Windows 11 puts 'disgusting' Remote Mailslots protocol out of its misery

It's simple, unreliable, insecure, and on its way out

Microsoft recently outlined several new features it is building into Windows 11, from file recommendations and one-keystroke shortcuts for the XAML context menu in File Explorer to Local Security Authority (LSA) protection against secrets and credential thefts.

However, buried at the bottom of the Windows 11 Insider Preview Build 25314 released to the Canary Channel was the first step in dismantling Remote Mailslots, a decades-old legacy interprocess communications protocol with roots in LAN Manager DOS – or the days before Windows NT.

Or as Ned Pyle, principal program manager at Microsoft, wrote, "It goes without saying that this protocol is disgusting. If you've been an IT Pro for a few decades, you might recall people using the anonymous NET SEND command (MAILSLOT\Messngr) to broadcast important messages to all logged on users," a process he called "crap."

Mailslots are used as a server-client interface. A server creates a mailslot and a client writes datagrams – or short message broadcasts to all computers on a network that are listening – to it using NetBIOS datagrams as a transport when running over a network with Windows, according to Pyle. As an example, the Proto-SMB1 Common Internet File System (CIFS) browser protocol uses "\MAILSLOT\LANMAN" and "\MAILSLOT\BROWSE," he wrote.

A client can write data to it by name and only the server can read the mailslot. The server creating a mailslot receives a mailslot handle, which is used when a process reads messages from the mailslot.

There are limitations. The data in a mailslot can't be larger than 424 bytes – and once the handles to a mailslot are closed, the mailslot and all the data inside are deleted.

The problem, Pyle wrote, is that Remote Mailslot is a "simple, unreliable, insecure, and unidirectional" protocol whose time has passed. Given that, starting with the latest Insider Preview Build, Remote Mailslot is being disabled by default.

Those using Remote Mailslot are seemingly tied to the old ways of doing things. To use Remote Mailslot, a person also needs to be using the Server Message Block (SMB) 1 protocol, which was disabled by default and superseded in 2007 by SMB2, so about 99.97 percent of Windows users are not affected, Pyle wrote.

Those who have manually re-enabled SMB1 and have an application that still uses a Remote Mailslot will see this message:

3025

ERROR_REMOTE_MAILSLOTS_DEPRECATED

"The requested operation failed. Remote mailslots have been deprecated."

Those getting the message should "contact your vendor about updating their software to join the 21st century, as it both requires SMB1 and Remote Mailslot," Pyle wrote. "This protocol is not secure, was replaced decades ago by better technology, and should not be used under any circumstances."

If a user needs "to re-enable Remote Mailslots temporarily while you yell at your vendor or developer," use the following PowerShell command:

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

In the next release of Windows and Windows Server, Remote Mailslots will be deprecated, the next step before being removed altogether. Pyle said that will take time and there will ample public warnings about it, similar to what users saw with SMB1. ®