A brief digression about SMB

So far none of this has involved any actual Shared Message Block protocol. The CIFS spec contains a detailed rundown on SMB packet formats. While SMB can run over various transports including IP, here we only discuss its usual interaction via TCP 139. A 4-byte block length is sent down the TCP stream followed by the block itself, so the transport handlers then know how much to read from or write to the network. SMB is thus independent of how IP-level packets split up the stream -- it doesn't care, it just keeps reading a connected socket until it satisfies the length's worth or times out. SMB blocks can be up to 65536 bytes long *excluding* the length integer, but in practice the blocks are usually smaller. SMB also trusts the TCP reliable transport layer to segregate different client sessions. In an alternate mode that uses UDP 138 the data blocks look almost the same, except that 12 bytes of unused "filler" are used under UDP to pass various session and sequencing context info. Many SMB request types support what is called the "AndX" mechanism, which provides a way to send several requests at once. Fields in these specify how to locate any subsequent SMB requests that were "batched" into this block. See the spec for more information.

The Samba code builds SMB blocks into buffers using a bunch of hairy macros with names like "SSVAL" to move short and long integers around and convert byte-ordering. [For a fun time, try unsnarling "byteorder.h".] Since Samba builds these internal buffers to include the 4-byte block length at offset 0, any other offsets described here are relative to that. After the block length comes the SMB header itself, starting at offset 4 in our reference frame with 0xFF, 'S', 'M', 'B'. A one-byte command code and several fixed-length fields follow, ending the SMB header proper. The command code indicates the type of SMB being requested or responded to. The request / response descriptions in CIFS exclude the header, and only detail what follows. After the header is a length byte and a variable-length bunch of two-byte "parameter words", and finally any associated buffers which can contain values, strings, file data, or whatever. A rough chart of this is given in Appendix C. The parameter words begin at offset 37 and are where most of the work gets done; in Samba they are called "smb_vwvN" where N is a number starting with 0. The buffers start at a variable offset depending on how many parameter words preceded; Samba has a routine called smb_buf() to dig through and find it. It should be noted that while the leading length bytes are in network order, all values inside the SMB blocks must be in "Intel" or little-endian order! In general both the block structure *and* what gets placed into it is all rather complex and confusing, and if it's any reassurance, the comments in earlier versions of Samba hint that much of it started as total guesswork and verbatim copying of block sections from packet dumps of sessions between MS boxes. As more SMB-savvy contributors came into the Samba development picture, these blind but somehow functional shots in the dark became better explained and recoded.

When working with NT we often encounter something called "unicode", a somewhat warped international character encoding standard. Strings are encoded into sequences of two-byte words, wasting twice the storage space required. This causes the string "ABC" to appear as "41 00 42 00 43 00" in hex dumps, and pops up in registry entries, SMB packets, and many other places. The lengths of unicode strings are usually stored elsewhere, such as but not always in an SMB parameter word, and there is sometimes confusion about precisely how long any string is. For example, is unicode "ABC" of size 3 or size 6? If we include and count a terminating null as required when sending passwords, is it then of size 4, 7, or perhaps even 8? To make matters even worse the strings must in theory be word-aligned in memory, and to force this to be true a leading null is supposed to be *inserted* ahead of the first character. The latest Samba server version contains a small fix for a common case where NT clients cannot quite decide consistently about the length of a null password string, and may send it as either 1 or 0.

One important part of the header we need to be aware of is two words beginning at offset 9 -- the response class and error codes, called smb_rcls and smb_err. These describe protocol errors in some detail, and there is a fairly large translation table of the most common errors near the end of client.c. The two error classes we usually ever see in practice are DOS and SERVER. There are several different possible class/error response combinations to describe any one kind of problem, such as failure to authenticate a user, and which pairs get sent back depends upon what type of platform the target is. The patch kit below includes a small routine called interpret_error() that boils an assortment of common errors down into a couple of standard return codes. This helps us distinguish between fatal errors, nonfatal errors and password problems, which figures significantly in a later phase of the attack. Some of the information here is not documented in CIFS, but can be found by doing "net helpmsg {smb_err #}" under NT, which seems to have a very complete set of error message texts available.

Phase 2: Dialect negotiation

Assuming an open TCP connection and successful session request, SMB request blocks may now be sent. The next step is for client and server to agree on the "dialect" of SMB protocol they can support. Over time, SMB has evolved from earliest "Microsoft networks" core protocol, through two types of Lan Manager and up to the current variant that NT uses. Each new dialect adds a couple of features, to support things like new authentication protocols and long filenames. The client sends a list of dialects it supports as [get this!] a bunch of null-terminated ASCII strings, including entries like

NT LM 0.12
which the server string-compares against dialects it recognizes and picks the "highest" common protocol level. There is a big comment in Samba's server.c just before reply_negprot() describing what most server platforms do with this. A response is built and sent back to the client, containing several important items: a numeric index into the dialect list to indicate which to use, some security-relevant flags, and an optional 8-byte "encryption key" to use for authentication. This "key" is a random challenge nonce that the server generates and temporarily remembers. A confusingly named "session key" is also sent, which is just some sort of unique but mostly unimportant identifier and *not* the same as the cryptkey.

Most SMB servers support backward dialect compatibility, and even if we support the latest NT we can always lie and exclude some of the later dialects from the list we send. Sessions between two NT machines involve more complex security protocols, so for our attack purposes it is definitely worth our while to convince a server that we are a dumb old client that can't handle the fancier stuff. Microsoft clients can't do this but smbclient can, with a settable max_protocol variable, and we should therefore plug "-m LANMAN2" into our command line to force the server to dumb itself down somewhat. Smbclient also parses dialects as strings here, not numeric levels.

The security mode flags appear in smb_vwv1, and we need to pay attention to the low two bits thereof. Smbclient tells us what this "sec mode" is at debug level 3. The earlier NetBIOS implementations optionally required a simple password to connect to a shared filesystem, and had no real concept of *who* was connecting as long as the correct password was supplied. Everyone using such a fileshare must know the single password for it, which is considered fairly lame from a security standpoint. This is called "share-level security", and is used by Windows for Workgroups, Samba if appropriately configured, and maybe some other Lan Manager platforms. Later dialects have a concept of individual user login, and indicate this "user-level security" by setting the LSB of the security flags. The next higher bit in the flags indicates whether the client should use "password encryption" or not. Thus if smbclient reports "sec mode 3" as it does when connecting to most NT servers, both of these bits are set. Sometimes we see a reference to "server-level security", but this simply means that authentication data is forwarded to a Domain Controller machine for validation and does not affect the mode bits.

Dialect negotiation must occur on a connection before other SMB types may be sent. If dialect negotiation fails for some reason, the server sends a FIN along with the response and the TCP connection must be closed and reopened. One way to observe this is to try negotiating the dialect either twice or not at all on a given connection. If a server is running in user-level security and a protocol is negotiated that does not support user login at all, the server will generally set the user-level bit anyway and wind up refusing to allow most other SMB transactions on that connection until successful user authentication is performed. This happens during the next phase.

Phase 3: SMB session setup

A server running in user-level security generally requires this step before allowing access to shared resources. This phase can be skipped entirely against share-level servers, or used anyway to pass additional info about buffer sizes and client capabilities. Normally here is where usernames and passwords get plugged in and the "attack" really begins. The official CIFS name for this phase is SessionSetupAndX, implying once again that additional SMB requests can and often are batched into this block. Note carefully that despite the unfortunately confusing name, this "session setup" is a very DIFFERENT animal from the RFC1001/1002-style TCP "session setup" done in phase 0! In general the different TCP sessions distinguish between client *machines*, while a "UID" determined during this SMB setup phase distinguishes an individual *user* on a given client. This implies that all SMB traffic between a given client and server pair may pass over a single TCP connection regardless of originating user, although this is not required behavior by any means since servers can support several concurrent TCP connections. It also implies that multiple SMB setup requests can be sent across the single connection instance, which is perhaps the key thing that throws it wide open to various attacks.

The contents of this block and the server response vary somewhat depending on the agreed dialect level and security flags. The most relevant items in the request are a username and either a plaintext password or a hash derived from it. Other items include maximum buffer sizes, various other client information such as its domain and running OS [all of which can be faked up], and perhaps further SMB commands via the AndX mechanism.

Microsoft boxes collect a username and password through one or another "logon" dialog. Under WFWG, the simplest command-line way to change them on the fly is "net logon {user}" which sets them up for a subsequent "net use". NT requires a login to use the client workstation and saves the username and password from that as default credentials for subsequent filesharing, but these can be overridden in its "net" command line with optional /USER and password arguments. Smbclient accepts "-U username", and asks for a password that it will plug in at the appropriate time. The unmodified version accepts a password on the command line as an optional argument after the sharename, or by using the format "-U user%passwd". In many cases the password must be in all UPPERCASE, but some servers may accept or even require mixed-case even in LANMAN-only dialect -- this is a bit of a crap shoot, so try it both ways. The Samba server has hooks to try a couple of different permutations in an effort to authenticate oddball clients, with appropriate warnings about reduced keyspace.

Under user-level security a successful login means we are basically "in" as either the target user or a guest. The SMB response contains some strings containing the server's OS and version, and an important SMB header field called the UID. This is not quite the same thing as a Unix UID, although for convenience Samba does use the Unix UID of the authenticating user here. Microsoft servers construct an internal set of user credentials and rights and assign the UID as a token that refers to it. The UID is in theory unique only within the context of the enclosing TCP connection -- if multiple SMB sessions are active across one TCP connection the UID distinguishes the separate users there, and in theory different users on different TCP sockets could wind up being assigned the same UID. There is also a process-ID or PID header field that the *client* initiates, but that seems to hold little relevance except for some file-locking calls. Again, regardless of server platform the UID is merely a reference token and while playing games with UID/GID values may be effective against NFS servers, trying it here it only produces "invalid UID" SMB errors or is simply ignored by the server. The server can optionally set a flag in the setup response that indicates that a given session is a "guest" login. Samba does this and NT does not, but the setting of this bit seems irrelevant to the rights a given active UID has on the server anyway.

There are several possible error responses here, which our interpret_error routine turns into something we can recognize to mean whether to continue the attack or give up. An unknown username and/or password in most cases comes back as "access denied" unless unknown/null users get mapped to GUEST. There are some errors that imply that the supplied credentials were right but there is some other problem, such as "account disabled" or "cannot log in from the network." In such cases further attempts with a given username will probably be unproductive, but remember that here the TCP connection remains open regardless of the return status, allowing ample opportunity for retries with any other username and password. Protocol errors or transient server problems can also occur, some of which may imply that a new TCP session is needed.

Two important usernames to try right off against Microsoft platforms are ADMINISTRATOR and GUEST, since these usually exist out of the box and all too often have null passwords. If the ADMINISTRATOR login has been renamed to "something obscure" as recommended in several texts, its new name may show up somewhere on the target network as a type 0x3 anyway. As mentioned before, any other base computernames and type 0x3 messaging names collected from the target network are all potential usernames. A machine running the Microsoft web server may have an account of the form IUSR_{basename} that got quietly created during setup, and it is said that the SQL server pulls similar stunts. A null username or one that is unknown to the server is often accepted as a guest login that allows some limited amount of poking around -- often enough access to at least read files from the server if not write to them. Any hint at an account used for disk backups in an NT environment should be pursued, since such an account probably has "backup" privileges to read the entire filesystem including the normally inaccessible SAM security database. If the server is running Samba itself, a null username and password may grant guest access. Try some Unix accounts that have known or null passwords -- Samba by default disallows logins by accounts with null passwords, but for any allowable ones does not check for a valid user shell like other daemons do. Try likely null ones anyway since some sites may be configured to allow them. An exception to the null-password rule is Samba's default "pcguest" account in smb.conf, which many sites remap to "nobody" or something rather than create a new /etc/passwd entry.

If the client supports password encryption, it uses the user's password as input to one or both of two possible encryption algorithms referred to as the LANMAN method and the NT method. These algorithms are described in CIFS in excruciating detail, and reviewed in Appendix A here. By deliberately dumbing down our negotiated protocol level we can eliminate the need for the NT-style field even if connecting to an NT-dialect server. For backward compatibility NT accepts the LANMAN password format, which completely obviates the increased security supposedly given by long case-sensitive passwords. It is important to understand that it is the CLIENT that chooses whether or not to use password encryption, and the server's "use encryption" security mode bit is just a gentle suggestion. If a server cannot authenticate via a 24-byte crypto response it is supposed to use whatever is given AS PLAINTEXT. This is another major weakness in the protocol spec, since a compliant server cannot enforce use of encryption! We therefore don't even need "libdes" or the Samba crypto support for our attack kit, we can just send plaintext passwords. Furthermore, since at this point we can send multiple SetupAndX exchanges REGARDLESS of whether they succeed or fail, the opportunity for brute-force guessing is obvious. Most stock client apps are not useful as brute-forcing engines since they exit after one or two failed authentications, but our patch kit modifies smbclient's send_login() routine to keep trying until it either succeeds or runs out of passwords to try.

While this phase is ripe for brute-force attacks, it is also where servers might start logging things. Entries wind up, relative to their respective system-root directories, in "audit.log" under Windows, "config\secevent.evt" under the NT system directory, and "var/log.smb" on a Samba server. Microsoft platforms [particularly NT] open their log files in an exclusive way that prevents other processes from directly reading or modifying them, and Samba's logfiles can be protected against normal users. Unfortunately the default setup for what *gets* logged is weak or nonexistent. Windows seems only to log full filesharing connection attempts, which do not happen at this phase, and the logging is controlled via simple SYSTEM.INI lines. NT out of the box logs NOTHING -- one must configure the NT "system policy" to *do* the logging for both failed and successful user logins, and only the name given by the connecting client is saved -- NOT its IP address. Other Microsoft platforms have the same problem. Unless someone actively runs "netstat -a" during the attack or provides some third-party enhanced logging facility, no useful backtracing information will be saved. The Samba server by default only logs successful filesharing connections. This pretty much lets an attacker guess at Unix user passwords all day and never be noticed, similar to what vanilla rexec allows. Setting up more meaningful logging gets rather involved and is covered later under "defenses." In all cases, recall also that any TCP connections can be run through an intermediate relay which will cause the relay's IP address to be observed instead of the real source of an attack.

NT servers exhibit several quirks worth mentioning, most of which reveal that the design of the authentication backend is at best naive. A cleartext unicode NT password can be sent in smb_vwv8 but if the alignment is screwed up or the length given as uneven, the returned error is "parameter incorrect" and the event log entry is just "unexpected error." If a properly formed NT password is given under NT LM dialect, encrypted or otherwise, any LANMAN style one in smb_vwv7 is apparently ignored. Upon valid authentication, other error codes returned can mean things like "account disabled", "network access denied", "cannot log in from this workstation", as well as several others that arguably give out too much information that could help guide an attack. Users can be configured such that they can only log in from certain named clients, but not only can the client send an arbitrary caller name, it turns out that using either a null name or even a single space handily bypasses this silly restriction and allows the login anyway.

NT has the capability to "lock out" accounts after some number of failed login attempts. While there is no specific error to indicate this, it is quite easy to remotely determine [at least against NT 4.0 with non-permanent user lockout policy] when a temporary account lockout happens. Any failed login usually causes the server to delay for 2 or 3 seconds before sending the SMB "access denied" error, to slow down brute-force attacks. Attempts on a valid username will elicit these delayed responses until the lockout threshold is reached, and then suddenly there is NO delay anymore and subsequent guesses on the same username are denied immediately! If account lockout is enabled, the default threshold is between 5 and 10 tries and the lockout time is 30 minutes. Therefore in most cases it doesn't take very long to make the lockout perceptibly happen.

If attempts on one known-to-exist username triggers login-failure lockout but another one does not, chances are that the second one is the administrator account. Conversely, if attempts on ADMINISTRATOR trigger lockout, it is probably a decoy and the real one has been renamed. Lockout does not apply to the administrative account, with the ostensible idea being prevention of *total* denial of service attacks. This leaves ADMINISTRATOR or the equivalent accountname open to unlimited guessing. Even the access-denied delay can be effectively bypassed. The delay is imposed per TCP connection, so by opening up 10 connections and pounding in different sets of passwords an attacker gets a tenfold increase in brute-force speed. Such an attack probably occupies significant server CPU time since not only does the event logging go crazy, but each plaintext guess must be re-hashed on the *server* side for comparison against the stored OWF. A workaround sometimes suggested to combat this is an obscure registry setting that causes the whole server to shut down when the event log fills, but that just allows an even worse denial of service.

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