CVE-2021-27065

Exploited in the wild. In CISA KEV since 2021‑11‑03. A vendor fix is available.

CVSS 3.17.8High EPSS percentile99th100% chance in 30 days CISA KEVListedsince 2021-11-03, due 2022-05-03
Public exploit: 2 sources FixVendor fixNo build named by the sources

Published 2021-03-02Updated 2026-08-19Sources: CVE.org, NVD, BDU

NVD CVE.org

What to do

The vendor has published a fix. Version details are below where the sources state them.

Steps

Written by AI from the record

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.

Vendor advisory Usually a quick update

Check it yourselfA free OpenVAS check exists for this CVE. Run it against your hosts.Is your organisation exposed?Request an assessment to establish which checks fit your systems.

What it is

From the CVE record

Microsoft Exchange Server Remote Code Execution Vulnerability

In plain language

Written by AI from the record

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.

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 you're affected

Full Exchange server takeover
Email account compromise
Mailbox data theft risk
Business outage and recovery costs

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.

Exploitation

Where each signal puts this CVE on the scale from published to confirmed exploited.

EPSS99th

Public exploit

CISA KEV

Listed as exploited in the wild since 2021-11-03.

US federal agencies must remediate by 2022-05-03.

Known use in ransomware campaigns.

Apply updates per vendor instructions.

Public exploits

2 sources with a proof of concept or module.

Exploit links, PoCs and Metasploit modules after sign-in

EPSS

100% chance of exploitation activity in the next 30 days, which ranks it in the 99th percentile of scored CVEs.

Exploit Prediction Scoring System, FIRST.org. A probability, not a confirmation.

Lifecycle

The patch came 79 days before any public exploit.

2026-08-19Patch availablerecord updated
2026-08-12Patch availablerecord updated
2026-06-25OpenVAS check added
2026-03-06Public exploit / PoCsource: packetstorm
2025-12-18Patch availablerecord updated
2021-11-03Added to CISA KEVransomware campaign

Affected products

Microsoft Corpcommercial · USaka microsoft, microsoft corporation

And 19 more affected products. See all after sign-in

Technical detail

CVSS 3.1 vector

Open in the CVSS calculator

CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Scored 7.8 by NVD.

How it is reached

Attack Vector LocalRequires local access to the vulnerable system (e.g. local login, malicious file)
Attack Complexity LowNo special conditions — the attack can be reliably reproduced
Privileges Required NoneNo authentication required — anyone can exploit this
User Interaction RequiredA user must click a link, open a file, or perform some action

Scope

Scope UnchangedImpact is limited to the vulnerable component itself

Impact if exploited

Confidentiality HighTotal information disclosure — all data in the component is compromised
Integrity HighTotal loss of integrity — attacker can modify any data in the component
Availability HighTotal denial of service — the component is completely unavailable