CVE-2021-27065

This is a Microsoft Exchange Server flaw that lets an attacker run code on your server; typical small businesses using affected Exchange versions should treat it as urgent and update immediately.

Executive summary

CVE-2021-27065 is a Microsoft Exchange Server remote code execution flaw (CWE-22) that is confirmed in the wild via CISA KEV and has public exploits available, meaning attackers can trigger it to take over an Exchange server remotely.

If affected, business impact

Full Exchange server takeoverEmail account compromiseMailbox data theft riskBusiness outage and recovery costs

What to do now

Check your Microsoft Exchange Server version against the affected list: Exchange Server 2013 CU 21/22/23, Exchange Server 2013 SP1, Exchange Server 2016 CU 10/11/12/13/14.
Verify whether you’re reachable from the internet on Exchange services (common default for email), and confirm you haven’t already applied the fixed Exchange updates.
Upgrade to the fixed version for your exact release line (see below).
If you can’t upgrade right away, immediately isolate Exchange from external access and follow Microsoft’s mitigation guidance until you can patch.
After patching, review Exchange and security logs for signs of compromise and hunt for suspicious persistence.

Patch / advisory Usually a quick update

What is it

Think of your Exchange server as the mailbox “mailroom” for your company. This vulnerability can let a bad actor remotely run commands on that mailroom server, which can lead to email being accessed or disrupted. Because there are public exploits and it’s been used in ransomware campaigns, you should not wait.

Who is affected

This matters if you run Microsoft Exchange Server on-premises (or otherwise host Exchange yourself) and your version matches one of the affected releases listed for CVE-2021-27065.

It’s primarily a concern when attackers can reach the Exchange services from outside your network—this is commonly possible for businesses that expose Exchange to the internet for email delivery—and especially since this vulnerability has been used in ransomware activity.

How urgent is it

Attackers are already using this vulnerability in real ransomware campaigns, and CISA listed it in the KEV catalog with a required remediation deadline that has long passed. Because public exploits exist and weaponization is available, you should treat this as RED and patch now rather than later.

What to do — in detail

Confirm whether we’re affected

Identify the exact Microsoft Exchange Server version and cumulative update level currently installed.
Compare it to the affected versions provided for CVE-2021-27065:
- Microsoft Exchange Server 2013 CU21, CU22, CU23, and Exchange Server 2013 Service Pack 1
- Microsoft Exchange Server 2016 CU10, CU11, CU12, CU13, and CU14
Check whether Exchange services are reachable from the internet (typical for email). If Exchange is externally reachable, assume exposure is possible if the vulnerable version is in use.

Upgrade to the fixed version (per exact branch)

Patch to the fixed version Microsoft lists for your current branch:

Exchange Server 2013 CU21 → 15.00.1395.012
Exchange Server 2013 CU22 → 15.00.1473.006
Exchange Server 2013 CU23 → 15.00.1497.012
Exchange Server 2013 Service Pack 1 → 15.00.0847.064
Exchange Server 2016 CU10 → 15.01.1531.012
Exchange Server 2016 CU11 → 15.01.1591.018
Exchange Server 2016 CU12 → 15.01.1713.010
Exchange Server 2016 CU13 → 15.01.1779.008
Exchange Server 2016 CU14 → 15.01.1847.012

Breaking-change / upgrade notes

Because Exchange patching can require a maintenance window and may involve server restarts, plan downtime according to Microsoft’s update instructions.
If you’re on a multi-server setup (DAG/HAS/etc.), follow Microsoft guidance for patch order and synchronization.

If patching is delayed (temporary containment)

Reduce exposure immediately by restricting external access to Exchange services (for example, block inbound access from the internet at the firewall/reverse proxy where feasible) until the fixed update is installed.
Apply Microsoft’s documented mitigations from the CVE-2021-27065 update guidance link, and keep the server isolated from untrusted networks.

Post-patch verification and compromise checks

Review Exchange and Windows/security logs for suspicious behavior around the time of exposure.
Hunt for signs of persistence (unexpected scheduled tasks/services, unusual admin account activity, new web/remote access artifacts), and validate that mail flow is正常.

KEV timeline

CISA KEV added: 2021-11-03
CISA remediation due date: 2022-05-03

What to monitor next

Ongoing authentication failures/successes targeting Exchange.
Any new admin-level actions shortly after the patch.
Mail flow anomalies (sudden forwarding rules, unexpected mailbox access patterns).

Technical context

Severity is high and the vulnerability is categorized as CWE-22. It’s a Microsoft Exchange Server remote code execution issue, and it is confirmed in the wild: CVE-2021-27065 is listed in CISA’s KEV catalog, tied to ransomware campaigns (meaning real-world attackers have used it). Public exploits are available, which lowers the barrier for attackers.

Patch maturity: Microsoft has provided fixed versions for multiple Exchange 2013 and Exchange 2016 cumulative update branches. Exploitation likelihood was predicted extremely high (EPSS) but, per the findings guidance, real-world KEV-listed exploitation is the decisive signal for urgency.

Attack mechanism (high level): the weakness can be triggered remotely against affected Exchange Server installations, enabling attackers to execute code on the server rather than just causing a crash or data exposure via a read-only flaw.

This is a general assessment based on public vulnerability data. It does not account for your specific infrastructure — when in doubt, consult a security specialist.

CVSS Vector Breakdown

Exploitability

AV:LAttack Vector

Local

AC:LAttack Complexity

Low

PR:NPrivileges Required

None

UI:RUser Interaction

Required

Scope

S:UScope

Unchanged

Impact

C:HConfidentiality

High

I:HIntegrity

High

A:HAvailability

High

Open in CVSS calculator →

Weaknesses

CWE-22

Affected Products

Microsoft Corp

commercial·USaka microsoft, microsoft corporation

exchange serverOn-prem

Operating Systems / windows

Microsoft Exchange Server 2013 Cumulative Update 21On-prem

Operating Systems / windows

Microsoft Exchange Server 2013 Cumulative Update 22On-prem

Operating Systems / windows

Microsoft Exchange Server 2013 Cumulative Update 23On-prem

Enterprise Software / itsm-monitoring

Microsoft Exchange Server 2013 Service Pack 1On-premWeb App

Communications / email-server-client

Microsoft Exchange Server 2016 Cumulative Update 10On-prem

Operating Systems / windows

Microsoft Exchange Server 2016 Cumulative Update 11On-prem

Operating Systems / windows

Microsoft Exchange Server 2016 Cumulative Update 12On-prem

Operating Systems / windows

and 19 more affected products View all →

Exploitability

CISA Known Exploited Vulnerability

Added to KEV:Nov 3, 2021

Remediation due:May 3, 2022

Ransomware:Known ransomware use

Required action: Apply updates per vendor instructions.

2 exploit sources identified

Exploit details including PoC links, Metasploit modules, and scanner templates are available after registration.

View exploit details

Official Patch Available

Is your org exposed to CVE-2021-27065?Request an assessment to establish which checks are appropriate for your systems. Check my exposure

Attack Graph

Products CVE Techniques Tactics

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MITRE ATT&CK

2 techniques

Collection

Discovery

View detailed technique mapping

References

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-27065

msrc.microsoft.com

http://packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.html

packetstormsecurity.com

http://packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.html

packetstormsecurity.com

and 6 more references View all →

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