How to Address CVE 2025 21307 Without a Patch Before the Weekend

Microsoft's January 2025 Patch Tuesday release addresses a critical vulnerability-CVE-2025-21307 -in the Windows Reliable Multicast Transport Driver (RMCAST). With a CVSS score of 9.8, this vulnerability poses a severe threat and is highly susceptible to exploitation.

What is CVE-2025-21307?

RMCAST is a kernel-level Windows component responsible for implementing reliable multicast communication. It specifically supports the Pragmatic General Multicast (PGM) protocol, which enables applications to send data packets simultaneously to multiple receivers while guaranteeing reliable, ordered delivery. The vulnerability allows an unauthenticated, remote attacker to execute arbitrary code on a vulnerable Windows system by sending specially crafted packets to a Windows PGM open socket. PGM is designed for reliable data transmission-commonly used in video streaming and financial data distribution-yet lacks inherent request authentication. This absence of authentication is central to the vulnerability.

Successful exploitation requires a program that actively listens to a PGM port.

Deploying the Patch Before the Weekend

On Patch Tuesday, Microsoft released a patch to solve this vulnerability, and some organizations may have a mature Patch Tuesday process that allows them to deploy those patches to, usually, end-users' machines; however, addressing this critical vulnerability on critical servers before the weekend is a different challenge altogether.

Address the Vulnerability Before the Weekend When Patching is Not an Option

As every IT knows, deploying a Microsoft patch that involves kernel-level components will require a reboot of the server and may even break the application running on that server; hence, intensive testing is required. As such, for most organizations, deploying patches to servers is a long process that may take weeks and, unfortunately, in some cases, months.

However, even though the business reasoning behind this long process of deploying patches to a production server is valid, bad actors see this as an opportunity to exploit those critical, unpatched vulnerabilities.

In Qualys, our goal is to help our customers address risk efficiently while acknowledging that most organizations cannot deploy patches to production servers as soon as the patch is released - i.e., before the weekend. To help our customers address vulnerabilities before the weekend, without the risk involved in deploying a patch and rebooting a server, the Qualys Threat Research Unit (TRU) researches critical vulnerabilities and mitigation techniques that can be applied to reduce risk until a patch can be deployed. In most cases, those mitigation techniques introduce minimal operational risk and hence are "safer" to deploy immediately.

CVE-2025-21307 is a good example. In this case, our TRU team suggests our customer choose from a few mitigation techniques that, in the right context, if applied to servers, will reduce the security risk with minimal operational risk to the servers. For example, one mitigation technique is to disable the MSMQ service if it is running. Customers can use the same Qualys agent to run a script written and tested by Qualys to disable this service and, as a result, mitigate CVE-2025-21307 until a patch can be deployed. As most servers do not need the MSMQ service, disabling it will cause no service disruption.

If you work for an organization that values a rapid response to critical vulnerabilities without deploying a patch, you may enable a free trial of TruRisk Eliminate today and deploy this mitigation technique ASAP.

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