How to disable LM authentication on Windows NT
Article ID: 147706 - View products that this article applies to.
This article was previously published under Q147706
Prior to Windows NT 4.0 Service Pack 4 (SP4), Windows NT supported two kinds of challenge/response authentication:
To allow access to servers that only support LM authentication, Windows NT clients prior to SP4 always use both, even to Windows NT servers that supported NTLM authentication.
LM authentication is not as strong as Windows NT authentication so some customers may want to disable its use, because an attacker eavesdropping on network traffic will attack the weaker protocol. A successful attack can compromise the user's password.
Microsoft has developed an enhancement to NTLM called NTLMv2 that significantly improves both the authentication and session security mechanisms.
In addition, the implementation of the NTLM Security Service Provider (SSP) has been enhanced to allow clients to control which variants of NTLM are used, and to allow servers to control which variants they will accept, by setting a new registry key appropriately. It also allows clients and servers to require the negotiation of message confidentiality (encryption), message integrity, 128-bit encryption, and NTLMv2 session security.
These changes affect the following Windows NT components: Any application that uses Microsoft remote procedure call (RPC) or that uses the NTLM SSP, use the authentication and session security described herein. The Workstation and Server services use authentication but support their own session security.
BackgroundLM authentication is not as strong as NTLM or NTLMv2 because the algorithm allows passwords longer than 7 characters to be attacked in 7 character chunks. This limits the effective password strength to 7 characters drawn from the set of uppercase alphabetic, numeric, and punctuation characters, plus 32 special ALT characters. Users often do not even avail themselves of anything more than alphabetic characters.
In contrast, NTLM authentication takes advantage of all 14 characters in the password and allows lowercase letters. Thus, even though an attacker eavesdropping on the Windows NT authentication protocol can attack it in the same way as the LM authentication protocol, it will take far longer for the attack to succeed. If the password is strong enough, it will take a single 200 MHz Pentium Pro computer an average of 2,200 years to find the keys derived from it and 5,500 years to find the password itself (or 2.2 years and 5.5 years with 1,000 such computers, and so forth).
NOTE: This estimate is based on the rate at which the recent RSA Labs "DES Challenge" code tests DES keys on a 200 MHz Pentium Pro: 1,000,000/second; and the number of DES keys: 2**56 or 7.2*10**16. For more information about the "DES Challenge", please see the following Web site:
http://www.rsa.comOn the other hand, if a password is not strong enough, a dictionary lookup can find it in seconds.
One possible way of getting a "strong enough" password is to have it be at least 11 characters in length, with at least 4 of those characters uppercase, numbers, or punctuation. Even if the remaining 7 characters are lowercase low randomness text with (say) 3 bits of randomness per character, this will give more than DES's key space of 7.2*10**16 possible combinations, and the password will not be in dictionaries.
However, hardware accelerators costing $250,000 have been built that can find either the LM or NTLM password-derived key in 3-6 days no matter how long the password is. These numbers change as technology gets better. For current statistics and details please see the following Web site:
http://www.eff.orgHaving the password-derived key of a user does not allow an attacker to log on interactively but, with special software, it is sufficient to access network resources as that user.
For NTLMv2, the key space for password-derived keys is 128 bits. This makes a brute force search infeasible, even with hardware accelerators, if the password is strong enough.
If both client and server are using SP4, the enhanced NTLMv2 session security is negotiated. It provides separate keys for message integrity and confidentiality, and client input into the challenge to prevent chosen plain text attacks, and makes use of the HMAC-MD5 algorithm (see RFC 2104) for message integrity checking.
Because the datagram variant of NTLM does not have a negotiation step, use of otherwise negotiated options, such as NTLMv2 session security and 128- bit encryption for message confidentiality, has to be configured.
Important This 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:
(http://support.microsoft.com/kb/322756/ )How to back up and restore the registry in Windows
Control of NTLM security is through the following registry key:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\control\LSANOTE: On Win9x, the valid registry key is LMCompatibility while on Windows NT it is LMCompatibilityLevel.
Choice of the authentication protocol variants used and accepted is through the following value of that key:
NOTE: Authentication is used to establish a session (username/password). Session security is used once a session is established using the apropriate type of authentication. Also system times should be within 30 mins of one another. Authentication can fail because the server will think the challenge from the client has expired.
Value: LMCompatibilityLevel Value Type: REG_DWORD - Number Valid Range: 0-5 Default: 0 Description: This parameter specifies the type of authentication to be used. Level 0 - Send LM response and NTLM response; never use NTLMv2 session security Level 1 - Use NTLMv2 session security if negotiated Level 2 - Send NTLM authentication only Level 3 - Send NTLMv2 authentication only Level 4 - DC refuses LM authentication Level 5 - DC refuses LM and NTLM authentication (accepts only NTLMv2)
Control over the minimum security negotiated for applications using NTLMSSP is through the following key:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\control\LSA\MSV1_0The following values are for this key:
Value: NtlmMinClientSec Value Type: REG_DWORD - Number Valid Range: the logical 'or' of any of the following values: 0x00000010 0x00000020 0x00080000 0x20000000 Default: 0 Value: NtlmMinServerSec Value Type: REG_DWORD - Number Valid Range: same as NtlmMinClientSec Default: 0 Description: This parameter specifies the minimum security to be used. 0x00000010 Message integrity 0x00000020 Message confidentiality 0x00080000 NTLMv2 session security 0x20000000 128 bit encryption
LMCompatibilityLevel - ClientsIMPORTANT: For an SP4 client to choose level 3 or greater, the domain controllers for the user's account domains for all users who will use the client (hereafter, "the users' domain controllers") MUST have been upgraded to SP4.
If an SP4 client chooses level 0, which is the default, it will interoperate with earlier servers exactly as it did with Service Pack 3 (SP3).
If an SP4 client chooses level 1, it will interoperate with earlier servers exactly as it did at Service Pack 3 (SP3). In addition, it will negotiate NTLMv2 session security with SP4 servers.
IMPORTANT: When using level 1 or greater, if the last password change came from a Windows for Workgroups or MS-DOS LanManager 2.x client or earlier, the data needed for NTLM and NTLMv2 authentication will not be available on the domain controller, and SP4 clients will not be able to connect to SP4 servers. The workaround is to use level 0, or always change passwords from a Windows NT, Windows 95, or Windows 98 client.
If an SP4 client chooses level 2, it will not be able to connect to servers that only support LM authentication, such as Windows 95, Windows 98, Windows for Workgroups and earlier (hereafter called "downlevel LM clients/servers"), unless the users' domain controllers have been upgraded.
If an SP4 client chooses level 3 or greater, it will always send the new NTLMv2 response. This response can pass through downlevel LM servers and SP3 or earlier Windows NT servers and their domain controllers as long as the users' domain controllers have been upgraded to SP4. For example, if an SP4 client chooses level 3 or greater, the users' domain controllers MUST have been upgraded to SP4 as well. However, this response will not work with downlevel LM servers in Share Level security mode with non-null passwords. The workaround is to configure them to use User Level security.
LMCompatibilityLevel - Servers/DCsIf an SP4 server chooses level 4 or greater, a user with a local account on that server will not be able to connect to it from a downlevel LM client using that local account.
If an SP4 domain controller chooses level 4 or greater, a user with an account in that domain will not be able to connect to any member server from a downlevel LM client using their domain account. Thus, level 4 means that all users with accounts on a server or domain have to be using Windows NT to connect.
If an SP4 server chooses level 5 or greater, a user with a local account on that server will not be able to connect to it from an SP3 or earlier Windows NT client using that local account.
SP4 clients choosing level 0 or 1 will still be able to connect to SP4 servers, even with levels 1 or 3 configured, but will be using both the Windows NT protocol and the weaker LM protocol. They will also be able to connect to downlevel LM servers, even if the users' domain controllers have not been upgraded.
DeploymentBecause of the above considerations, if it is preferred to deploy NTLMv2, the following steps should be taken:
NtlmMinClientSec and NtlmMinServerSecIf the bit with value 0x00000010 is set in the NtlmMinClientSec or NtlmMinServerSec value, the connection will fail if message integrity is not negotiated.
If the bit with value 0x00000020 is set in the NtlmMinClientSec or NtlmMinServerSec value, the connection will fail if message confidentiality is not negotiated.
If the bit with value 0x00080000 is set in the NtlmMinClientSec or NtlmMinServerSec value, the connection will fail if NTLMv2 session security is not negotiated.
If the bit with value 0x20000000 is set in the NtlmMinClientSec or NtlmMinServerSec value, the connection will fail if 128-bit encryption is not negotiated.
NOTE: These settings will not guarantee that the NTLM SSP is actually used by every application, or that message integrity or confidentiality will actually be used by an application even when they are negotiated.
To resolve this problem, obtain the latest service pack for Windows NT 4.0 or Windows NT Server 4.0, Terminal Server Edition. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
(http://support.microsoft.com/kb/152734/ )How to obtain the latest Windows NT 4.0 service pack
After installing SP4, perform the following steps to configure LM Compatibility level on Windows NT workstations and servers:
Article ID: 147706 - Last Review: September 16, 2006 - Revision: 3.4