CVE-2021-42305Network attack vector

Content spoofing requiring user interaction

Published Nov 10, 2021 · Updated Aug 19, 2026

Content spoofing in affected Microsoft Exchange Server 2013, 2016, and 2019 builds allows remote attackers to alter displayed content. Microsoft has not published the affected component, crafted input, or processing error that permits the spoofed content. Exploitation is network-based and unauthenticated but requires interaction with attacker-controlled content; the documented impact is limited to integrity.

CVSS severity6.5
Medium
EPSS probability7.87%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Content spoofing in affected Microsoft Exchange Server 2013, 2016, and 2019 builds allows remote attackers to alter displayed content. Microsoft has not published the affected component, crafted input, or processing error that permits the spoofed content. Exploitation is network-based and unauthenticated but requires interaction with attacker-controlled content; the documented impact is limited to integrity.

The record

CVE
CVE-2021-42305
Published
Nov 10, 2021
Updated
Aug 19, 2026
Vendor
Microsoft
Product
Microsoft Exchange Server 2016 Cumulative Update 21
Classifications
Unavailable
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Nov 10, 2021CVE publishedPublication date reported by the CVE source.
  2. 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=15.01.0 <15.01.2308.020

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Microsoft · Microsoft Exchange Server 2016 Cumulative Update 21version=15.01.0 <15.01.2308.020
Microsoft · Microsoft Exchange Server 2013 Cumulative Update 23version=15.00.0 <15.00.1497.026
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 11version=15.02.0 <15.02.0986.014
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 10version=15.02.0 <15.02.0792.019
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 22version=15.0.0 <15.01.2375.017

Published CVSS scores

4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:P/A:N
6.5Microsoft CorporationCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

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 exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

No sourced classifications are available.

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.

Technologies

Your stack

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Your stack

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