⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● Critical · CVSS 9.9

How to Fix CVE-2026-42898: Code Injection RCE in Microsoft Dynamics 365 (on-premises) version 9.1

Other vulnerabilities in the same area that are worth patching alongside this one:

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 9.9 - Critical
Actively exploited?Not currently listed in CISA KEV
Affected9.0 < 9.1.45.11
Fixed inSee vendor advisory
Type (CWE)CWE-94: Improper Control of Generation of Code ('Code Injection')

What is CVE-2026-42898?

CVE-2026-42898 is a code injection flaw in Microsoft Dynamics 365 (on-premises) version 9.1. Attacker-controlled input is evaluated as code by the application runtime, giving the attacker arbitrary execution inside the process. Vendor description: Improper control of generation of code ('code injection') in Microsoft Dynamics 365 (on-premises) allows an authorized attacker to execute code over a network.

Why this CVE matters

Code injection against an application server is a direct path to remote code execution. The attacker executes inside the application runtime, which means database credentials, integration keys, and any secrets the process has loaded in memory are all exposed.

For deployments of Microsoft Dynamics 365 (on-premises) version 9.1 that have been exposed to the public internet during the disclosure window, the operating assumption should be that scanning has already happened. Even where exploitation has not been publicly observed, scanning for the vulnerable fingerprint is cheap and routine. Patching closes the door; log review and credential rotation close out the rest of the response.

Am I affected?

You are affected if your installation matches any of these version ranges:

Check your installed version against the list above. If you cannot determine the version, treat the system as affected and follow the upgrade path below.

On Windows, check the product's installed version via Settings - Apps - Installed apps, or run Get-Package from PowerShell to enumerate installed versions.

How to fix CVE-2026-42898

  1. Read the vendor advisory in full: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-42898
  2. Upgrade Microsoft Dynamics 365 (on-premises) version 9.1 to the patched build listed in the vendor advisory.
  3. Back up the configuration (and database, where applicable) before upgrading.
  4. Rotate any credentials, API keys, or session tokens that the vulnerable service touched. An unauthenticated RCE-class flaw means anything the process could see should be treated as exposed.
  5. Apply the patch in a maintenance window. For HA pairs, upgrade the standby node first, fail over, then upgrade the former primary.
  6. Restart the affected service so the patched binary loads, then verify the new version (see verification section).

Apply the Microsoft security update


# CVE-2026-42898 affects Microsoft Dynamics 365 (on-premises) version 9.1. Affected build range: 9.0 < 9.1.45.11.
# Fixed in build: 9.1.45.11.
# Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-42898

# 1. Check the current build on the host.
[System.Environment]::OSVersion.Version
Get-ComputerInfo | Select-Object OsName, OsVersion, OsBuildNumber

# 2. Install the cumulative + security rollup that ships the fix.
Install-Module -Name PSWindowsUpdate -Force -SkipPublisherCheck -Confirm:$false
Import-Module PSWindowsUpdate
Get-WindowsUpdate -AcceptAll
Install-WindowsUpdate -AcceptAll -AutoReboot

# 3. Verify the patched build is present.
[System.Environment]::OSVersion.Version
# The build number must be >= 9.1.45.11 for the patch listed in the advisory.


# Inventory missing patches across a Windows fleet via Ansible (winrm).
ansible windows -m win_updates -a "category_names=SecurityUpdates state=installed"


# Re-run the version check after reboot and log the result.
$log = "C:\Logs\CVE-2026-42898-fix.log"
New-Item -ItemType Directory -Force -Path (Split-Path $log) | Out-Null
$build = [System.Environment]::OSVersion.Version
"$(Get-Date -Format s) post-patch build: $build" | Out-File $log -Append

Verify the fix landed


# CVE-2026-42898 verification checklist.

# 1. Confirm the running version matches 9.1.45.11 (replace the version probe with
#    the platform-specific command shown above).

# 2. Re-scan the host with your vulnerability scanner (Nessus, Qualys, Tenable,
#    OpenVAS, Wazuh). The scanner must no longer flag CVE-2026-42898.

# 3. Inspect recent service and kernel logs for crash-loops or rollback events.
journalctl -u <service-name> --since "10 minutes ago"
dmesg --since "10 minutes ago"

# 4. Cross-check the running build against the vendor advisory:
#    https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-42898

If you cannot patch immediately

No official workaround exists beyond restricting network exposure to the affected component. Apply the vendor patch as the primary remediation.

How to verify the fix worked

If your installation was internet-reachable during the disclosure window, treat log review as part of the remediation rather than an optional follow-up. Look for unexpected administrator accounts in Microsoft Dynamics 365 (on-premises) version 9.1, scheduled tasks or cron jobs you did not create, new files in web-accessible directories, and outbound connections to addresses not in your baseline. Suspicious requests to the vulnerable endpoint immediately followed by successful 200-class responses with unusually large bodies are a strong indicator of exploitation.

Frequently asked questions

Is CVE-2026-42898 being exploited in the wild?

Public exploitation has not been confirmed by CISA at the time of writing. Treat the patch as time-sensitive anyway; reports often lag actual abuse.

Will a WAF or IDS rule fully mitigate CVE-2026-42898?

No. Network-layer filters can reduce noise and slow opportunistic scanners, but they will not stop a determined attacker. The vendor patch is the only durable fix.

Do I need to assume compromise if my Microsoft Dynamics 365 (on-premises) version 9.1 was internet-facing and unpatched?

For an unauthenticated RCE-class flaw exposed to the public internet during the known exploitation window, yes. Review logs, rotate credentials the process could access, and look for unexpected accounts, scheduled tasks, or outbound connections.

References

*This guide was assembled from the official vendor advisory, the NVD record, and the CISA KEV catalog entry on 2026-05-25. Always confirm against the vendor advisory before applying changes in production.*