123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345346347348349350351352353354355356357358359360361362363364365366367368369370371372373374375376377378379380381382383384385386387388389390391392393394395396397398399400401402403404405406407408409410411412413414415416417418419420421422423424425426427428429 HttpInspect ----------- Daniel Roelker -- 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.