IRONSMITHINTEL
HIGHCVSS8.8
|
Actively Exploited
|CISA KEV|CVE-2023-21529|Auth: low — authenticated user|Reboot: required|Manual only

KB5023038: Windows Server Security Update (February 2023)

Microsoft Exchange Server contains a deserialization of untrusted data that allows an authenticated attacker to achieve remote code execution.

Published Feb 14, 2023 · Updated May 16, 2026
Why patchRisk explained in plain English
Worst-case scenarioIf unpatched

A remote attacker, with a low-privilege account, can achieve full data confidentiality loss, arbitrary modification of data, complete denial of service or system unavailability. CISA has confirmed use of this vulnerability in known ransomware campaigns — treat as high priority for remediation. Federal agencies are required to remediate by 2026-04-27 under CISA BOD 22-01.

How the attack worksNo clicks needed

This is a Deserialization of Untrusted Data (CWE-502) vulnerability in Microsoft Exchange Server. Exploitation requires remote network access, low attack complexity, a low-privilege authenticated account, and no user interaction required.

Am I affected?Quick check

Probably yes if any of these apply:

●Exchange Administrators
●Messaging Team
●IT Security
●Running exchange server: 2013, 2016, 2019
Fixed inKB5023038 (applies to 4 product versions) — build 15.00.1497.047, 15.01.2507.021+
Real-world incidentsWhat we've seen

Used in known ransomware campaigns. Threat-research write-up: https://www.microsoft.com/en-us/security/blog/2026/04/06/storm-1175-focuses-gaze-on-vulnerable-web-facing-assets-in-high-tempo-medusa-ransomware-operations/

How to patch

Manual download

For air-gapped servers or out-of-band deployment. Microsoft Update Catalog returns every OS-version variant of this update.

↗ Microsoft Update CatalogKB5023038

Manual remediation steps

Apply the Microsoft Security Update

Microsoft has released an official security update that fixes this vulnerability.

Required KB Update

    1
    KB5023038 — https://support.microsoft.com/help/5023038

Supersedes: KB5019758

Affected Products

    1
    Microsoft Exchange Server 2013 Cumulative Update 23
    1
    Microsoft Exchange Server 2016 Cumulative Update 23
    1
    Microsoft Exchange Server 2019 Cumulative Update 11
    1
    Microsoft Exchange Server 2019 Cumulative Update 12

Fixed Build Numbers

    1
    15.00.1497.047
    1
    15.01.2507.021
    1
    15.02.0986.041
    1
    15.02.1118.025

Installation Methods

Windows Update (recommended)

1
Settings → Windows Update → Check for updates
2
The security update is offered if your system is in scope
3
Restart when prompted — a reboot IS required to complete the install

Microsoft Update Catalog (manual download)

1
Open https://catalog.update.microsoft.com
2
Search for KB5023038
3
Download the package matching your OS architecture and Windows build
4
Run the .msu installer with administrator privileges
5
Restart when prompted

WSUS / SCCM / Intune

Approve KB5023038 for the affected products in your update management console.

Microsoft Download Center Links

    1
    https://www.microsoft.com/download/details.aspx?familyid=1b7a2ea2-c8af-4ea5-837a-e5a9fcc60bcd
    1
    https://www.microsoft.com/download/details.aspx?familyid=30d02c29-3f58-48b5-8dc6-6b54af690732
    1
    https://www.microsoft.com/download/details.aspx?familyid=68684ee7-ed06-4819-92d4-33b46eb56093
    1
    https://www.microsoft.com/download/details.aspx?familyid=80de331a-fc22-450e-b951-cc3f933897e4

Verification

Confirm the update is installed:

Get-HotFix | Where-Object { $_.HotFixID -in @('KB5023038') }

References

    1
    Microsoft Security Response Center: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2023-21529
    1
    KB article: https://support.microsoft.com/help/5023038
    1
    NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2023-21529
    1
    CISA KEV: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2023-21529

Discovery Credit

Piotr Bazydlo (@chudypb) of Trend Micro Zero Day Initiative, zcgonvh with 360 noah lab

PowerShell automationComing soon

No tested PowerShell script for this entry yet. We’re prioritising automation based on user demand.

References

↗ CVE-2023-21529↗ MSRC Advisory↗ NVD↗ KBKB5023038