CVE-2021-42321Known exploitationPublic exploitNetwork attack vector

Authenticated code execution via unsafe mailbox deserialization

Published Nov 10, 2021 · Updated Aug 19, 2026

Remote code execution in Microsoft Exchange Server 2016 and 2019 allows remote authenticated users to execute code through Exchange Web Services. Exchange accepts crafted serialized data in the ExtensionMasterTable mailbox configuration, and incomplete ChainedSerializationBinder restrictions permit a ClaimsPrincipal gadget chain. A valid mailbox account and access to Exchange Web Services are required; successful exploitation runs attacker-controlled commands on the server.

CVSS severity8.8
High
EPSS probability91.74%
Next 30 days · Sep 17, 2026
Known exploitationReported
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Remote code execution in Microsoft Exchange Server 2016 and 2019 allows remote authenticated users to execute code through Exchange Web Services. Exchange accepts crafted serialized data in the ExtensionMasterTable mailbox configuration, and incomplete ChainedSerializationBinder restrictions permit a ClaimsPrincipal gadget chain. A valid mailbox account and access to Exchange Web Services are required; successful exploitation runs attacker-controlled commands on the server.

The record

CVE
CVE-2021-42321
Published
Nov 10, 2021
Updated
Aug 19, 2026
Vendor
Microsoft
Product
Microsoft Exchange Server 2016 Cumulative Update 21
Classifications
CWE-502, CWE-184, T1190
Attack vector
network
Privileges
authenticated

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 privilegesauthenticated

What is affected?

Microsoft · Microsoft Exchange Server 2016 Cumulative Update 21version=15.01.0 <15.01.2308.020
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

8.8NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
8.8MicrosoftCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
6.5NVDAV:N/AC:L/Au:S/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-502Deserialization of Untrusted Data
CWE-184Incomplete List of Disallowed Inputs
T1190Exploit Public-Facing Application

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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