Control SMB signing behavior (preview) (2024)

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Important

Windows Insider and Windows Server Insider builds are in PREVIEW. This information relates to a prerelease product that may be substantially modified before it's released. Microsoft makes no warranties, expressed or implied, with respect to the information provided here.

How SMB signing works

Server Message Block (SMB) signing is a security feature that uses the session key and cipher suite to add a signature to a message going across a connection. This signature contains a hash of the entire message in the SMB header. If someone tampers with the message in transit, the data in the tampered message doesn't match the hash in the signature. The hash also includes the identities of the original sender and the intended recipient. Signature mismatches alert users to possible foul play, helping them protect their deployments from relay and spoofing attacks.

SMB signing requirements can involve both outbound signing, which covers traffic from the SMB client, and inbound signing, which covers traffic to the server. Windows and Windows Server can require outbound signing only, inbound signing only, both, or neither. For example:

  • Windows 11 Insiders Enterprise, Pro, and Education require both outbound and inbound SMB signing.

  • Windows Server Insiders requires outbound SMB signing only.

  • Windows 11 Insider Home edition doesn't require outbound or inbound SMB signing.

SMB signing behavior

Although all versions of Windows and Windows Server support SMB signing, a third-party may opt to disable or not support it. If you try to connect to a remote share on a third-party SMB server that doesn't allow SMB signing, you may encounter one of the following error messages:

0xc000a000-1073700864STATUS_INVALID_SIGNATURE The cryptographic signature is invalid.

To resolve this issue, adjust the settings on your third-party SMB server to allow (enable) SMB signing.

When you try to connect to third-party devices that use guest accounts to simplify access, you may receive one of these error messages:

You can't access this shared folder because your organization's security policies blockunauthenticated guest access. These policies help protect your PC from unsafe or maliciousdevices on the network.
Error code: 0x80070035The network path was not found.
System error 3227320323 has occurred.

Disabling SMB signing may be necessary if you're unable to disable guest usage for your third-party. However, this means that you're using guest access and preventing your client from ensuring signing to a trusted device.

Caution

We don't recommend disabling SMB signing as a workaround for third-party servers. We also don't recommend trying to sign with guest accounts.

Prerequisites

In order to control SMB signing behavior and maximize its capabilities, your system must be running one of the following two operating systems:

  • Windows 11 Insider Preview Build 25905 or later
  • Windows Server Preview Build 26010 or later

You should also follow these recommendations to ensure your SMB signatures are effective at securing your data:

  • Use Kerberos instead of NTLMv2.
  • Don't connect to shares using IP addresses.
  • Don't use CNAME DNS records. Instead, assign alternate computer names with NETDOM.EXE.

Disable SMB signing

SMB signing is required by default on the latest Insider Preview builds of Windows 11 and Windows Server. All Windows environments support SMB signing. However, if your environment uses third-party servers and the third-party server doesn't support SMB signing, you can't connect to the remote share.

Requiring SMB signing also disables guest access to shares. In these cases, you must disable SMB signing manually to restore access for guest accounts. You can manually disable SMB signing through Group Policy, PowerShell, and Windows Admin Center.

Note

If you need to modify the Active Directory domain-based group policy, use Group Policy Management (gpmc.msc).

  • Group Policy
  • PowerShell
  • Windows Admin Center

To disable SMB signing in Group Policy, perform the following steps:

  1. Select Start, type gpedit.msc, then hit Enter.

  2. In the Local Group Policy Editor, navigate to Computer Configuration\Windows Settings\Security Settings\Local Policies\Security Options.

  3. Open Microsoft network client: Digitally sign communications (always), select Disabled, then select OK.

Enable SMB signing

SMB signing ensures data integrity by verifying that data isn't tampered with during transmission. Additionally, SMB signing provides authentication by verifying the identity of the server and client, which helps prevent adversary-in-the-middle attacks.

  • Group Policy
  • PowerShell
  • Windows Admin Center

To enable SMB signing in Group Policy, perform the following steps:

  1. Select Start, type gpedit.msc, then hit Enter.

  2. In the Local Group Policy Editor, navigate to Computer Configuration\Windows Settings\Security Settings\Local Policies\Security Options.

  3. Open Microsoft network client: Digitally sign communications (always), select Enabled, then select OK.

Verify SMB signing status

To check if SMB signing is enabled or disabled on your SMB client or SMB server, run the following command:

Get-SmbClientConfiguration | FL RequireSecuritySignature
Get-SmbServerConfiguration | FL RequireSecuritySignature

If the returned information is True, then SMB signing is enabled, otherwise, if the returned information is False, then SMB signing is disabled.

Related content

  • Overview of File Sharing using the SMB 3 protocol in Windows Server

  • SMB over QUIC

  • SMB security enhancements

  • How to enable insecure guest logons in SMB2 and SMB3

Control SMB signing behavior (preview) (2024)

FAQs

Control SMB signing behavior (preview)? ›

In the Local Group Policy Editor, navigate to Computer Configuration\Windows Settings\Security Settings\Local Policies\Security Options. Open Microsoft network client: Digitally sign communications (always), select Enabled, then select OK.

What does enabling SMB signing do? ›

SMB signing essentially signs each packet with a digital signature so the client and server can confirm where they originated from as well as the authenticity of the call.

What is the risk of SMB signing? ›

In combination with systems where SMB signing is disabled, an attacker or malicious person can, by performing an NTLM relay attack, increase the privileges within the network. Depending on the network environment an attacker may be able to increase privileges to the highest level.

How do I know if my SMB signing is enforced? ›

The policies for SMB signing are located in Computer Configuration > Windows Settings > Security Settings > Local Policies > Security Options.
  1. Microsoft network client: Digitally sign communications (always) ...
  2. Microsoft network client: Digitally sign communications (if server agrees)
Dec 26, 2023

What is the difference between SMB and SMB signing? ›

Smb encryption is much more secure than smb signatures and smb 3.1 introduced functionality to make smb encryption faster than smb signatures. Smb signatures can be breached by MITM attacks which means your data is compromised. Encryption cannot.

Should I disable SMB signing? ›

Disabling SMB signing may be necessary if you're unable to disable guest usage for your third-party. However, this means that you're using guest access and preventing your client from ensuring signing to a trusted device. We don't recommend disabling SMB signing as a workaround for third-party servers.

Is it safe to enable SMB? ›

Security concerns

The SMBv1 protocol is not safe to use. By using this old protocol, you lose protections such as pre-authentication integrity, secure dialect negotiation, encryption, disabling insecure guest logins, and improved message signing.

What is the risk of SMB? ›

SMB protocol is vulnerable to cyberattacks because its old versions do not use encryption, which means that any hacker who knows how to exploit it can gain access to your files and data.

Does SMB signing slow network performance? ›

SMB signing and SMB encryption are known to slow down SMB transfers. The amount of the performance loss depends greatly on the capabilities of the hardware involved. The primary factors are the count and speed of the CPU core, and how much CPU time is dedicated to other workloads.

What are the famous SMB vulnerabilities? ›

Most common SMB exploits
  • EternalBlue. The EternalBlue vulnerability was discovered by the US National Security Agency (NSA) and published in 2017 by The Shadow Brokers (TSB) hacker group. ...
  • EternalRomance. ...
  • EternalChampion. ...
  • EternalSynergy. ...
  • SMBGhost (CoronaBlue) ...
  • EternalRocks. ...
  • WannaCry. ...
  • Petya and NotPetya.
Nov 11, 2023

How do I know if my SMB is being used? ›

SMB1 - Audit Active Usage using Message Analyzer

I would check on your servers , if they have got it then turn it off. Give it about 10 mins or so , then you will find out what devices are using it. I usually check the active SMB sessions on the servers to try and determine what might be affected.

What is the vulnerability of SMB? ›

The SMB vulnerability can let an unauthorized attacker to run any code as part of an application. According to the Microsoft advisory, “To exploit the vulnerability against an SMB Server, an unauthenticated attacker could send a specially crafted packet to a targeted SMBv3 Server.

How SMB signing can improve the security of a network? ›

By implementing SMB signing, organizations can ensure the integrity of their data and detect potential attacks. Key benefits of enabling this measure include: Increased security: SMB signing helps detect unauthorized access to data and protect against potential attacks.

What is SMB signing vulnerability? ›

SMB signing disabled vulnerability is a security vulnerability that allows an attacker to bypass SMB signing and modify the data in transit. This vulnerability can be exploited by attackers to gain unauthorized access to sensitive information or to carry out other malicious activities.

What port is SMB signing? ›

SMB uses either IP port 139 or 445. Port 139: SMB originally ran on top of NetBIOS using port 139. NetBIOS is an older transport layer that allows Windows computers to talk to each other on the same network. Port 445: Later versions of SMB (after Windows 2000) began to use port 445 on top of a TCP stack.

Is SMB Secure or not? ›

SMB Encryption offers an end-to-end privacy and integrity assurance between the file server and the client. It provides this security regardless of the networks traversed, such as wide area network (WAN) connections maintained by non-Microsoft providers.

What are the benefits of enabling SMB direct on an interface? ›

Using SMB Direct provides: Increased throughput: Leverages the full throughput of high speed networks where the network adapters coordinate the transfer of large amounts of data at line speed.

What is SMB Signing Disabled vulnerability? ›

SMB Signing Disabled is a Medium risk vulnerability that is one of the most frequently found on networks around the world. This issue has been around since at long time but has proven either difficult to detect, difficult to resolve or prone to being overlooked entirely.

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