Transformations

Transformation keywords turn the data at a sticky buffer into something else. Some transformations support options for greater control over the transformation process

Example::

alert http any any -> any any (file_data; strip_whitespace; \
    content:"window.navigate("; sid:1;)

This example will match on traffic even if there are one or more spaces between the navigate and (.

The transforms can be chained. They are processed in the order in which they appear in a rule. Each transform's output acts as input for the next one.

Example::

alert http any any -> any any (http_request_line; compress_whitespace; to_sha256; \
    content:"|54A9 7A8A B09C 1B81 3725 2214 51D3 F997 F015 9DD7 049E E5AD CED3 945A FC79 7401|"; sid:1;)

.. note:: not all sticky buffers support transformations yet

dotprefix

Takes the buffer, and prepends a . character to help facilitate concise domain checks. For example, an input string of hello.google.com would be modified and become .hello.google.com. Additionally, adding the dot allows google.com to match against content:".google.com"

Example::

alert dns any any -> any any (dns.query; dotprefix; \
    content:".microsoft.com"; sid:1;)

This example will match on windows.update.microsoft.com and maps.microsoft.com.au but not windows.update.fakemicrosoft.com.

This rule can be used to match on the domain only; example::

alert dns any any -> any any (dns.query; dotprefix; \
    content:".microsoft.com"; endswith; sid:1;)

This example will match on windows.update.microsoft.com but not windows.update.microsoft.com.au.

Finally, this rule can be used to match on the TLD only; example::

alert dns any any -> any any (dns.query; dotprefix; \
    content:".co.uk"; endswith; sid:1;)

This example will match on maps.google.co.uk but not maps.google.co.nl.

domain

Takes the buffer and extracts the domain name from it. The domain name is defined using Mozilla’s Public Suffix List <https://publicsuffix.org/>_. This implies that it is using traditional top level domain such as .com but also some specific domain like airport.aero or execute-api.cn-north-1.amazonaws.com.cn where declaration of sub domain by users below the domain is possible.

Example::

alert tls any any -> any any (tls.sni; domain; \
    dataset:isset,domains,type string,load domains.lst; sid:1;)

This example will match on all domains contained in the file domains.lst. For example, if domains.lst contains oisf.net then webshop.oisf.net will match.

tld

Takes the buffer and extracts the Top Level Domain (TLD) name from it. The TLD name is defined using Mozilla’s Public Suffix List <https://publicsuffix.org/>_. This implies that it is will have traditional TLD such as com but also some specific domain like airport.aero or execute-api.cn-north-1.amazonaws.com.cn where declaration of sub domain by users below the domain is possible.

Example::

alert tls any any -> any any (tls.sni; tld; \
    dataset:isset,tlds,type string,load tlds.lst; sid:1;)

This example will match on all TLDs contained in the file tlds.lst. For example, if tlds.lst contains net then oisf.net will match.

strip_whitespace

Strips all whitespace as considered by the isspace() call in C.

Example::

alert http any any -> any any (file_data; strip_whitespace; \
    content:"window.navigate("; sid:1;)

compress_whitespace

Compresses all consecutive whitespace into a single space.

to_lowercase

Converts the buffer to lowercase and passes the value on.

This example alerts if http.uri contains this text has been converted to lowercase

Example::

alert http any any -> any any (http.uri; to_lowercase; \
    content:"this text has been converted to lowercase"; sid:1;)

to_md5

Takes the buffer, calculates the MD5 hash and passes the raw hash value on.

Example::

alert http any any -> any any (http_request_line; to_md5; \
    content:"|54 A9 7A 8A B0 9C 1B 81 37 25 22 14 51 D3 F9 97|"; sid:1;)

to_uppercase

Converts the buffer to uppercase and passes the value on.

This example alerts if http.uri contains THIS TEXT HAS BEEN CONVERTED TO UPPERCASE

Example::

alert http any any -> any any (http.uri; to_uppercase; \
    content:"THIS TEXT HAS BEEN CONVERTED TO UPPERCASE"; sid:1;)

to_sha1

Takes the buffer, calculates the SHA-1 hash and passes the raw hash value on.

Example::

alert http any any -> any any (http_request_line; to_sha1; \
    content:"|54A9 7A8A B09C 1B81 3725 2214 51D3 F997 F015 9DD7|"; sid:1;)

to_sha256

Takes the buffer, calculates the SHA-256 hash and passes the raw hash value on.

Example::

alert http any any -> any any (http_request_line; to_sha256; \
    content:"|54A9 7A8A B09C 1B81 3725 2214 51D3 F997 F015 9DD7 049E E5AD CED3 945A FC79 7401|"; sid:1;)

pcrexform

Takes the buffer, applies the required regular expression, and outputs the first captured expression.

.. note:: this transform requires a mandatory option string containing a regular expression.

This example alerts if http.request_line contains /dropper.php Example::

alert http any any -> any any (msg:"HTTP with pcrexform"; http.request_line; \
    pcrexform:"[a-zA-Z]+\s+(.*)\s+HTTP"; content:"/dropper.php"; sid:1;)

url_decode

Decodes url-encoded data, ie replacing '+' with space and '%HH' with its value. This does not decode unicode '%uZZZZ' encoding

xor

Takes the buffer, applies xor decoding.

.. note:: this transform requires a mandatory option which is the hexadecimal encoded xor key.

This example alerts if http.uri contains password= xored with 4-bytes key 0d0ac8ff Example::

alert http any any -> any any (msg:"HTTP with xor"; http.uri; \
    xor:"0d0ac8ff"; content:"password="; sid:1;)

header_lowercase

This transform is meant for HTTP/1 HTTP/2 header names normalization. It lowercases the header names, while keeping untouched the header values.

The implementation uses a state machine :

it lowercases until it finds ``:```
it does not change until it finds a new line and switch back to first state

This example alerts for both HTTP/1 and HTTP/2 with a authorization header Example::

alert http any any -> any any (msg:"HTTP authorization"; http.header_names; \
    header_lowercase; content:"authorization:"; sid:1;)

strip_pseudo_headers

This transform is meant for HTTP/1 HTTP/2 header names normalization. It strips HTTP2 pseudo-headers (names and values).

The implementation just strips every line beginning by :.

This example alerts for both HTTP/1 and HTTP/2 with only a user agent Example::

alert http any any -> any any (msg:"HTTP ua only"; http.header_names; \
   bsize:16; content:"|0d 0a|User-Agent|0d 0a 0d 0a|"; nocase; sid:1;)

.. _from_base64:

from_base64

This transform is similar to the keyword base64_decode: the buffer is decoded using the optional values for mode, offset and bytes and is available for matching on the decoded data.

After this transform completes, the buffer will contain only bytes that could be bases64-decoded. If the decoding process encountered invalid bytes, those will not be included in the buffer.

The option values must be , separated and can appear in any order.

.. note:: from_base64 follows RFC 4648 by default i.e. encounter with any character that is not found in the base64 alphabet leads to rejection of that character and the rest of the string.

Format::

from_base64: [[bytes <value>] [, offset <offset_value> [, mode: strict|rfc4648|rfc2045]]]
from_base64

There are defaults for each of the options:

bytes defaults to the length of the input buffer
offset defaults to 0 and must be less than 65536
mode defaults to rfc4648

The second example shows the rule keyword only which will cause the default values for each option to be used.

Note that both bytes and offset may be variables from byte_extract and/or byte_math in later versions of Suricata. They are not supported yet.

Mode rfc4648 applies RFC 4648 decoding logic which is suitable for encoding binary data that can be safely sent by email, used in a URL, or included with HTTP POST requests.

Mode rfc2045 applies RFC 2045 decoding logic which supports strings, including those with embedded spaces, line breaks, and any non base64 alphabet.

Mode strict will fail if an invalid character is found in the encoded bytes.

The following examples will alert when the buffer contents match (see the last content value for the expected strings).

This example uses the defaults and transforms "VGhpcyBpcyBTdXJpY2F0YQ==" to "This is Suricata"::

   content: "VGhpcyBpcyBTdXJpY2F0YQ=="; from_base64; content:"This is Suricata";

This example transforms "dGhpc2lzYXRlc3QK" to "thisisatest"::

   content:"/?arg=dGhpc2lzYXRlc3QK"; from_base64: offset 6, mode rfc4648; \
   content:"thisisatest";

This example transforms "Zm 9v Ym Fy" to "foobar"::

   content:"/?arg=Zm 9v Ym Fy"; from_base64: offset 6, mode rfc2045; \
   content:"foobar";

.. _lua-transform:

luaxform

This transform allows a Lua script to apply a transformation to a buffer.

Lua scripts that are used for transformations must contain a function named transform. The function signature is::

-- transform function arguments:
--   input - the buffer to be transformed. Use #input to determine byte count
--   args - The arguments from the rule. Access each as args[0], args[1], ...
function transform(input, args)

Lua scripts can have other functions; Suricata only invokes the transform function.

Lua transforms can be passed optional arguments -- see the examples below -- but they are not required to do so. Specify comma-separated arguments in the rule, e.g::

luaxform:transform.lua, bytes 0, offset 2

A Lua transform function is not invoked if the buffer is empty or the Lua framework is not accessible (rare).

Lua transform functions must return two values (see below) or the buffer is not modified.

Note that the arguments and values are passed without validation nor interpretation. There is a maximum of 10 arguments.

The Lua transform function is invoked with these parameters:

input The buffer provided to the transform
arguments The list of arguments.

Lua transform functions must return two values [Lua datatypes shown]:

buffer [Lua string] The return buffer containing the original input buffer or buffer modified by the transform.
bytes [Lua integer] Number of bytes in return buffer.

This example supplies the HTTP data to a Lua transform and the transform results are checked with content.

Example::

alert http any any -> any any (msg:"Lua Xform example"; flow:established;  \
        file.data; luaxform:./lua/lua-transform.lua; content: "abc"; sid: 2;)

This example supplies the HTTP data to a Lua transform with with arguments that specify the offset and byte count for the transform. The resulting buffer is then checked with a content match.

Example::

alert http any any -> any any (msg:"Lua Xform example"; flow:established; \
        file.data; luaxform:./lua/lua-transform.lua, bytes 12, offset 13; content: "abc"; sid: 1;)

The following Lua script shows a transform that handles arguments: bytes and offset and uses those values (or defaults, if there are no arguments) for applying the uppercase transform to the buffer.

.. code-block:: lua

local function get_value(item, key) if string.find(item, key) then local _, value = string.match(item, "(%a+)%s*(%d*)") if value ~= "" then return tonumber(value) end end

   return nil

end

-- Arguments supported local bytes_key = "bytes" local offset_key = "offset"

function transform(input, args) local bytes = #input local offset = 0

   -- Look for optional bytes and offset arguments
   for i, item in ipairs(args) do
       local value = get_value(item, bytes_key)
       if value ~= nil then
           bytes = value
       else
           local value = get_value(item, offset_key)
           if value ~= nil then
               offset = value
           end
       end
   end
   local str_len = #input
   if offset < 0 or offset > str_len then
       print("offset is out of bounds: " .. offset)
       return nil
   end
   str_len = str_len - offset
   if bytes < 0 or bytes > str_len then
       print("invalid bytes " ..  bytes .. " or bytes > length " .. bytes .. " length " .. str_len)
       return nil
   end
   local sub = string.sub(input, offset + 1, offset + bytes)
   return string.upper(sub), bytes

end

gunzip

Takes the buffer, applies gunzip decompression.

This transform takes an optional argument which is a comma-separated list of key-values. Key and value are separated by a space. The only key being interperted is max-size, which is the max output size. Default for max-size is 1024. If the decompressed data were to be larger than max-size, the transform will decompress data up to max-size. Value 0 is forbidden for max-size (there is no unlimited value).

This example alerts if http.uri contains base64-encoded gzipped value Example::

alert http any any -> any any (msg:"from_base64 + gunzip";
        http.uri; content:"/gzb64?value="; fast_pattern;
        from_base64: offset 13 ;
        gunzip: max-size 1024; content:"This is compressed then base64-encoded"; startswith; endswith;
        sid:2; rev:1;)

zlib_deflate

Takes the buffer, applies zlib decompression.

This example alerts if http.uri contains base64-encoded zlib-compressed value Example::

alert http any any -> any any (msg:"from_base64 + gunzip";
        http.uri; content:"/zb64?value="; fast_pattern;
        from_base64: offset 12 ;
        zlib_deflate; content:"This is compressed then base64-encoded"; startswith; endswith;
        sid:2; rev:1;)