Security researchers have identified a critical vulnerability in the Windows Message Queuing (MSMQ) middleware service, with the potential to expose hundreds of thousands of systems to attack. The flaw, CVE-2023-21554, enables unauthenticated attackers to remotely execute code on unpatched Windows servers.
The vulnerability has been patched by Microsoft, but all supported versions of Windows, including the latest Windows 11 22H2 and Windows Server 2022, are affected. Microsoft has warned that the flaw has previously been exploited, making it a priority for customers who have reviewed the security update and determined its applicability within their environment.
Security firm Check Point Research has estimated that over 360,000 internet-exposed MSMQ servers are vulnerable to the flaw. The actual number of unpatched systems is expected to be higher, as the estimate only includes devices running the MSMQ service that are reachable over the internet.
While the MSMQ service is not enabled by default on most systems, it is a middleware service used by other software, which can be toggled on in the background when installing enterprise apps, and will remain running after uninstallation.
Microsoft has advised organisations that can’t immediately deploy the patch to disable the Windows MSMQ service if possible. Admins can check if there is a service running named Message Queuing and TCP port 1801 is listening on their machine.
Organisations that cannot disable MSMQ or deploy Microsoft’s patch can also block 1801/TCP connections from untrusted sources using firewall rules. The flaw was reported to Microsoft by researchers Wayne Low of Fortinet’s FortiGuard Lab and Haifei Li of Check Point Research.
