Snort FAQ

## HttpInspect

Originally authored by Daniel Roelker Updated by members of Snort Team

## 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.

## 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.

## 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.

compress_depth *

This option specifies the maximum amount of packet payload to decompress. This value can be set from 1 to 65535. The default for this option is 1460. Please note, in case of multiple policies, the value specified in the default policy is used and this value overwrites the values specified in the other policies. In case of unlimited_decompress this should be set to its max value. This value should be specified in the default policy even when the HTTP inspect preprocessor is turned off using the disabled keyword.

decompress_depth *

This option specifies the maximum amount of decompressed data to obtain from the compressed packet payload. This value can be set from 1 to 65535. The default for this option is 2920.

Please note, in case of multiple policies, the value specified in the default policy is used and this value overwrites the values specified in the other policies. In case of unlimited_decompress this should be set to its max value. This value should be specified in the default policy even when the HTTP inspect preprocessor is turned off using the disabled keyword.

max_gzip_mem *

This option determines (in bytes) the maximum amount of memory the HTTP Inspect preprocessor will use for decompression. The minimum allowed value for this option is 3276 bytes. This option determines the number of concurrent sessions that can be decompressed at any given instant. The default value for this option is 838860. Additionally, this value is used as the memory limit for the options SWF/PDF file decompression functionality. If these features are enabled, then a second block of memory of the same size is allocated for decompression session state information.

Note: This value should be specified in the default policy even when the HTTP inspect preprocessor is turned off using the disabled keyword.

memcap <num> *

This option determines (in bytes) the maximum amount of memory the HTTP Inspect preprocessor will use for logging the URI and Hostname data. This value can be set from 2304 to 603979776 (576 MB). This option along with the maximum uri and hostname logging size (which is defined in snort) will determine the maximum HTTP sessions that will log the URI and hostname data at any given instant. The maximum size for logging URI data is 2048 and for hostname is 256. The default value for this option is 150994944 (144 MB).

Note: This value should be specified in the default policy even when the HTTP inspect preprocessor is turned off using the “disabled” keyword. In case of multiple policies, the value specified in the default policy will overwrite the value specified in other policies.

• max http sessions logged = memcap /( max uri logging size + max hostname logging size )
• max uri logging size defined in snort : 2048
• max hostname logging size defined in snort : 256

disabled *

This optional keyword is allowed with any policy to avoid packet processing. This option disables the preprocessor. When the preprocessor is disabled only the “memcap”, “max_gzip_mem”, “compress_depth” and “decompress_depth” options are applied when specified with the configuration. Other options are parsed but not used. Any valid configuration may have “disabled” added to it.

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]

Multiple server IPs (and/or networks using CIDR notation) can be specified by using a space separated list of IPs enclosed in braces {}’s:

preprocessor http_inspect_server: server { 10.1.1.1 10.2.2.0/24 } [server options]

NOTE: There is a limit of 40 IPs or networks per http_inspect_server line.

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/iis4_0/iis5_0] *

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 * server_flow_depth * client_flow_depth * post_depth * no_alerts * inspect_uri_only * oversize_dir_length * normalize_headers * normalize_cookies * normalize_utf * max_header_length * max_headers * max_spaces * enable_cookie * extended_response_inspection * inspect_gzip * normalize_javascript * max_javascript_whitespaces * enable_xff * unlimited_decompress * http_methods * log_uri * log_hostname * decompress_swf * decompress_pdf 

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 five 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, backslashes, etc.

iis4_0, iis5_0: In IIS 4.0 and 5.0, there was a double decoding vulnerability. These two profiles are identical to IIS, except they will alert by default if a URL has a double encoding. Double decode is not supported in IIS 5.1 and beyond, so it’s disabled in Snort.

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.

extended_response_inspection *

This enables the extended HTTP response inspection. The default http response inspection does not inspect the various fields of a HTTP response. By turning this option the HTTP response will be thoroughly inspected. The different fields of a HTTP response such as status code, status message, headers, cookie (when enable_cookie is configured) and body are extracted and saved into buffers. Different rule options are provided to inspect these buffers. When this option is turned on, if the HTTP response packet has a body then any content pattern matches ( without http modifiers ) will search the response body (decompressed in case of gzip) and not the entire packet payload. To search for patterns in the header of the response, one should use the http modifiers with content such as http_header, http_stat_code, http_stat_msg and http_cookie.

enable_cookie *

Ex: Set-Cookie: mycookie \r\n

In this case, Set-Cookie: \r\n will be in the HTTP header buffer and the pattern mycookie will be in the HTTP cookie buffer.

inspect_gzip *

This option specifies the HTTP inspect module to uncompress the compressed data(gzip/deflate) in HTTP response. You should select the config option “extended_response_inspection” before configuring this option. Decompression is done across packets. So the decompression will end when either the ‘compress_depth’ or ‘decompress_depth’ is reached or when the compressed data ends. When the compressed data is spanned across multiple packets, the state of the last decompressed packet is used to decompressed the data of the next packet. But the decompressed data are individually inspected. (i.e. the decompressed data from different packets are not combined while inspecting). Also the amount of decompressed data that will be inspected depends on the server_flow_depth configured. Http Inspect generates a preprocessor alert with gid 120 and sid 6 when the decompression fails. When the decompression fails due to a CRC error encountered by zlib, HTTP Inspect will also provide the detection module with the data that was decompressed by zlib.

unlimited_decompress *

This option enables the user to decompress unlimited gzip data (across multiple packets). Decompression will stop when the compressed data ends or when a out of sequence packet is received. To ensure unlimited decompression, user should set the ‘compress_depth’ and ‘decompress_depth’ to its maximum values in the default policy. The decompression in a single packet is still limited by the ‘compress_depth’ and ‘decompress_depth’.

decompress_swf { deflate lzma } *

This option will enable decompression of compressed SWF (Adobe Flash content) files encountered as the HTTP Response body in a GET transaction. A prerequisite is enabling extended_response_inspection (described above). When enabled, the preprocessor will examine the response body for the corresponding file signature. ‘CWS’ for Deflate/ZLIB compressed and ‘ZWS’ for LZMA compressed. Each decompression mode can be individually enabled. e.g. … { lzma } or { deflate } or { lzma deflate }. The compressed content is decompressed ‘in-place’ with the content made available to the detection/rules ‘file_data’ option. If enabled and located, the compressed SWF file signature is converted to ‘FWS’ to indicate an uncompressed file. The ‘decompress_depth’, ‘compress_depth’, and ‘unlimited_decompress’ are optionally used to place limits on the decompression process. The semantics for SWF files are similar to the gzip decompression process. LZMA decompression is only available if Snort is built with the liblzma package present and functional. If the LZMA package is not present, then the { lzma } option will indicate a fatal parsing error. If the liblzma package IS present, but one desires to disable LZMA support, then the --disable-lzma option on configure will disable usage of the library. During the decompression process, the preprocessor may generate alert 120:12 if Deflate decompression fails or alert 120:13 if LZMA decompression fails.

decompress_pdf { deflate } *

This option will enable decompression of the compressed portions of PDF files encountered as the HTTP Response body in a GET transaction. A prerequisite is enabling extended_response_inspection (described above). When enabled, the preprocessor will examine the response body for the ‘%PDF-‘ file signature. PDF files are then parsed, locating PDF ‘streams’ with a single ‘/FlateDecode’ filter. These streams are decompressed in-place, replacing the compressed content. The ‘decompress_depth’, ‘compress_depth’, and ‘unlimited_decompress’ are optionally used to place limits on the decompression process. The semantics for PDF files are similar to the gzip decompression process. During the file parsing/decompression process, the preprocessor may generate several alerts:

 120:14 - Deflate decompression error 120:15 - Located a 'stream' with an unsupported compression ('/Filter') algorithm 120:16 - Located a 'stream' with cascaded '/FlateDecode' options (e.g. /Filter [ /FlateDecode /FlateDecode ]) 120:17 - PDF File parsing error 

normalize_javascript *

This option enables the normalization of Javascript within the HTTP response body. You should select the config option “extended_response_inspection” before configuring this option. When this option is turned on, Http Inspect searches for a Javascript within the HTTP response body by searching for the <script> tags and starts normalizing it. When Http Inspect sees the <script> tag without a type, it is considered as a javascript. The obfuscated data within the javascript functions such as unescape, String.fromCharCode, decodeURI, decodeURIComponent will be normalized. The different encodings handled within the unescape/ decodeURI/decodeURIComponent are %XX, %uXXXX, \XX and \uXXXX. Apart from these encodings, Http Inspect will also detect the consecutive whitespaces and normalize it to a single space. Http Inspect will also normalize the plus and concatenate the strings. The rule option file_data  can be used to access this normalized buffer from the rule.A preprocessor alert with SID 9 and GID 120 is generated when the obfuscation levels within the Http Inspect is equal to or greater than 2.

 Example:

HTTP/1.1 200 OK\r\n Date: Wed, 29 Jul 2009 13:35:26 GMT\r\n Server: Apache/2.2.3 (Debian) PHP/5.2.0-8+etch10 mod_ssl/2.2.3 OpenSSL/0.9.8c\r\n Last-Modified: Sun, 20 Jan 2008 12:01:21 GMT\r\n Accept-Ranges: bytes\r\n Content-Length: 214\r\n Keep-Alive: timeout=15, max=99\r\n Connection: Keep-Alive\r\n Content-Type: application/octet-stream\r\n\r\n <html xmlns="http://www.w3.org/1999/xhtml">

FIXME

</html> 

The above javascript will generate the preprocessor alert with SID 9 and GIDF 120 when normalize_javascript is turned on.

Http Inspect will also generate a preprocessor alert with GID 120 and SID 11 when there are more than one type of encodings within the escaped/encoded data.

For example:

unescape("%48\x65%6C%6C%6F%2C%20%73%6E%6F%72%74%20%74%65%61%6D%21"); String.fromCharCode(0x48, 0x65, 0x6c, 0x6c, 111, 44, 32, 115, 110, 111, 114, 116, 32, 116, 101, 97, 109, 33)  The above obfuscation will generate the preprocessor alert with GID 120 and SID 11.

This option is turned off by default in HTTP Inspect.

max_javascript_whitespaces [positive integer] *

This option takes an integer as an argument. The integer determines the maximum number of consecutive whitespaces allowed within the Javascript obfuscated data in a HTTP response body. The config option “normalize_javascript” should be turned on before configuring this config option. When the whitespaces in the javascript obfuscated data is equal to or more than this value a preprocessor alert with GID 120 and SID 10 is generated. The default value for this option is 200. To enable, specify an integer argument to max_spaces of 1 to 65535. Specifying a value of 0 is treated as disabling the alert.

enable_xff *

This option enables Snort to parse and log the original client IP present in the X-Forwarded-For or True-Client-IP HTTP request headers along with the generated events. The XFF/True-Client-IP Original client IP address is logged only with unified2 output and is not logged with console (-A cmg) output. NOTE: The original client IP from XFF/True-Client-IP in unified2 logs can be viewed using the tool u2spewfoo. This tool is present in the tools/u2spewfoo directory of snort source tree.

xff_headers *

If/When the enable_xff option is present, the xff_headers option specifies a set of custom ‘xff’ headers. This option allows the definition of up to six custom headers in addition to the two default (and always present) X-Forwarded-For and True-Client-IP headers. The option permits both the custom and default headers to be prioritized. The headers/priority pairs are specified as a list. Lower numerical values imply a higher priority. The headers do not need to be specified in priority order. Nor do the priorities need to be contiguous. Priority values can range from 1 to 255. The priority values and header names must be unique. The header names must not collide with known http headers such as ‘host’, ‘cookie’, ‘content-length’, etc.

A example of the xff_header syntax is:

 xff_headers { [ x-forwarded-highest-priority 1 ] [ x-forwarded-second-highest-priority 2 ] \ [ x-forwarded-lowest-priority-custom 3 ] } 

HTTP_Inspect will locate the highest priority xff header and return that one address in the unified2 log file. From the example, if both exff_2 and exff_1 exist in the request header, then the exff_1 value (which has the higher priority) will be logged.

The default X-Forwarded-For and True-Client-IP headers are always present. They may be explicitly specified in the xff_headers config in order to determine their priority. If not specified, they will be automatically added to the xff list as the lowest priority headers.

For example, let us say that we have the following (abbreviated) HTTP request header:

 ... Host: www.snort.org X-Forwarded-For: 192.168.1.1 X-Was-Originally-Forwarded-From: 10.1.1.1 ... 

With the default xff behavior (no xff_headers), the ‘X-Forwarded-For’ header would be used to provide a 192.168.1.1 Original Client IP address in the unified2 log. Custom headers are not parsed.

With:

 xff_headers { [ x-was-originally-forwarded-from 1 ] [ x-another-forwarding-header 2 ] [ x-forwarded-for 3 ] } 

The X-Was-Originally-Forwarded-From header is the highest priority present and its value of 10.1.1.1 will be logged as the Original Client IP in the unified2 log.

But with:

 xff_headers { [ x-was-originally-forwarded-from 3 ] [ x-another-forwarding-header 2 ] [ x-forwarded-for 1 ] } 

Now the X-Forwarded-For header is the highest priority and its value of 192.168.1.1 is logged.

• server_flow_depth [integer] *
• flow_depth [integer] * (to be deprecated)

This specifies the amount of server response payload to inspect. When extended_response_inspection is turned on, it is applied to the HTTP response body (decompressed data when inspect_gzip is turned on) and not the HTTP headers. When extended_response_inspection is turned off the server_flow_depth is applied to the entire HTTP response (including headers). Unlike client_flow_depth this option is applied per TCP session. This option can be used to balance the needs of IDS performance and level of inspection of HTTP server response data. Snort rules are targeted at HTTP server response traffic and when used with a small flow_depth value may cause false negatives. 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. It is suggested to set the server_flow_depth to its maximum value.

This value can be set from -1 to 65535. A value of -1 causes Snort to ignore all server side traffic for ports defined in “ports” when extended_response_inspection is turned off. When the extended_response_inspection is turned on, value of -1 causes Snort to ignore the HTTP response body data and not the HTTP headers. Inversely, a value of 0 causes Snort to inspect all HTTP server payloads defined in “ports” (note that this will likely slow down IDS performance). Values above 0 tell Snort the number of bytes to inspect of the server response (excluding the HTTP headers when extended_response_inspection is turned on) in a given HTTP session. Only packets payloads starting with ‘HTTP’ will be considered as the first packet of a server response. If less than flow_depth bytes are in the payload of the HTTP response packets in a given session, the entire payload will be inspected. If more than flow_depth bytes are in the payload of the HTTP response packet in a session only flow_depth bytes of the payload will be inspected for that session. Rules that are meant to inspect data in the payload of the HTTP response packets in a session beyond 65535 bytes will be ineffective unless flow_depth is set to 0. The default value for server_flow_depth is 300. Note that the 65535 byte maximum flow_depth applies to stream reassembled packets as well. It is suggested to set the server_flow_depth to its maximum value.

client_flow_depth [integer] *

This specifies the amount of raw client request payload to inspect. This value can be set from -1 to 1460. Unlike server_flow_depth this value is applied to the first packet of the HTTP request. It is not a session based flow depth. It has a default value of 300. It primarily eliminates Snort from inspecting larger HTTP Cookies that appear at the end of many client request Headers. A value of -1 causes Snort to ignore all client side traffic for ports defined in “ports.” Inversely, a value of 0 causes Snort to inspect all HTTP client side traffic defined in “ports” (note that this will likely slow down IDS performance). Values above 0 tell Snort the number of bytes to inspect in the first packet of the client request. If less than flow_depth bytes are in the TCP payload (HTTP request) of the first packet, the entire payload will be inspected.
If more than flow_depth bytes are in the payload of the first packet only flow_depth bytes of the payload will be inspected. Rules that are meant to inspect data in the payload of the first packet of a client request beyond 1460 bytes will be ineffective unless flow_depth is set to 0. Note that the 1460 byte maximum flow_depth applies to stream reassembled packets as well. It is suggested to set the client_flow_depth to its maximum value.

post_depth [integer] *

This specifies the amount of data to inspect in a client post message. The value can be set from -1 to 65495. The default value is -1. A value of -1 causes Snort to ignore all the data in the post message. Inversely, a value of 0 causes Snort to inspect all the client post message. This increases the performance by inspecting only specified bytes in the post message.

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.

extended_ascii_uri *

This option enables the support for extended ascii codes in the HTTP request URI. This option is turned off by default and is not supported with any of the profiles.

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.

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.

* 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 specific to IIS 4.0 and 5.0. The versions of IIS do 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 or carriage return for a space delimiter. Apache accepts 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.

small_chunk_length { <chunk size> <consecutive chunks> } *

This option is an evasion detector for consecutive small chunk sizes when either the client or server use Transfer-Encoding: chunked. specifies the maximum chunk size for which a chunk will be considered small. specifies the number of consecutive small chunks <= before an event will be generated. This option is turned off by default. Maximum values for each are 255 and a of 0 disables. Events generated are gid:119,sid:27 for client small chunks and gid:120,sid:7 for server small chunks. Example: small_chunk_length { 10 5 } Meaning alert if we see 5 consecutive chunk sizes of 10 or less.

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 then 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.

max_header_length [positive integer] * This option takes an integer as an argument. The integer is the maximum length allowed for an HTTP client request header field. Requests that exceed this length will cause a “Long Header” alert. This alert is off by default. To enable, specify an integer argument to max_header_length of 1 to 65535. Specifying a value of 0 is treated as disabling the alert.

max_spaces [positive integer] *

This option takes an integer as an argument. The integer determines the maximum number of whitespaces allowed with HTTP client request line folding. Requests headers folded with whitespaces equal to or more than this value will cause a “Space Saturation” alert with SID 26 and GID 119. The default value for this option is 200. To enable, specify an integer argument to max_spaces of 1 to 65535. Specifying a value of 0 is treated as disabling the alert.

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.

tab_uri_delimiter *

Both Apache and newer versions of IIS accept tabs as delimiters. However, this option is deprecated and has been replaced by, and is enabled by default with, the whitespace_chars option. For more details on its use, see the whitespace_chars section above.

normalize_headers *

This option turns on normalization for HTTP Header Fields, not including Cookies (using the same configuration parameters as the URI normalization (i.e., multi-slash, directory, etc.). It is useful for normalizing Referrer URIs that may appear in the HTTP Header.

normalize_cookies *

This option turns on normalization for HTTP Cookie Fields (using the same configuration parameters as the URI normalization (i.e., multi-slash, directory, etc.). It is useful for normalizing data in HTTP Cookies that may be encoded.

normalize_utf *

This option turns on normalization of HTTP response bodies where the Content-Type header lists the character set as “utf-16le”, “utf-16be”, “utf-32le”, or “utf-32be”. HTTP Inspect will attempt to normalize these back into 8-bit encoding, generating an alert if the extra bytes are non-zero.

max_headers [positive integer] *

This option takes an integer as an argument. The integer is the maximum number of HTTP client request header fields. Requests that contain more HTTP Headers than this value will cause a “Max Header” alert. The alert is off by default. To enable, specify an integer argument to max_headers of 1 to 1024. Specifying a value of 0 is treated as disabling the alert.

http_methods { <CMD1> <CMD2> } *

This specifies additional HTTP Request Methods outside of those checked by default within the preprocessor (GET and POST). The list should be enclosed within braces and delimited by spaces or \t\n\r. The config option, braces and methods also needs to be separated by braces.

Example : http_methods { PUT CONNECT }

Please note the maximum length for a method name is 256.

log_uri *

This option enables HTTP Inspect preprocessor to parse the URI data from the HTTP request and log it along with all the generated events for that session. Stream5 reassembly needs to be turned on HTTP ports to enable the logging. If there are multiple HTTP requests in the session, the URI data of the most recent HTTP request during the alert will be logged. The maximum URI logged is 2048.

Please note, this is logged only with the unified2 output and is not logged with console output (-A cmg). u2spewfoo can be used to read this data from the unified2.

log_hostname *

This option enables HTTP Inspect preprocessor to parse the hostname data from the “Host” header of the HTTP request and log it along with all the generated events for that session. Stream5 reassembly needs to be turned on HTTP ports to enable the logging. If there are multiple HTTP requests in the session, the Hostname data of the most recent HTTP request during the alert will be logged. In case of multiple “Host” headers within one HTTP request, a preprocessor alert with sid 24 is generated. The maximum hostname length logged is 256.

]Please note, this is logged only with the unified2 output and is not logged with console output (-A cmg). u2spewfoo can be used to read this data from the unified2.

### 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 *

 server_flow_depth 300 non_strict URL parsing is set 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) max_header_length 0 (header length not checked) max_headers 0 (number of headers not checked) max_sapces 200 (number of allowed white spaces) 

IIS *

 server_flow_depth 300 non_strict URL parsing is set 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) %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) max_header_length 0 (header length not checked) max_headers 0 (number of headers not checked) 

IIS4_0 and IIS5_0 *

 server_flow_depth 300 non_strict URL parsing is set 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) max_header_length 0 (header length not checked) max_headers 0 (number of headers not checked) 

All *

 server_flow_depth 300 non_strict URL parsing is set 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) max_header_length 0 (header length not checked) max_headers 0 (number of headers not checked) 

## Pattern Match With HTTP Buffers

### 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.

## Http Content Match Keywords

http_client_body *

The http_client_body keyword is a content modifier that restricts the search to the body of an HTTP client request. As this keyword is a modifier to the previous ‘content’ keyword, there must be a content in the rule before ‘http_client_body’ is specified. The amount of data that is inspected with this option depends on the post_depth config option of HttpInspect. Pattern matches with this keyword wont work when post_depth is set to -1.

http_cookie *

The http_cookie keyword is a content modifier that restricts the search to the extracted Cookie Header field (excluding the header name itself and the CRLF terminating the header line) of a HTTP client request or a HTTP server response. The Cookie buffer does not include the header names (“Cookie:” for HTTP requests or “Set-Cookie:” for HTTP responses) or leading spaces and the CRLF terminating the header line. These are included in the HTTP header buffer. As this keyword is a modifier to the previous ‘content’ keyword, there must be a content in the rule before ‘http_cookie’ is specified. This keyword is dependent on the ‘enable_cookie’ config option. The Cookie Header field will be extracted only when this option is configured. If enable_cookie is not specified, the cookie still ends up in HTTP header. When enable_cookie is not specified, using http_cookie is the same as using http_header. The extracted Cookie Header field will be NORMALIZED if the normalize_cookies is configured with HttpInspect.

http_raw_cookie *

The http_raw_cookie keyword is a content modifier that restricts the search to the extracted UNNORMALIZED Cookie Header field of a HTTP client request or a HTTP server response. As this keyword is a modifier to the previous ‘content’ keyword, there must be a content in the rule before ‘http_raw_cookie’ is specified. This keyword is dependent on the ‘enable_cookie’ config option. The Cookie Header field will be extracted only when this option is configured.

http_header *

The http_header keyword is a content modifier that restricts the search to the extracted Header fields of a HTTP client request or a HTTP server response.
As this keyword is a modifier to the previous ‘content’ keyword, there must be a content in the rule before ‘http_header’ is specified.

http_raw_header *

The http_raw_header keyword is a content modifier that restricts the search to the extracted UNNORMALIZED Header fields of a HTTP client request or a HTTP server response. As this keyword is a modifier to the previous ‘content’ keyword, there must be a content in the rule before ‘http_raw_header’ is specified.

http_method *

The http_method keyword is a content modifier that restricts the search to the extracted Method from a HTTP client request. As this keyword is a modifier to the previous ‘content’ keyword, there must be a content in the rule before ‘http_method’ is specified. NOTE: This field is not fast_pattern eligible.

http_uri *

The http_uri keyword is a content modifier that restricts the search to the NORMALIZED request URI field . Using a content rule option followed by a http_uri modifier is the same as using a uricontent by itself. As this keyword is a modifier to the previous ‘content’ keyword, there must be a content in the rule before ‘http_uri’ is specified.

http_raw_uri *

The http_raw_uri keyword is a content modifier that restricts the search to the UNNORMALIZED request URI field . As this keyword is a modifier to the previous ‘content’ keyword, there must be a content in the rule before ‘http_raw_uri’ is specified.

http_stat_code *

The http_stat_code keyword is a content modifier that restricts the search to the extracted Status code field from a HTTP server response. As this keyword is a modifier to the previous ‘content’ keyword, there must be a content in the rule before ‘http_stat_code’ is specified. The Status Code field will be extracted only if the extended_response_inspection is configured for the HttpInspect.

http_stat_msg *

The http_stat_msg keyword is a content modifier that restricts the search to the extracted Status Message field from a HTTP server response. As this keyword is a modifier to the previous ‘content’ keyword, there must be a content in the rule before ‘http_stat_msg’ is specified.

The Status Message field will be extracted only if the extended_response_inspection is configured for the HttpInspect.

http_encode *

The http_encode keyword will enable alerting based on encoding type present in a HTTP client request or a HTTP server response.

There are several keywords associated with http_encode. The keywords ‘uri’, ‘header’ and ‘cookie’ determine the HTTP fields used to search for a particular encoding type. The keywords ‘utf8’, ‘double_encode’, ‘non_ascii’, ‘uencode’, ‘ascii’, ‘iis_encode’ and ‘bare_byte’ determine the encoding type which would trigger the alert. These keywords can be combined using a OR operation. Negation is allowed on these keywords.

The config option ‘normalize_headers’ needs to be turned on for rules to work with keyword ‘header’. The keyword ‘cookie’ is depedent on config options ‘enable_cookie’ and ‘normalize_cookies’ This rule option will not be able to detect encodings if the specified HTTP fields are not NORMALIZED.

## Conclusion

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.

 SID Description —— —— 1 ASCII encoding 2 Double decoding attack 3 U encoding 4 Bare byte Unicode encoding 5 Base36 encoding # Deprecated in Snort 2.9.1 6 UTF-8 encoding 7 IIS Unicode codepoint encoding 8 multi-slash encoding 9 IIS backslash evasion 10 self-directory traversal 11 directory traversal 12 Apache whitespace (tab) 13 Non-RFC HTTP delimiter 14 Non-RFC defined char 15 Oversize request-URI directory 16 Oversize chunk encoding 17 Unauthorized proxy use detected 18 Webroot directory traversal 19 Long header 20 Max headers 21 Multiple Content-Length headers 22 Chunk size mismatch 23 Invalid True-IP/XFF Orginal Client IP 24 Multiple Host headers 25 Hostname exceeds 255 characters 27 Chunked encoding - excessive consecutive small chunks 28 Unbounded POST (without Content-Length or Transfer-Encoding: chunked) 29 multiple true IPs in a session 30 both true_client_ip and XFF hdrs present 31 unknown method 32 simple request (HTTP/0.9)