Why did Windows 11 24H2 break my NAS connection?

Windows 11 version 24H2 tightened SMB security, and that's why a NAS that worked for years suddenly stopped after the update. Two default changes are responsible.

1. SMB signing is now required

Per Microsoft: "Windows 11, version 24H2 Enterprise, Pro, and Education require both outbound and inbound SMB signing." SMB signing adds a tamper-proof signature to each message to block relay and spoofing attacks. If your NAS doesn't sign, the connection is refused, typically with STATUS_INVALID_SIGNATURE (0xc000a000). (Windows 11 24H2 Home doesn't require signing.)

2. Requiring signing also disables guest access

Microsoft notes that "requiring SMB signing also disables guest access to shares." So a NAS share set up for guest/anonymous access stops working the moment signing is required.

The secure fix (on the NAS)

1. Enable SMB signing on the NAS (DSM: Control Panel → File Services → SMB → Advanced; update firmware first).
2. Use a real user account with a password instead of guest access.
3. Connect by hostname, ensure SMB2/3 is enabled, and prefer Kerberos over NTLM.

What not to do

Microsoft is explicit: "We don't recommend disabling SMB signing as a workaround for third-party servers." Disabling signing or enabling insecure guest logons on Windows reopens relay and ransomware exposure. Configure the NAS to meet the new baseline instead — and keep the data on it backed up.