the period of exploitation, assume possible compromise: isolate affected hosts, collect memory and disk images, hunt for webshells or unexpected persistence, reset privileged credentials, and engage incident-response procedures. Review mail-flow and authentication logs for signs of data access or exfiltration.

As an additional check for personal or corporate email addresses that may appear in known breach data sets, you can run a free exposure scan of your email to see whether those addresses have already surfaced in public breach corpora.