[REVS] Recovering Windows Password Cache Entries

From: SecuriTeam (support_at_securiteam.com)
Date: 03/29/05

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Date: 29 Mar 2005 19:31:55 +0200

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      Recovering Windows Password Cache Entries
    ------------------------------------------------------------------------

    SUMMARY

    Users authenticate themselves on a Domain Controller (DC) using
    NTLM/NTLMv2. However the DC sometimes goes offline or the network cable is
    unplugged; in this situation, the Local Security Authority System Service
    (LSASS) uses password cache entries from the registry to perform offline
    logon.

    This whitepaper explains the technical issues underneath Windows password
    cache entries, which are undocumented by Microsoft. This paper aims at:

     * Helping pentesters or security bso retrieving the password cache
    entries (hash value) for auditing purposes;

     * Providing more compatibility for programs that may require access to
    these entries without using the LSA API.

    DETAILS

    Description of the Authentication Process
    The WINLOGON process displays the msgina dialog and prompts for the
    username, password and domain. The authentication process itself is
    handled by LSASS:

       WinLogon ---> LSASS ---> LSASRV -> MSV1_0 -> [Registry Cache Entries]
          ||
        MSGina

    The most important part of the authentication process happens in
    MSV1_0.dll. LSASS calls the LSAApLogonUserEx2 function which first checks
    if the DC is unavailable; in this case, it attempts to match the password
    entered by the user against the cached password.

    The cache entries do not include the authentication credentials in the
    clear:
    a LSA key is used to decrypt them. Credentials are stored in
    HKLM\SECURITY\CACHE\NL$n with n ranging between 1 and 10. The default ACL
    does not allow Administrators to read these registry values, which can
    only be accessed with SYSTEM privileges.

    The size of these values may differ but they are roughly composed of 4
    parts:

                      MD CH T EDATA
    NL$ = [ metadata in the clear ][ Text ][ Text ][ Encrypted Data ]
                    64 bytes 16 Bytes 16 bytes > 100 bytes

     * MD contains several informations about elements of the cache entry
    structure, such as the username size in the first 2 bytes.

     * CH is an array of 16 random(?) bytes used to generate a RC4 key.

     * EDATA contains encrypted authentication credential: username (Unicode),
    domain name (Unicode), NT-hash, LM-hash (optional). It can be decrypted
    using the decrypted LSA secret NL$KM. specific to each computer.

    EDATA is decrypted by performing these steps:
    0. LSA keyB = DES( NL$KM, static in-memory LSA keyA )
    1. RC4 keyC = HMAC_MD5( LSA keyB, CH )
    2. DATA = RC4( EDATA, RC4 keyC );

    DATA contains the following informations:
     * [ 96, 102 ] : MSCASH = MD4( MD4(password ) || lowercase(username) )
     * [ 168, 168 + username_length * 2 ] : username
     * [ 168 + username_length * 2 + 2, ... ] : domain name

    The password hash is salted with the Unicode username.

    The CacheDump Tool
    CacheDump, licensed under the GPL, demonstrates how to recover cache entry
    information: username and MSCASH. Administrators or security consultants
    are welcomed to use this program; malicious users can't do anything with
    it as long as they do not have Administrator privileges.

    CacheDump does not rely on the dll-injection method used in pwdump or
    lsadump2; it creates a NT service on the fly in order to read the static
    LSA key from LSASS.EXE's process memory, and deciphers the cache entries
    to expose the MSCASH values.

    CacheDump's output is similar to pwdump's, with of course a different hash
    function; a plugin for john the ripper password cracker has been developed
    for offline dictionary and bruteforce cracking.

    These flags are helpful for troubleshooting:
        -v : Verbose mode;
        -vv : Very Verbose mode - displays every step of the dump process;
        -K : Kill and remove the service, in case a previous run of CacheDump
    died unexpectedly.

    Bug reports are welcomed. CacheDump is still in development stage, so
    please report any bug you encounter, including your Windows version,
    service pack level, language, and the output of "cachedump -vv".

    Prevention
    In order to prevent a malicious user from recovering cached passwords, we
    recommend to:

     * Revoke local administrator privileges from all users;
     * Reduce the number of cached password. Change to 1 the following
    registry key: HKEY_LOCAL_MACHINE\SOFTWARE\MICROSOFT\WINDOWS
    NT\CURRENTVERSION\WINLOGON\CACHEDLOGONSCOUNT

    ADDITIONAL INFORMATION

    The information has been provided by Thales Security Systems (TSS).
    The original article can be found at:
    <http://www.cr0.net:8040/misc/cachedump.html>
    http://www.cr0.net:8040/misc/cachedump.html
    To keep updated with the tool visit the project's homepage at:
    <http://www.cr0.net:8040/misc/cachedump-1.1.zip>
    http://www.cr0.net:8040/misc/cachedump-1.1.zip

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