MS16-101: Description of the security update for Windows authentication methods: August 9, 2016

• All future security and non-security updates for Windows 8.1 and Windows Server 2012 R2 require update 2919355 to be installed. We recommend that you install update 2919355 on your Windows 8.1-based or Windows Server 2012 R2-based computer so that you receive future updates. 
• If you install a language pack after you install this update, you must reinstall this update. Therefore, we recommend that you install any language packs that you need before you install this update. For more information, see Add language packs to Windows.
  

Known issues in this security update

• Known issue 1
  
  The security updates that are provided in MS16-101 and newer updates disable the ability of the Negotiate process to fall back to NTLM when Kerberos authentication fails for password change operations with the STATUS_NO_LOGON_SERVERS (0xc000005e) error code. In this situation, you may receive one of the following error codes.
  
  

  Hexadecimal	Decimal	Symbolic	Friendly
  0xc0000388	1073740920	STATUS_DOWNGRADE_DETECTED	The system detected a possible attempt to compromise security. Please make sure that you can contact the server that authenticated you.
  0x4f1	1265	ERROR_DOWNGRADE_DETECTED	The system detected a possible attempt to compromise security. Please make sure that you can contact the server that authenticated you.

  
  
  Workaround
  
  If password changes that previously succeeded fail after the installation of MS16-101, it's likely that password changes were previously relying on NTLM fallback because Kerberos was failing. In order to change passwords successfully by using Kerberos protocols, follow these steps:
  
  
  
  1. Configure open communication on TCP port 464 between clients that have MS16-101 installed and the domain controller that is servicing password resets.
     
     Read-only domain controllers (RODCs) can service self-service password resets if the user is allowed by the RODCs password replication policy. Users who are not allowed by the RODC password policy require network connectivity to a read/write domain controller (RWDC) in the user account domain.
     
     Note To check whether TCP port 464 is open, follow these steps:
     
     
     
     1. Create an equivalent display filter for your network monitor parser. For example:
        

        ipv4.address== <ip address of client> && tcp.port==464

     2. In the results, look for the "TCP:[SynReTransmit" frame.
        
        
  2. Make sure that the target Kerberos names are valid. (IP addresses are not valid for the Kerberos protocol. Kerberos supports short names and fully qualified domain names.)
  3. Make sure that service principal names (SPNs) are registered correctly.
     
     For more information, see Kerberos and Self-Service Password Reset.
• Known issue 2
  
  We know about an issue in which programmatic password resets of domain user accounts fail and return the STATUS_DOWNGRADE_DETECTED (0x800704F1) error code if the expected failure is one of the following:
  
  
  • ERROR_INVALID_PASSWORD
  • ERROR_PWD_TOO_SHORT (rarely returned)
  • STATUS_WRONG_PASSWORD
  • STATUS_PASSWORD_RESTRICTION
  
  The following table shows the full error mapping.
  
  

  Hexadecimal	Decimal	Symbolic	Friendly
  0x56	86	ERROR_INVALID_PASSWORD	The specified network password is not correct.
  0x267	615	ERROR_PWD_TOO_SHORT	The password that was provided is too short to meet the policy of your user account. Please provide a longer password.
  0xc000006a	-1073741718	STATUS_WRONG_PASSWORD	When you try to update a password, this return status indicates that the value that was provided as the current password is incorrect.
  0xc000006c	-1073741716	STATUS_PASSWORD_RESTRICTION	When you try to update a password, this return status indicates that some password update rule was violated. For example, the password may not meet the length criteria.
  0x800704F1	1265	STATUS_DOWNGRADE_DETECTED	The system cannot contact a domain controller to service the authentication request. Please try again later.
  0xc0000388	-1073740920	STATUS_DOWNGRADE_DETECTED	The system cannot contact a domain controller to service the authentication request. Please try again later.

  
  
  Resolution
  
  MS16-101 has been re-released to address this issue. Install the latest version of the updates for this bulletin to resolve this issue.
  
  
• Known issue 3
  
  We know about an issue in which programmatic resets of local user account password changes may fail and return the STATUS_DOWNGRADE_DETECTED (0x800704F1) error code.
  
  The following table shows the full error mapping.
  
  

  Hexadecimal	Decimal	Symbolic	Friendly
  0x4f1	1265	ERROR_DOWNGRADE_DETECTED	The system cannot contact a domain controller to service the authentication request. Please try again later.

  
  
  Resolution
  
  MS16-101 has been re-released to address this issue. Install the latest version of the updates for this bulletin to resolve this issue.
  
  
• Known issue 4
  
  Passwords for disabled and locked-out user accounts cannot be changed using the negotiate package.
  
  Password changes for disabled and locked-out accounts will still work when using other methods such as when using an LDAP modify operation directly. For example, the PowerShell cmdlet Set-ADAccountPassword uses an "LDAP Modify" operation to change the password and remains unaffected.
  
  Workaround
  
  These accounts require an administrator to make password resets. This behavior is by design after you install MS16-101 and later fixes.
  
  
• Known issue 5
  
  Applications that use the NetUserChangePassword API and that pass a servername in the domainname parameter will no longer work after MS16-101 and later updates are installed.
  
  Microsoft documentation states that providing a remote server name in the domainname parameter of the NetUserChangePassword function is supported. For example, the NetUserChangePassword function MSDN topic states the following:
  
  domainname [in]
  
  A pointer to a constant string that specifies the DNS or NetBIOS name of a remote server or domain on which the function is to execute. If this parameter is NULL, the logon domain of the caller is used.
  However, this guidance has been superseded by MS16-101, unless the password reset is for a local account on the local computer. Post MS16-101, in order for domain user password changes to work, you must pass a valid DNS Domain Name to the NetUserChangePassword API.
• Known issue 6
  
  After you install the security updates that are described in MS16-101, remote, programmatic changes of a local user account password, and password changes across untrusted forest fail.
  
  
  This operation fails because the operation relies on NTLM fall-back which is no longer supported for nonlocal accounts after MS16-101 is installed.
  
  
  A registry entry is provided that you can use to disable this change.
  
  Warning This workaround may make a computer or a network more vulnerable to attack by malicious users or by malicious software such as viruses. We do not recommend this workaround but are providing this information so that you can implement this workaround at your own discretion. Use this workaround at your own risk.
  
  ImportantThis section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
  322756 How to back up and restore the registry in Windows
  
  
  To disable this change, set the NegoAllowNtlmPwdChangeFallback DWORD entry to use a value of 1 (one).
  
  
  Important Setting the NegoAllowNtlmPwdChangeFallback registry entry to a value of 1 will disable this security fix:
  

  Registry value	Description
  0	Default value. Fallback is prevented.
  1	Fallback is always allowed. The security fix is turned off. Customers that are having issues with remote local accounts or untrusted forest scenarios can set the registry to this value.

  To add these registry values, follow these steps:
  
  1. Click Start, click Run, type regedit in the Open box, and then click OK.
  2. Locate and then click the following subkey in the registry:
     
     HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Lsa
     
  3. On the Edit menu, point to New, and then click DWORD Value.
  4. Type NegoAllowNtlmPwdChangeFallback for the name of the DWORD, and then press ENTER.
  5. Right-click NegoAllowNtlmPwdChangeFallback, and then click Modify.
  6. In the Value data box, type 1 to disable this change, and then click OK.
     
     
     Note To restore the default value, type 0 (zero), and then click OK.
  Status
  
  The root cause of this issue is understood. This article will be updated with additional details as they become available.