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Microsoft is getting rid of the Remote Mailslot legacy protocol in Windows 11 soon

A couple of days ago, Microsoft released build 25314 to its new Canary Channel. The build contains a lot of new features including access key shortcuts and recommendations in File Explorer, along with LSA protection enablement. There are indications of upcoming improvements to Taskbar and Windows Settings too. Notably on the security side, the company is getting rid of the Remote Mailslot protocol.

A broadcast message from Remote Mailslot with a vomiting emoji on top

In a dedicated blog post, Microsoft's Principal Program Manager Ned Pyle has noted that the legacy Remote Mailslot protocol has already been disabled by default in the latest Canary Channel release. It will be deprecated shortly and will eventually be removed permanently.

For those unaware, Remote Mailslot is a protocol belonging to the days of LAN Manager DOS, that is, older than Windows NT. It follows a communication process where the mailslot server creates a mailslot that clients can write data to, but no longer than 424 bytes. A mailslot is ephemeral, once all the handles to it are closed, the mailslot and all the data in it gets deleted. Mailslots were used to broadcast messages within a domain, with Pyle having particularly strong opinions about the legacy protocol, calling it "disgusting" and "crap".

Starting with build 25314 in the Canary Channel, Remote Mailslot is disabled by default and you'll be greeted with an error if you attempt to access it. If you come across errors, Pyle has recommended shouting at your software vendor and telling them to upgrade their software "to join the 21st century". While your vendor asks for time to concede to your demand, you can use run the following PowerShell command to get Remote Mailslot running again, with the knowledge that it is a major security risk to your organization:

PS C:\> Set-SmbClientConfiguration -EnableMailslots $true

The removal of Remote Mailslot will not affect 99.97% of Windows customers since it is also used in tandem with the SMB1 protocol that has been disabled for the past several years too. Microsoft has noted that more deprecations for similar and even bigger legacy protocols are on the way too.

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