CVE-2021-27065Known exploitationPublic exploitNetwork attack vector

Code execution via ECP arbitrary file write

Published Mar 2, 2021 · Updated Aug 19, 2026

Arbitrary file write in Microsoft Exchange Server 2013, 2016, and 2019 allows remote authenticated users to execute code through ECP requests. ECP fails to constrain virtual-directory URL properties, letting crafted Set-OabVirtualDirectory data traverse to a web-accessible path and persist attacker-controlled ASPX content. Authenticated ECP access is required; observed ProxyLogon attacks supplied that context through a separate flaw and used the written web shell for persistent server control.

CVSS severity7.8
High
EPSS probability99.88%
Next 30 days · Sep 16, 2026
Known exploitationReported
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Arbitrary file write in Microsoft Exchange Server 2013, 2016, and 2019 allows remote authenticated users to execute code through ECP requests. ECP fails to constrain virtual-directory URL properties, letting crafted Set-OabVirtualDirectory data traverse to a web-accessible path and persist attacker-controlled ASPX content. Authenticated ECP access is required; observed ProxyLogon attacks supplied that context through a separate flaw and used the written web shell for persistent server control.

The record

CVE
CVE-2021-27065
Published
Mar 2, 2021
Updated
Aug 19, 2026
Vendor
Microsoft
Product
Microsoft Exchange Server 2013 Cumulative Update 22
Classifications
CWE-22, T1190, T1505.003
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jan 1, 2021Exploitation reportedHAFNIUM exploitation of on-premises Exchange Server
  2. Mar 2, 2021CVE publishedPublication date reported by the CVE source.
  3. Aug 19, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=15.00.0 <15.00.1473.006

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Microsoft · Microsoft Exchange Server 2013 Cumulative Update 22version=15.00.0 <15.00.1473.006
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 10version=15.01.0 <15.01.1531.012
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 4version=15.02.0 <15.02.0659.012
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 18version=15.01.0 <15.01.2106.013
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 16version=15.01.0 <15.01.1979.008
Microsoft · Microsoft Exchange Server 2013 Cumulative Update 23version=15.00.0 <15.00.1497.012
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 3version=15.02.0 <15.02.0464.015
Microsoft · Microsoft Exchange Server 2013 Cumulative Update 21version=15.00.0 <15.00.1395.012
Microsoft · Microsoft Exchange Server 2019version=15.02.0 <15.02.0221.018
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 14version=15.01.0 <15.01.1847.012
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 8version=15.01.0 <15.01.1415.010
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 19version=15.01.0 <15.01.2176.009
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 15version=15.01.0 <15.01.1913.012
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 11version=15.01.0 <15.01.1591.018
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 13version=15.01.0 <15.01.1779.008
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 6version=15.02.0 <15.02.0529.013
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 5version=15.02.0 <15.02.0595.008
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 1version=15.02.0 <15.01.1979.006
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 17version=15.01.0 <15.01.2044.013
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 2version=15.02.0 <15.02.0397.011
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 8version=15.02.0 <15.02.0792.010
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 7version=15.02.0 <15.02.0721.013
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 12version=15.01.0 <15.01.1713.010
Microsoft · Microsoft Exchange Server 2013 Service Pack 1version=15.00.0 <15.00.0847.064
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 9version=15.01.0 <15.01.1466.016

Published CVSS scores

7.8Microsoft CorporationCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
6.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationReported
Public exploitPublished

Weakness, pattern, technique

CWE-22Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
T1190Exploit Public-Facing Application
T1505.003Web Shell

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Reported exploitation

Technologies

Your stack

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Your stack

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