File: README.http_inspect

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HttpInspect
-----------
Daniel Roelker <droelker@sourcefire.com>

-- Overview --
HttpInspect is a generic HTTP decoder for user applications.  Given a data 
buffer, HttpInspect will decode the buffer, find HTTP fields, and normalize 
the fields.  HttpInspect works on both client requests and server responses.

This initial version of HttpInspect only handles stateless processing.  This
means that HttpInspect looks for HTTP fields on a packet by packet basis, and 
will be fooled if packets are not reassembled.  This works fine when there is 
another module handling the reassembly, but there are limitations in analyzing 
the protocol.  That's why future versions will have a stateful processing mode 
which will hook into various reassembly modules.

-- Configuration --
HttpInspect has a very "rich" user configuration.  Users can configure 
individual HTTP servers with a variety of options, which should allow the 
user to emulate any type of web server.

It is VERY IMPORTANT to learn the configuration semantics, so you know what to 
expect from the normalization routines.  So read this section over.

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

preprocessor http_inspect: global [followed by the configuration options]

You can only have a single global configuration, you'll get an error if 
you try otherwise.

The global configuration options are described below:

* iis_unicode_map [filename (located in the config dir)] [codemap (integer)] *
This is the global iis_unicode_map file.  THIS ALWAYS NEEDS TO BE SPECIFIED IN 
THE GLOBAL CONFIGURATION, otherwise you get an error.  The Microsoft US
unicode codepoint map is located in the snort /etc directory as a default.
It is called unicode.map and should be used if no other is available.  You
can generate your own unicode maps by using the program
ms_unicode_generator.c located in the HttpInspect utils directory.
Remember that this configuration is for the global IIS unicode map.  
Individual servers can reference their own IIS unicode map.

* detect_anomalous_servers *
This global configuration option enables generic HTTP server traffic inspection
on non-HTTP configured ports, and alerts if HTTP traffic is seen.  DON'T turn
this on if you don't have a default server configuration that encompasses all
of the HTTP server ports that your users might go to.  In the future we
want to limit this to particular networks so it's more useful, but for right
now this inspects all network traffic.

* proxy_alert *
This enables global alerting on HTTP server proxy usage.  By configuring
HttpInspect servers and enabling allow_proxy_use, you will only receive proxy
use alerts for web users that aren't using the configured proxies or are using
a rogue proxy server.

Please note that if users aren't required to configure web proxy use, then
you may get a lot of proxy alerts.  So, please only use this feature with
traditional proxy environments. Blind firewall proxies don't count.

** Server Configuration **
This is where the fun stuff begins.  There are two types of server 
configurations: default and [IP].  The default configuration:
  - preprocessor http_inspect_server: server default [server options]
  
This configuration supplies the default server configuration for any server 
that is not individually configured.  Most of your web servers will most 
likely end up using this default configuration.  Most of the time I would 
suggest setting your default server to:
  - preprocessor http_inspect_server: server default profile all ports { [whatever ports you want] }

In the case of individual IPs the configuration is very similar:
  - preprocessor http_inspect_server: server [IP] [server options]

Now we'll talk about the server options.  Some configuration options have
an argument of 'yes' or 'no'.  This argument specifies whether the user wants
the configuration option to generate an alert or not.  

IMPORTANT: 
The 'yes/no' argument does not specify whether the configuration option 
itself is on or off, only the alerting functionality.

* profile [all/apache/iis] *
Users can configure HttpInspect by using pre-defined HTTP server
profiles.  Profiles must be specified as the first server option and
cannot be combined with any other options except:
  - ports
  - iis_unicode_map
  - allow_proxy_use
  - flow_depth 
  - no_alerts
  - inspect_uri_only
  - oversize_dir_length
These options must be specified after the 'profile' option.

Example:

preprocessor http_inspect_server: server 1.1.1.1 profile all ports { 80 3128 }
 
There are three profiles available:
  - all: The "all" profile is meant to normalize the URI using most of the
  	     common tricks available.  We alert on the more serious forms of 
	     evasions.  This is a great profile for detecting all the types of	
	     attacks regardless of the HTTP server.

  - apache: The "apache" profile is used for apache web servers.  This differs
	     from the 'iis' profile by only accepting utf-8 standard
	     unicode encoding and not accepting backslashes as
	     legitimate slashes, like IIS does.  Apache also accepts 
	     tabs as whitespace 

  - iis: The "iis" profile mimics IIS servers.  So that means we use IIS
  	     unicode codemaps for each server, %u encoding, bare-byte encoding,
	     double decoding, backslashes, etc.

Profiles are not required by http_inspect.
 
* ports { [port] [port] . . . } *
This is how the user configures what ports to decode on the HTTP server. 
Encrypted traffic (SSL) cannot be decoded, so adding ports 443 will only 
yield encoding false positives.

* iis_unicode_map [file (located in config dir(] [codemap (integer)] *
The IIS Unicode Map is generated by the program ms_unicode_generator.c.  This
program is located in src/preprocessors/HttpInspect/util.  Executing this
program generates a unicode map for the system that it was run on.  So to get
the specific unicode mappings for an IIS web server, you run this program on
that server and use that unicode map in this configuration.

When using this option, the user needs to specify the file that contains the
IIS unicode map and also specify the unicode map to use.  For US servers, this
is usually 1252.  But the ms_unicode_generator program tells you which codemap
to use for you server, it's the ANSI codepage.  You can select the correct
code page by looking at the available code pages that the ms_unicode_generator
outputs.

* flow_depth [integer] *
This specifies the amount of server response payload to inspect.  This option
significantly increases IDS performance because we are ignoring a large part of
the network traffic (HTTP server response payloads).  A small percentage of
snort rules are targeted at this traffic and a small flow_depth value may
cause false negatives in some of these rules.  Most of these rules target
either the HTTP header, or the content that is likely to be in the first
hundred or so bytes of non-header data.  Headers are usually under 300 bytes
long, but your mileage may vary.

This value can be set from 0 to 1460, with a value of 0 inspecting all HTTP
server payloads (note that this will likely slow down IDS performance).  Values
above 0 tell http_inspect the number of bytes to inspect in the first packet
of the server response.

* ascii [yes/no] *
The ASCII decode option tells us whether to decode encoded ASCII chars, a.k.a
%2f = /, %2e = ., etc.  I suggest you don't log alerts for ASCII since it is 
very common to see normal ASCII encoding usage in URLs.

* utf_8 [yes/no] *
The UTF-8 decode option tells us to decode standard UTF-8 unicode sequences that
are in the URI.  This abides by the unicode standard and only uses % encoding.
Apache uses this standard, so for any apache servers, make sure you have this
option turned on.  As for alerting, you may be interested in knowing when you
have an utf-8 encoded URI, but this will be prone to false positives as
legitimate web clients use this type of encoding.  When utf_8 is enabled,
ascii decoding is also enabled to enforce correct functioning.

* u_encode [yes/no] *
This option emulates the IIS %u encoding scheme.  How the %u encoding scheme
works is as follows:  The encoding scheme is started by a %u followed by 4
chars, like %uXXXX.  The XXXX is a hex encoded value that correlates to an
IIS unicode codepoint.  This value can most definitely be ASCII.  An ASCII
char is encoded like, %u002f = /, %u002e = ., etc.  If no iis_unicode_map is
specified before or after this option, the default codemap is used.

You should alert on %u encodings, because I'm not aware of any legitimate 
clients that use this encoding.  So it is most likely someone trying to be
covert.

* bare_byte [yes/no] *
Bare byte encoding is an IIS trick that uses non-ASCII chars as valid values in
decoding UTF-8 values.  This is NOT in the HTTP standard, as all non-ASCII
values have to be encoded with a %.  Bare byte encoding allows the user to 
emulate an IIS server and interpret non-standard encodings correctly.

The alert on this decoding should be enabled, because there are no legitimate
clients that encoded UTF-8 this way, since it is non-standard.

* base36 [yes/no] *
This is an option to decode base36 encoded chars.  I didn't have access to 
a server with this option, since it appears that this is related to certain
Asian versions of windows.  I'm going off of info from:
http://www.yk.rim.or.jp/~shikap/patch/spp_http_decode.patch
So I hope that works for any of you with this option.  Please note that if you
have enabled %u encoding, this option will not work.  You have to use the
base36 option with the utf_8 option.  Don't use the %u option, because base36
won't work.  When base36 is enabled, so is ascii encoding to enforce correct
behavior.

* iis_unicode [yes/no] *
The iis_unicode option turns on the unicode codepoint mapping.  If there is no
iis_unicode_map option specified with the server config, iis_unicode uses the
default codemap.  The iis_unicode option handles the mapping of non-ascii
codepoints that the IIS server accepts and decodes normal UTF-8 request.

Users should alert on the iis_unicode option, because it is seen mainly in 
attacks and evasion attempts.  When iis_unicode is enabled, so is ascii and
utf-8 decoding to enforce correct decoding.  To alert on utf-8 decoding, the
user must enable also enable 'utf_8 yes'. 

* double_decode [yes/no] *
The double_decode option is once again IIS specific and emulates IIS 
functionality.  How this works is that IIS does two passes through the request 
URI, doing decodes in each one.  In the first pass, it seems that all types of 
IIS encoding is done: UTF-8 unicode, ASCII, bare byte, and %u.  In the second
pass the following encodings are done:  ASCII, bare byte, and %u.  We leave out
UTF-8 because I think how this works is that the % encoded UTF-8 is decoded to
the unicode byte in the first pass, and then UTF-8 decoded in the second stage.
Anyway, this is really complex and adds tons of different encodings for one
char.  When double_decode is enabled, so is ascii to enforce correct decoding.

* non_rfc_char { [byte] [0x00] . . . } *
This option lets users receive an alert if certain non-RFC chars are used in
a request URI.  For instance, a user may not want to see NULL bytes in the
request-URI and we can give an alert on that.  Please use this option with
care, because you could configure it to say, alert on all '/' or something
like that.  It's flexible, so be careful.

* multi_slash [yes/no] *
This option normalizes multiple slashes in a row, so something like:
"foo/////////bar" get normalized to "foo/bar".

If you want an alert when multiple slashes are seen, then configure with a yes,
otherwise a no.

* iis_backslash [yes/no] *
Normalize backslashes to slashes.  This is again an IIS emulation.  So a
request-URI of "/foo\bar" gets normalized to "/foo/bar".

* directory [yes/no] *
This option normalizes directory traversals and self-referential directories.
So, "/foo/this_is_not_a_real_dir/../bar" get normalized to "/foo/bar".  Also,
"/foo/./bar" gets normalized to "/foo/bar".  If a user wants to configure an
alert, then specify "yes", otherwise "no".  This alert may give false positives
since some web sites refer to files using directory traversals.

* apache_whitespace [yes/no] *
This option deals with non-RFC standard of tab for a space delimiter.  Apache
uses this, so if the emulated web server is Apache you need to enable this
option.  Alerts on this option may be interesting, but may also be false
positive prone.

* iis_delimiter [yes/no] *
I originally started out with \n being IIS specific, but Apache takes this
non-standard delimiter was well.  Since this is common, we always take this 
as standard since the most popular web servers accept it.  But you can still
get an alert on this option.

* chunk_length [non-zero positive integer] *
This option is an anomaly detector for abnormally large chunk sizes.  This picks
up the apache chunk encoding exploits, and may also alert on HTTP tunneling that
uses chunk encoding.

* no_pipeline_req *
This option turns HTTP pipeline decoding off, and is a performance enhancement
if needed.  By default pipeline requests are inspected for attacks, but when
this option is enabled, pipeline requests are not decoded and analyzed per HTTP
protocol field.  It is only inspected with the generic pattern matching.

* non_strict *
This option turns on non-strict URI parsing for the broken way in which
Apache servers will decode a URI.  Only use this option on servers that will
accept URIs like this "GET /index.html alsjdfk alsj lj aj  la jsj s\n".  The
non_strict option assumes the URI is between the first and second space
even if there is no valid HTTP identifier after the second space.

* allow_proxy_use *
By specifying this keyword, the user is allowing proxy use on this server.
This means that no alert will be generated if the proxy_alert global keyword
has been used.  If the proxy_alert keyword is not enabled, then this option
does nothing.  The allow_proxy_use keyword is just a way to suppress 
unauthorized proxy use for an authorized server.  

* no_alerts *
This option turns off all alerts that are generated by the HttpInspect
preprocessor module.  This has no effect on http rules in the ruleset.
No argument is specified.

* oversize_dir_length [non-zero positive integer] *
This option takes a non-zero positive integer as an argument.  The
argument specifies the max char directory length for URL directory.  If a 
URL directory is larger than this argument size, an alert is generated.  
A good argument value is 300 chars.  This should limit the alerts
to IDS evasion type attacks, like whisker -I 4.

* inspect_uri_only *
This is a performance optimization.  When enabled, only the URI portion of HTTP
requests will be inspected for attacks.  As this field usually contains 90-95%
of the web attacks, you'll catch most of the attacks.  So if you need extra
performance, then enable this optimization.  It's important to note that
if this option is used without any uricontent rules, then no inspection will
take place.  This is obvious since the uri is only inspected with uricontent
rules, and if there are none available then there is nothing to inspect.

For example, if we have the following rule set:

alert tcp any any -> any 80 ( msg:"content"; content: "foo"; )

and the we inspect the following URI:

GET /foo.htm HTTP/1.0\r\n\r\n

No alert will be generated when 'inspect_uri_only' is enabled.  The 
'inspect_uri_only' configuration turns off all forms of detection except 
uricontent inspection.

* webroot *
This option generates an alert when a directory traversal traverses past
the web server root directory.  This generates much less false positives than 
the directory option, because it doesn't alert on directory traversals that 
stay within the web server directory structure.  It only alerts when the 
directory traversals go past the web server root directory, which
is associated with certain web attacks.

-- Profile Breakout --
There are three profiles that users can select.  Only the configuration 
that are listed under the profiles are turned on.  If there is no mention 
of alert on or off, then that means there is no alert associated with the 
configuration.

* Apache *

flow_depth 300
non_strict URL parsing
chunk encoding (alert on chunks larger than 500000 bytes)
ascii decoding is on (alert off)
multiple slash (alert off)
directory normalization (alert off)
webroot (alert on)
apache whitespace (alert off)
utf_8 encoding (alert off)

* IIS *

flow_depth 300
chunk encoding (alert on chunks larger than 500000 bytes)
iis_unicode_map is set to the codepoint map in the global configuration
ascii decoding (alert off)
multiple slash (alert off)
directory normalization (alert off)
webroot (alert on)
double decoding (alert on)
%u decoding (alert on)
bare byte decoding (alert on)
iis unicode codepoints (alert on)
iis backslash (alert off)
iis delimiter (alert off)
apache whitespace (alert off)
non_strict URL parsing

* All * 

flow_depth 300
chunk encoding (alert on chunks larger than 500000 bytes)
iis_unicode_map is set to the codepoint map in the global configuration
ascii decoding is on (alert off)
multiple slash (alert off)
directory normalization (alert off)
apache whitespace (alert off)
double decoding (alert on)
%u decoding (alert on)
bare byte decoding (alert on)
iis unicode codepoints (alert on)
iis backslash (alert off)
iis delimiter (alert off)
webroot (alert on)
non_strict URL parsing

The following lists the defaults:

Port 80
flow_depth 300
chunk encoding (alert on chunks larger than 500000 bytes)
ascii decoding is on (alert off)
utf_8 encoding (alert off)
multiple slash (alert off)
directory normalization (alert off)
webroot (alert on)
apache whitespace (alert off)
iis delimiter (alert off)
non_strict URL parsing

-- Writing uricontent rules --
The uricontent parameter in the snort rule language searches the NORMALIZED
request URI field.  This means that if you are writing rules that include
things that are normalized, such as %2f or directory traversals, these
rules will not alert.  The reason is that the things you are looking for
are normalized out of the URI buffer.  For example, the URI:

/scripts/..%c0%af../winnt/system32/cmd.exe?/c+ver

will get normalized into:

/winnt/system32/cmd.exe?/c+ver

Another example,

/cgi-bin/aaaaaaaaaaaaaaaaaaaaaaaaaa/..%252fp%68f?

into:

/cgi-bin/phf?

So when you are writing a uricontent rule, you should write the content that
you want to find in the context that the URI will be normalized.  Don't include
directory traversals (if you normalize directories) and don't look for encode
characters.  You can accomplish this type of detection by using the 'content'
rule parameter, since this rule inspects the unnormalized buffer.

-- Conclusion --
So you got to the end?  Good for you.  I'm sure you know more about HTTP
encodings and evasions then you ever wanted to.  My suggestions are to stick
with the "profile" options, since they are much easier to read and have been
researched.

If you feel like giving us profiles for other web servers, please do.
We'll incorporate them into the default server profiles for HttpInspect.