IRONSMITHINTEL
CRITICALCVSS9.8
|CVE-2023-35349|Auth: none|Reboot: required|Manual only

KB5031362: Windows Server 2016 Security Update (October 2023)

A crafted packet to TCP port 1801 on a Windows Server with MSMQ installed can give an attacker SYSTEM-level code execution.

Published Oct 10, 2023 · Updated May 21, 2026
Why patchRisk explained in plain English
Worst-case scenarioIf unpatched

An attacker who can reach TCP 1801 on a Windows Server with MSMQ installed can send a single crafted packet and execute code in the MSMQ service context (typically Network Service or SYSTEM). No authentication, no user interaction. Internet-exposed MSMQ hosts at QueueJumper disclosure numbered ~360,000 — internal exposure is far higher because MSMQ is installed by many enterprise applications without administrators realising it.

How the attack works

Microsoft Message Queuing (MSMQ) is a Windows messaging service used by line-of-business applications for asynchronous communication. When installed, MSMQ listens on TCP 1801. A flaw in how the MSMQ packet parser validates incoming messages lets an attacker craft a packet that corrupts memory inside the MSMQ service process (mqsvc.exe), leading to unauthenticated remote code execution.

Am I affected?Quick check

Probably yes if any of these apply:

●Any Windows Server with the MSMQ feature installed
●Frequently includes Exchange (legacy), SCCM, and many third-party LOB application servers

Affected OS versions

Windows Server 2016
Real-world incidentsWhat we've seen

A penetration tester scanning a corporate network finds half a dozen servers listening on TCP 1801 — none of which the IT team knew were running MSMQ. The CVE's exploit-development effort is amortised across multiple companion CVEs in the same parser, so working code is widely available. One crafted packet per host gives the tester SYSTEM on each. Internal MSMQ exposure is one of the most under-tracked attack surfaces in modern Windows environments.

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 CatalogKB5031362

Manual remediation steps

Check whether MSMQ is installed and exposed

MSMQ is sometimes installed by other software without administrators realising it. Check explicitly:

Get-WindowsFeature -Name MSMQ* | Where-Object Installed
Get-NetTCPConnection -LocalPort 1801 -State Listen -ErrorAction SilentlyContinue

If MSMQ is not needed, removing the feature is the strongest mitigation — patch is required regardless if the feature remains.

Prerequisites

    1
    Local administrator on the target server
    1
    Maintenance window with reboot capacity
    1
    Current backup or snapshot you can roll back to
    1
    Network path to Windows Update / WSUS / Microsoft Update Catalog

Estimated time

20–40 minutes per server (download + install + reboot)

Reboot required

Yes — install the cumulative update and reboot the server before the fix is active.

Steps

1. Confirm the server is missing the patch

Get-HotFix -Id KB5031362 -ErrorAction SilentlyContinue

2. Install the update — pick one channel

Windows Update / WSUS (preferred):

UsoClient ScanInstallWait
# (or use your standard WSUS / SCCM / Intune deployment for KB5031362)

Manual download (offline / air-gapped):

1
Open Microsoft Update Catalog: https://catalog.update.microsoft.com/Search.aspx?q=KB5031362
2
Download the MSU for Windows Server 2016 that matches your architecture (x64).
3
Copy the .msu file to the server and run as Administrator.

3. Reboot

Restart-Computer -Force

Verification

Get-HotFix -Id KB5031362
[System.Environment]::OSVersion.Version

Rollback

wusa.exe /uninstall /kb:5031362 /quiet /norestart
# Reboot after uninstall

Removing a cumulative update also removes every fix it delivered — prefer rolling forward.

Notes

    1
    This entry covers Windows Server 2016 specifically (KB5031362). Other Windows Server versions have their own KB for CVE-2023-35349.
    1
    Reference advisories: MSRC https://msrc.microsoft.com/update-guide/vulnerability/CVE-2023-35349 and NVD https://nvd.nist.gov/vuln/detail/CVE-2023-35349.
PowerShell automationComing soon

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References

↗ CVE-2023-35349↗ MSRC Advisory↗ NVD↗ KBKB5031362