A POSIX implementation of NTLM2 client authentication. 😭
Switch branches/tags
Nothing to show
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.



Travis Build Status Coverity Build Status

ntlmclient is a pure C library that supports NTLM2 authentication for POSIX systems. It is generally used to support authentication to Windows-based servers that do not enable other authentication algorithms.

🚨🚨🚨 NTLM2 support should be deprecated 🚨🚨🚨

This library is provided for compatibility with systems that do not offer any other authentication algorithms. NTLM2 is often enabled because it provides simple, integrated access to Windows systems, often called "single sign-on". This allows authentication to remote systems with the currently logged-in user credentials, without users being forced to re-enter their password. While convenient, NTLM2 is built on outdated cryptographic systems and should not be preferred.

For "single sign-on" support, you should instead prefer Kerberos, as it is an industry standard built on modern ciphers. If you do not require single sign-on, using a simple authentication mechanism like HTTP Basic is adequate, provided the connection is encrypted with transport layer security.

This library is provided since many systems are configured to support authentication using only NTLM2. Not all systems can upgrade to Kerberos.

Regardless, you should be providing security at the transport layer, using IPsec or HTTPS.


NTLM is a "challenge/response" authentication mechanism that allows a server to authenticate a client without it having to provide the actual password. Briefly:

  1. The client creates an initial NTLM authentication negotiation message, called a "negotiation" message (sometimes called a "Type 1" message).

    The ntlmclient library returns the negotiation message as a raw stream of bytes. If you require the message in base64 format (for example, to use with SPNEGO over HTTPS), then you must encode it before sending it over the HTTPS connection.

  2. The remote server will respond with a "challenge" message (sometimes called a "Type 2" message). This message includes capabilities and information from the remote server that ntlmclient will use to complete authentication.

    The ntlmclient library expects the challenge message as a raw stream of bytes. If it was provided to you in base64 format (for example, over HTTP) then you must decode it before providing it to the library.

  3. The client creates the final message, a "response" message (sometimes called a "Type 3" message). This message includes a hash of the challenge that was given to the client, using the password as a key. Given this message, the server will decide whether authentication succeeded or failed.

    Like the other messages, the response message is a raw stream of bytes and should be encoded as base64, if necessary.

Getting Started

You should read the full documentation, but a simple usage example is:

  1. Initialize the NTLM client context. You can specify option flags, or pass NTLM_CLIENT_DEFAULTS (or 0) to the option flags argument to accept the defaults.

    ntlm_client *ntlm;
    /* Create an NTLM client context, using the default options.  This
     * will return an NTLM context on success, or NULL on failure.
    if ((ntlm = ntlm_client_init(NTLM_CLIENT_DEFAULTS)) == NULL) {
        /* Can only fail on out of memory. */
        fprintf(stderr, "out of memory");
  2. Set the local hostname, the user's credentials to authenticate with, and the authentication "target" (the name of the remote machine). The strings provided are expected to be in UTF-8.

    (Functions return 0 on success and non-zero on error.)

    if (ntlm_client_set_hostname(ntlm, "hostname", "DOMAIN") != 0 ||
        ntlm_client_set_credentials(ntlm, "user", "DOMAIN", "secret") != 0) {
        ntlm_client_set_target(ntlm, "SERVER") != 0) {
        /* Get the error message from the NTLM context. */
        fprintf(stderr, "%s\n", ntlm_client_get_errmsg());
  3. Compute the negotiate message and deliver it to the server.

    const unsigned char *negotiate_msg;
    size_t negotiate_len;
    if (ntlm_client_negotiate(&negotiate_msg, &negotiate_len, ntlm) != 0) {
        fprintf(stderr, "%s\n", ntlm_client_get_errmsg());
    /* For HTTP, base64 encode the negotiate message. */
  4. Read the challenge message from the server, provide it to the library.

     * Read the NTLM challenge message from the remote host.  For HTTP,
     * this will be in the `Authorization` header, following the SPNEGO
     * mechanism name ("NTLM" or "Negotiate"), and should be base64 decoded.
    if (ntlm_client_parse_challenge(ntlm, challenge, challenge_len) != 0) {
        fprintf(stderr, "%s\n", ntlm_client_get_errmsg());
  5. Compute the response message and deliver it to the server.

    const unsigned char *response;
    size_t response_len;
    if (ntlm_client_response(&response, &response_len, ntlm) != 0) {
        fprintf(stderr, "%s\n", ntlm_client_get_errmsg());
     * For HTTP, base64 encode the response message and set it as the
     * `WWW-Authenticate` header.
  6. Determine if authentication has succeeded; for example, for HTTP transports, the server will return a 401 when authentication fails. In a failure, you should restart the authentication process, either with a new authentication context, or after calling:

  7. Free the NTLM context.



ntlmclient supports:

  • NTLM2 authentication This is the most recent "single sign-on" authentication mechanism that Microsoft developed before adopting Kerberos. This system provides authentication without actually transmitting the password. However, it relies on outdated encryption algorithms, so this mechanism should not be used without transport encryption (IPsec, TLS, etc).

  • LM and NTLM authentication (optionally) These are older "single sign-on" authentication mechanisms and rely on weaker encryption algorithms. Most Windows systems (client and server) have disabled both LM and NTLM in favor of NTLM2. Generally, these should not be used. And even more than NTLM2, these should never be used without transport encryption.

  • macOS and Linux Support Cryptographic primitives are provided by CommonCrypto on macOS, and OpenSSL or mbedTLS on non-macOS platforms.

    Unicode functionality (UTF8 to UTF16 conversion) is provided by iconv when available, falling back to conversion routines provided by Unicode, Inc.

What's Not Supported

ntlmclient does not support:

  • Session Security NTLM "session security", or "signing and sealing" is a message signature and encryption scheme. Key exchange is performed during the NTLM authentication process. There is no support for this; it should be not be used, and should be deprecated in favor of TLS.

  • Windows Support ntlmclient is POSIX-only. Windows users are encouraged to use the system's NTLM support.

Further Reading

MS-NLMP: NT LAN Manager (NTLM) Authentication Protocol Specification Microsoft's published specification of the NTLM authentication protocol.

The NTLM Authentication Protocol and Security Support Provider An independent reference of the NTLM authentication system, based on the research from the Samba team and used as the basis for jCIFS.

Simple and Protected GSSAPI Negotiation Mechanism (SPNEGO) The HTTP authentication mechanism for NTLM and Kerberos.


ntlmclient is released under the MIT license. This software is based on the NTLM2 implementation in Microsoft Team Explorer Everywhere.

See the license file for the full license text.