Windows Services for NFS: use-after-free in the ONCRPC XDR driver allows unauthenticated remote code execution

Impact

A use-after-free in the ONCRPC XDR driver behind Windows Services for NFS lets an unauthenticated attacker execute code over the network, scored 9.8. The bug is in a kernel-mode driver, so success means code in the kernel of the file server, not in a user session. This matters to a fleet only where a Windows Server host with the NFS Server or Client role serves or mounts shares - typically a legacy file server holding datasets or scratch space that Linux GPU nodes mount. Such a server is usually reachable from the compute network by design, and compromising it puts the attacker in the path of the data the fleet trains on. Microsoft assigned two ids for the same flaw class in the same driver in the same monthly update; they are covered together here because the fix and the maintenance action are identical.

Who can reach it

Anyone who can send RPC traffic to the NFS ports (typically 111 and 2049) of an affected Windows Server. No authentication and no user interaction. Only hosts with Services for NFS installed are exposed; the affected list is Windows Server 2012 through 2019.

What to do

Install the September 2026 Windows security update for the affected Windows Server builds and reboot - a kernel-mode driver fix, so the node goes down for the restart. Where NFS on Windows is not actually used, removing the Services for NFS role removes the exposure outright. Where it is used, restrict the NFS ports to the subnets that need them until the reboot window; 2012 and 2012 R2 are out of standard support, so confirm you have an update channel for them at all before planning around one.

Also covers 1 CVE

The vendor assigned a separate id to each affected code path. They share this advisory, this score and this fix, so they are one entry here.

References

Related entries

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