CVE-2021-33766Known exploitationNetwork attack vector

Unauthenticated information disclosure via ECP authentication bypass

Published Jul 14, 2021 · Updated Aug 10, 2026

Information disclosure in Microsoft Exchange Server 2013, 2016, and 2019 allows remote attackers to access sensitive information via ECP web services. The ECP web application fails to authenticate crafted web-service requests, allowing an unauthenticated caller to bypass authentication and retrieve server information. Exploitation requires network access to ECP; confidentiality impact is established, while integrity and availability effects are not established by the cited advisory.

CVSS severity5.0
Medium
EPSS probability98.14%
Next 30 days · Sep 16, 2026
Known exploitationReported
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Information disclosure in Microsoft Exchange Server 2013, 2016, and 2019 allows remote attackers to access sensitive information via ECP web services. The ECP web application fails to authenticate crafted web-service requests, allowing an unauthenticated caller to bypass authentication and retrieve server information. Exploitation requires network access to ECP; confidentiality impact is established, while integrity and availability effects are not established by the cited advisory.

The record

CVE
CVE-2021-33766
Published
Jul 14, 2021
Updated
Aug 10, 2026
Vendor
Microsoft
Product
Microsoft Exchange Server 2013 Cumulative Update 23
Classifications
T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 14, 2021CVE publishedPublication date reported by the CVE source.
  2. Jan 18, 2022Exploitation reportedCISA Known Exploited Vulnerability listing
  3. Aug 10, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

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

  1. Affected versionversion=15.00.0 <15.00.1497.015

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Microsoft · Microsoft Exchange Server 2013 Cumulative Update 23version=15.00.0 <15.00.1497.015
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 19version=15.01.0 <15.01.2176.012
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 8version=15.02.0 <15.02.0792.013
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 9version=15.02.0 <15.02.0858.010
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 20version=15.01.0 <15.01.2242.008

Published CVSS scores

7.5[email protected]CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
5.0[email protected]AV:N/AC:L/Au:N/C:P/I:N/A:N
7.3[email protected]CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L

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
Privilegesunauthenticated
Known exploitationReported
Public exploitUnconfirmed

Weakness, pattern, technique

T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

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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