CVE-2021-31209

Update tampering via missing download integrity check

Published May 11, 2021 · Updated Feb 28, 2025

Spoofing in Microsoft Exchange Server 2013, 2016, and 2019 allows attackers to tamper with Exchange Help update data. Exchange Server Help update handling downloads update content without verifying its integrity, permitting modified data to be accepted as trusted. Exploitation requires network-adjacent access and user interaction; chained with another flaw, the tampered update can execute code as SYSTEM.

CVSS severity6.5
Medium
EPSS probability2.63%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Spoofing in Microsoft Exchange Server 2013, 2016, and 2019 allows attackers to tamper with Exchange Help update data. Exchange Server Help update handling downloads update content without verifying its integrity, permitting modified data to be accepted as trusted. Exploitation requires network-adjacent access and user interaction; chained with another flaw, the tampered update can execute code as SYSTEM.

The record

CVE
CVE-2021-31209
Published
May 11, 2021
Updated
Feb 28, 2025
Vendor
Microsoft
Product
Microsoft Exchange Server 2013 Cumulative Update 23
Classifications
CWE-290, T1557
Attack vector
adjacent
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 11, 2021CVE publishedPublication date reported by the CVE source.
  2. Feb 28, 2025Record 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 <15.00.1497.018

What conditions does exploitation require?

Attack vectoradjacent
Required privilegesunauthenticated

What is affected?

Microsoft · Microsoft Exchange Server 2013 Cumulative Update 23version=15.00.0 <15.00.1497.018
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 19version=15.01.0 <15.01.2176.014
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 8version=15.02.0 <15.02.0792.015
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 9version=15.02.0 <15.02.0858.012
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 20version=15.01.0 <15.01.2242.010

Published CVSS scores

3.5Zero Day InitiativeCVSS:3.0/AV:A/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N
6.5Microsoft CorporationCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:L/A:N
8.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
5.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/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)
Vectoradjacent
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-290Authentication Bypass by Spoofing
T1557Adversary-in-the-Middle

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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