CVE-2021-31198Network attack vector

SYSTEM code execution via OAB arbitrary file write

Published May 11, 2021 · Updated Feb 28, 2025

Code injection in Microsoft Exchange Server 2013, 2016, and 2019 allows remote attackers to execute code through the OAB service. The service fails to validate user-supplied data before writing arbitrary files into OAB folders. Authentication is normally required but can be bypassed, after which planted files execute in the SYSTEM context.

CVSS severity7.8
High
EPSS probability4.87%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Code injection in Microsoft Exchange Server 2013, 2016, and 2019 allows remote attackers to execute code through the OAB service. The service fails to validate user-supplied data before writing arbitrary files into OAB folders. Authentication is normally required but can be bypassed, after which planted files execute in the SYSTEM context.

The record

CVE
CVE-2021-31198
Published
May 11, 2021
Updated
Feb 28, 2025
Vendor
Microsoft
Product
Microsoft Exchange Server 2013 Cumulative Update 23
Classifications
CWE-94, CWE-20, T1190
Attack vector
network
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 vectornetwork
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

8.8Zero Day InitiativeCVSS:3.0/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
7.8Microsoft CorporationCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

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

CWE-94Improper Control of Generation of Code ('Code Injection')
CWE-20Improper Input Validation
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.

Technologies

Your stack

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Your stack

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