FTPTelnet
=========
Steven Sturges <ssturges@sourcefire.com>
Thanks to Dan Roelker <droelker@sourcefire.com>,
Marc Norton <mnorton@sourcefire.com>, and Andy Mullican
<amullican@sourcefire.com> for their help with the design.

== Overview ==
FTPTelnet serves two purposes, decoding and normalizing Telnet traffic,
and decoding and interpreting FTP traffic.

FTPTelnet handles stateful and stateless processing.  It saves state
between individual packets.  However maintaining correct state is
dependent on the reassembly of the client side of the stream (ie, a
loss of coherent stream data results in a loss of state).

-- Telnet -- 
Given a telnet data buffer, FTPTelnet will normalize the buffer with
respect to telnet commands and option negotiation, eliminating telnet
command sequences per RFC 854.  It will also determine when a
telnet connection is encrypted, per the use of the telnet encryption
option per RFC 2946.

-- FTP --
Given an FTP command channel buffer, FTPTelnet will interpret the data,
identifying FTP commands and parameters, as well as FTP response codes
and messages.  It will enforce correctness of the parameters, determine
when an FTP command connection is encrypted, and determine when an FTP
data channel is opened.

== Configuration ==
FTPTelnet has a very "rich" user configuration.  Users can configure 
checks for a few specific telnet exploits, and specify individual FTP
servers and clients with a variety of options, which should allow the 
user to emulate any type of web server or client.

It is VERY IMPORTANT to understand the configuration semantics, so an
administrator can know what to expect from the normalization routines and
protocol correctness checks.

Within the discussions of each option, occurance of a / in the option
parameters implies that one or the other parameter must be specified
when that option is used.  For example, with a yes/no option, the presence
of the option causes detection to occur when tracking state.  A yes
results in an alert being generated.

-- Global Configuration --
The global configuration deals with configuration options that determine the 
global functioning of FTPTelnet.  The following example gives the generic
global configuration format:

preprocessor ftp_telnet: global [followed by the configuration options]

There can only be a single global configuration.  An error will be reported
otherwise.

The global configuration options are described below:

*inspection_type stateful/stateless *
Indicate whether to operate in stateful stateless mode.

* encrypted_traffic yes/no *
Detect and alert on encrypted telnet and FTP sessions.

* check_encrypted *
Instructs the preprocessor to continue checking a data stream after it
is encrypted, looking for an eventual decrypted data.  This is really
only applicable if the encrypted_traffic is used, however that is not
enforced.

-- Telnet configuration --
The telnet configuration deals with configuration options relating to
the telnet protocol.  The following example gives the generic telnet
configuration format:

preprocessor ftp_telnet_protocol: telnet [followed by the options]

There should only be a single telnet configuration, however, subsequent
ones simply override configuration options established prior.

The telnet configuration options are described below:

* ports { port [port] ... } *
This specifies on what ports to check for telnet data.  Typically, this will
only include 23, however, additional ports may be specified since certain
network routers and other devices employ telnet as a means of administration.
Because encrypted traffic (SSL) cannot be decoded, adding ports 22 (SSH) could
yield unexpected results.

* normalize *
This turns on normalization for the telnet connections.  Typically, it should
be turned on.

* ayt_attack_thresh number *
Detect and alert on consecutive are you there [AYT] commands beyond the
threshold number specified.  This addresses a few specific vulnerabilities
relating to bsd-based implementations of telnet.

* detect_anomalies *
In order to support certain options, Telnet supports subnegotiation. Per the 
Telnet RFC, subnegotiation begins with SB (subnegotiation begin) and must
end with an SE (subnegotiation end). However, certain implementations of
Telnet servers will ignore the SB without a cooresponding SE. This is
anomalous behavior which could be an evasion case. Being that FTP uses the
Telnet protocol on the control connection, it is also susceptible to this
behavior. The detect_anomalies option enables alerting on Telnet SB without
the corresponding SE.

-- FTP configuration --
The FTP configuration deals with configuration options relating to
FTP servers and FTP clients.  Each of these will be discussed invidually
below, as the configurations are different.

** Server Configuration **
There are two types of server configurations: default and [IP].  The
default configuration:
  - preprocessor ftp_telnet_protocol: ftp server default [server options]

This configuration supplies the default FTP server configuration for any
server that is not individually configured.  It is suggested that you
start with the default configuration and craft individual server
configurations as required by the environment.

For specific IP's (ie, FTP servers with known behaviour), the configuration
is very similar:
  - preprocessor ftp_telnet_protocol: ftp server [IP] [server options]

The server options described below may be used in both the default and
specific IP configuration.

* ports { port [port] ... } *
This specifies on what ports to check for FTP data.  Typically, this will
only include 21, however, additional ports may be specified.

* print_cmds
Useful for debugging.  This turns on printing of the configuration
information for each ftp command listed for this server.  By default
it is not in use, as it generates a substantial amount of output when
snort is run from the commandline.

* ftp_cmds { cmd [cmd] ... } *
This specifies additional FTP commands outside of those checked by
default within the preprocessor.  The preprocessor may be configured
to generate an alert when it sees a command it does not recognize.

Aside from the default commands recognized, it may be necessary to
allow the use of the "X" commands, specified in RFC 775.  To do so, use
the following ftp_cmds option.  Since these are rarely used by FTP
client implementations, they are not included in the defaults.

ftp_cmds { XPWD XCWD XCUP XMKD XRMD }

* def_max_param_len number *
This specifies the default maximum parameter length for all commands
in bytes.  If the parameter for an FTP command exceeds that length,
and the preprocessor is configured to do so, an alert will be generated.
This is used to check for buffer overflow exploits within FTP servers.

* alt_max_param_len number { cmd [cmd] ... } *
This specifies the maximum parameter length for the specified commands
in bytes, overriding the default.  If the parameter for that FTP command
exceeds that length, and the preprocessor is configured to do so, an
alert will be generated.  It can be used to restrict specific commands to
small parameter values.  For example the USER command -- usernames may
be no longer than 16 bytes, so the appropriate configuration would be:
alt_max_param_len 16 { USER }

* chk_str_fmt { cmd [cmd] ... } *
This causes the preprocessor to check for string format attacks on
the specified commands.

* cmd_validity cmd < fmt > *
This specifies the valid format for parameters of a given command.
fmt is as follows:
    int            Param must be an integer
    number         Param must be an integer between 1 and 255
    char _chars    Param must be a single char, and one of _chars
    date _datefmt  Param follows format specified where
                   # = Number, C=Char, []=optional, |=OR, {}=choice,
                   anything else=literal (ie, .+- )
    string         Param is string (effectively unrestricted)
    host_port      Param must a host port specifier, per RFC 959.
    {}, |          One of, alternate values enclosed within
    []             Optional value enclosed within

Examples of the cmd_validity option are shown below.  These examples
are the default checks (per RFC 959 and others) performed by the
preprocessor.
cmd_validity MODE < char SBC >
cmd_validity STRU < char FRP >
cmd_validity ALLO < int [ char R int ] >
cmd_validity TYPE < { char AE [ char NTC ] | char I | char L [ number ] } >
cmd_validity PORT < host_port >

A cmd_validity line in the configuration can be used to override these
defaults and/or add a check for other commands.  A few examples follow

# This allows additional modes, including mode Z which allows for
# zip-style compression.
cmd_validity MODE < char ASBCZ >

# Allow for a date in the MDTM command.
cmd_validity MDTM < [ date nnnnnnnnnnnnnn[.n[n[n]]] ] string >

MDTM is an odd case that is worth discussing...

While not part of an established standard, certain FTP servers accept
MDTM commands that set the modification time on a file.  The most common
among servers that do, accept a format using YYYYMMDDHHmmss[.uuu].  Some
others accept a format using YYYYMMDDHHmmss[+|-]TZ format.  The example
above is for the first case (time format as specified in
http://www.ietf.org/internet-drafts/draft-ietf-ftpext-mlst-16.txt)

To check validity for a server that uses the TZ format, use the following:

cmd_validity MDTM < [ date nnnnnnnnnnnnnn[{+|-}n[n]] ] string >

* telnet_cmds yes/no *
Detect and alert when telnet cmds are seen on the FTP command channel.

* data_chan *
Causes the FTP preprocessor to force the rest of snort to ignore 
the FTP data channel connections.  This means that NO INSPECTION
other than state (preprocessor AND rules) will be performed on that
data channel.  It can turned on to improve performance -- especially
with respect to large file transfers -- by ignoring traffic.

** Client Configuration **
There are two types of client configurations: default and [IP].  The
default configuration:
  - preprocessor ftp_telnet_protocol: ftp client default [client options]

This configuration supplies the default FTP client configuration for any
client that is not individually configured.  It is suggested that you
start with the default configuration and craft individual client
configurations as required by the environment.

For specific IP's (ie, FTP clients with known behaviour), the configuration
is very similar:
  - preprocessor ftp_telnet_protocol: ftp client [IP] [client options]

The client options described below may be used in both the default and
specific IP configuration.

* max_resp_len number *
This specifies the maximum length for all response messages in bytes.
If the message for an FTP response (everything after the 3 digit code)
exceeds that length, and the preprocessor is configured to do so, an
alert will be generated.  This is used to check for buffer overflow
exploits within FTP clients.

* bounce yes/no *
Detect and alert on FTP bounce attacks.  An FTP bounce attack occurs
when the FTP PORT command is issued and the specified host does not
match the host of the client.

* bounce_to { CIDR,[port|portlow,porthi] } *
When bounce is turned on, this allows the PORT command to use the
IP address [CIDR format] and port (or inclusive port range ) without
generating an alert.  It can be used to deal with proxied FTP
connections where the FTP data channel is different from the client.
Two examples of the bounce_to command:
1) This allows bounces to 192.168.1.1 port 20020 -- ie, the use
of PORT 192,168,1,1,78,52.
  bounce_to { 192.168.1.1,20020 }
2) This allows bounces to 192.168.1.1 ports 20020 to 20040 -- ie,
the use of PORT 192,168,1,1,78,xx, where xx is 52 through 72, inclusive.
  bounce_to { 192.168.1.1,20020,20040 } 
3) This allows bounces to 192.168.1.1 port 20020 and 192.168.1.2 port 20030
  bounce_to { 192.168.1.1,20020 192.168.1.2,20030 }

* telnet_cmds yes/no *
Detect and alert when telnet cmds are seen on the FTP command channel.

-- Default Configuration --
The following gives the default configurations:

# Global
preprocessor ftp_telnet: global \
  inspection_type stateless

# Telnet protocol
preprocessor ftp_telnet_protocol: telnet \
  ports { 23 } 

# Default FTP Server
# The default FTP server configuration for FTP command validation.
# Most of this comes from RFC 959, with additional commands being
# drawn from other RFCs/Internet Drafts that are commonly in use.
# 
# The default_max_param_len is somewhat arbitrary, but is taken
# from the majority of the snort FTP rules that alert on parameter
# size > 100 characters.  This was the case as of 18 Sep 2004.
preprocessor ftp_telnet_protocol: ftp server default \
  ports { 21 } \
  def_max_param_len 100 \
  ftp_cmds { USER PASS ACCT CWD CDUP SMNT \
    QUIT REIN PORT PASV TYPE STRU MODE RETR STOR STOU APPE ALLO REST \
    RNFR RNTO ABOR DELE RMD MKD PWD LIST NLST SITE SYST STAT HELP NOOP } \
  ftp_cmds { AUTH ADAT PROT PBSZ CONF ENC } \
  ftp_cmds { FEAT OPTS } \
  ftp_cmds { MDTM REST SIZE MLST MLSD } \
  alt_max_param_len 0 { CDUP QUIT REIN PASV STOU ABOR PWD SYST NOOP } \
  cmd_validity MODE < char SBC > \
  cmd_validity STRU < char FRP > \
  cmd_validity ALLO < int [ char R int ] > \
  cmd_validity TYPE < { char AE [ char NTC ] | char I | char L [ number ] } > \
  cmd_validity PORT < host_port > 

# Default FTP Client
preprocessor ftp_telnet_protocol: ftp client default \
   max_resp_len 100

== Writing rules ==
{TBD?} Writing rules for normalized telnet changes from the telnet_decode
preprocessor.

The content parameter in the snort rule language searches the NORMALIZED
telnet and FTP buffers.  This means that rules that include things that
are normalized, such as telnet option negotiation for encryption,
ie |FF FA 26|, these rules will not alert.  They should be changed to
add the rawbytes parameter to specify look at the raw data, not the
normalized data.

== Conclusion ==
While some of this configuration information can be overwhelming, the
defaults are sufficient to get started.  Some of the examples provided,
especially with respect to the FTP command validation, are derived from
tests conducted with traffic generated from various publicly available
freeware and/or trialware telnet & FTP client and server implementations.

== Alerts ==

FTP Telnet uses generator ID 125 for FTP and generator ID 126 for telnet.

The following alerts will be generated for FTP:

SID   Description
---   -----------
1     Telnet command on FTP command channel
2     Invalid FTP command
3     FTP parameter length overflow
4     FTP malformed parameter
5     Possible string format attempt in FTP command/parameter
6     FTP response length overflow
7     FTP command channel encrypted
8     FTP bounce attack
9     Evasive Telnet command on FTP command channel

The following alerts will be generated for Telnet:

SID   Description
---   -----------
1     Telnet consecutive AYT overflow
2     Telnet data encrypted
3     Subnegotiation Begin without matching Subnegotiation End

