Thank you for the heads up, this is helpful information.

Any more details? Versions of SMB or back-end server info?

FWIW, having no problems here connecting to SMB or CIFS shares from 10.12.5, servers are restricted to SMB2 connections. We just pushed 10.12.5 out to our test group last week, so very curious before we push to production.

Updated the workaround does work. My account was being locked out for another reason.

Excerpt of an email from the Apple engineers:
I understand that after updating to 10.12.5 clients may hang indefinitely at connecting to an SMB server. If my understanding is incorrect, let me know.

We have received other reports of this behavior, however it is unusual that specifying a colon after the username is not successful workaround for your organization (smb://username:@server.domain.name/sharename). To assist in our investigation,
please provide the following:

We've seen the same issue, but it is very sporadic. One machine which wasn't working consistently, began working the following day when we tried to test more and reproduce the issue. My case # for reference: 100195322890. Most of our users have moved on to OneDrive for file access, and most aren't on 10.12.5 yet as we disabled updating via AppStore after we heard about this issue. So it is tricky to track down. Trying to gather more info and the logs (sysdiagnose, tcpdump, etc) Apple wants. They also suggested these workarounds/tests:
a. Connecting without the share name, smb://servername
b. Connecting to smb://username:@servername/shareName
c. Connecting via command line ‘mount –t smbfs //user@servername/share /tmp/mountpoint’

We appear to be able to use smb based connections with macOS 10.12.5 without any issues.

Although there are some smb based systems we haven't tested it with yet…

Hi All, I too am seeing this behavior on 2 updated 10.12.4 -> 10.12.5 Unit test computers. The workaround (smb://username:@server.domain.name/sharename) is working for my account on both test computers. I am reluctant to push this out to anything more than 1 or 2 pilot computers. I will be keeping an eye on this thread to see what may come of it. in the mean time I will try to research it further.

We are seeing this issue too in 10.12.5 and in the 10.12.6 beta 1. I've opened a bug report with Apple and am waiting to hear back. The workaround presented above is helping in the meantime.

We have this issue on 4 Macs running 10.12.5. It is very sporadic as we have 30+ Macs running 10.12.5. I've opened a bug report with Apple as well. We are using the aforementioned workaround until we hear back from Apple Hopefully this is addressed in 10.12.6 before it ships.

We did hear back on our case that Engineering is working on a fix; hopefully we'll see it in 10.12.6 Beta 2.

We are having this issue on 10.12.5 and the (smb://username:@server.domain.name/sharename) workaround functions intermittently. I have found, however, that killing the AppleIDAuthAgent process will allow connections when the workaround will not. If you are scripting this, you will need to run it with the -9 flag ( for a non-catchable, non-ignorable kill):

pkill -9 AppleIDAuthAgent

To see if this is your issue, try to mount the server, then take a look in Activity Monitor and see if this process is running. Force quit it and the mount should occur instantly.

I too am seeing this SMB bug(albeit sporadic) on a couple of our systems. Other machines are working fine after updating to 10.12.5. Thanks for the info, will be testing workaround(s) today.

You saved my butt, thanks! smb://username:@server.domain.name/sharename

This worked well in testing. pkill -9 AppleIDAuthAgent

However, after ~ 3 minute, needed to run it again.

... didn't matter if it was printing, or SMB connection the command worked.

So now, need to figure out a long term implementation of the command, when where and how.

We'd been having zero problems with SMB on our 10.12.5 machines until yesterday, when I couldn't install software via policy because the target machine couldn't mount the share. This morning, my primary workstation just stopped talking to SMB shares about an hour ago. Checked to make sure the shares weren't still connected, rebooted, etc...no joy. However, the "pkill -9 AppleIDAuthAgent" command as suggested by @annamentzer has allowed me to get reconnected. THANK YOU!

Anyone know of macOS 10.12.6 beta 2 (released yesterday) resolves this issue?

Heard back from Apple. This is apparently going to be a future feature update. iSMB will retail for about 5k for the company in order to allow for SMB connections again.

What is iSMB? ANd does "future featuer update" mean 10.12.6 for example?

What is iSMB? And does "future feature update" mean 10.12.6 for example?

I'm just fucking around. I wouldn't be surprised if they pulled some shit like that though.

Starting in 10.12.5, I have seen this Finder dilog box appearing before any SMB volumes are mounted from scripts (including a Jamf login script). It appears to be intermittent. Definitely not consistent.

Is this new behavior in 10.12.5? I dont recall seeing it before...

Edit /etc/nsmb.conf
and add
signing_required=no

@dstranathan I've been seeing that randomly since 10.12.4. It doesn't happen every time and it doesn't happen to every server or even every user. It's very inconsistent. It doesn't stop users, but it does slow them down a little when they have to process this unexpected message.

Edit /etc/nsmb.conf
and add
signing_required=no

@AVmcclint @Dinnerticketboy see https://www.jamf.com/jamf-nation/discussions/23565/you-are-attempting-to-connect-to-the-server-dialog-box-since-10-12-4

Should the /etc/nsmb.conf file exist by default? I don't see it on my 10.12.5 Mac...does this also help with the hangs while mounting SMB shares and/or printing to Windows-based printers?