8.11. Transformations

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

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.

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

8.11.1. 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"

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.

8.11.2. domain

Takes the buffer and extracts the domain name from it. The domain name is defined using Mozilla’s Public Suffix List. 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.

8.11.3. 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. 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.

8.11.4. strip_whitespace

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

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

8.11.5. compress_whitespace

Compresses all consecutive whitespace into a single space.

8.11.6. 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

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

8.11.7. to_md5

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

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;)

8.11.8. 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

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

8.11.9. to_sha1

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

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

8.11.10. to_sha256

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

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;)

8.11.11. 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 .. container:: example-rule

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;)

8.11.12. url_decode

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

8.11.13. 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 .. container:: example-rule

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

8.11.14. 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 .. container:: example-rule

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

8.11.15. 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 .. container:: example-rule

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;)

8.11.16. 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";

8.11.17. 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.

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

8.11.18. 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 .. container:: example-rule

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;)

8.11.19. zlib_deflate

Takes the buffer, applies zlib 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 zlib-compressed value .. container:: example-rule

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;)

8.11.20. subslice

This transform creates a slice of the input buffer.

The subslice transform requires parameters:

offset Specifies the starting offset at which to create the subslice. When negative, expresses how far from the end of the input buffer to begin. If the absolute value of a negative offset exceeds the buffer length and truncate is not present, the transform will produce an empty buffer. When truncate is present, the starting position is reset to the beginning of the buffer. [REQUIRED]

nbytes Specifies the size of the subslice. When negative, specifies that the subslice ends nbytes bytes from the end of the input buffer. If the absolute value of a negative nbytes exceeds the buffer length and truncate is not present, the transform will produce an empty buffer. When truncate is present, the endpoint is limited to the beginning of the buffer. The default value is the size of the input buffer minus the value of offset. [OPTIONAL]

truncate Specifies behavior when offset + nbytes is larger than the input buffer size, or when the absolute value of a negative offset or negative nbytes exceeds the buffer length. When present, the result is trimmed as though offset + nbytes == buffer_length and excessive negative values are reset to the buffer boundaries. When not present (DEFAULT), a subslice larger than offset + nbytes produces an empty buffer, on which bsize:0 will match. [OPTIONAL]

8.11.20.1. Usage

Specify the subslice desired -- nbytes and truncate are optional:

Format:

subslice: offset <, nbytes>, <, truncate>;

When nbytes is not present, the size of the subslice will be the size of the input buffer minus the offset value.

When truncate is not present and the value of offset + nbytes exceeds the buffer length, an empty buffer will be produced such that bsize: 0 will match.

8.11.20.2. Examples

The following examples use an input buffer of This is Suricata.

The subslice will be a copy of the input buffer but omit the input buffer's first byte. The subslice is his is Suricata:

subslice: 1;

This example creates the subslice This is Suric:

subslice: 0, 13;

This example starts at offset 10 and ends at 5 bytes from the end of the buffer which creates a subslice from offset 10 to offset 12. The length of the input buffer is 17 bytes; 5 bytes from the end is 12. The subslice is "r"

subslice: 10, -5;

This example will create a subslice from the last 3 bytes of the input buffer and create ata:

subslice: -3;

8.11.20.3. Summary of Truncate Behavior: Edge Cases

The following table summarizes how truncate handles edge cases with the input buffer curl/7.64.1 (11 bytes):

Transform	No truncate	With truncate
`subslice: 5;`	7.64.1 (6 bytes)	7.64.1 (6 bytes)
`subslice: -20;`	Empty buffer	Full buffer (start at 0)
`subslice: -20, 5;`	Empty buffer	curl/ (5 bytes)
`subslice: 0, -30;`	Empty buffer	Empty buffer (end at 0)
`subslice: 0, -8;`	cur (3 bytes)	cur (3 bytes)
`subslice: -20, -30;`	Empty buffer	Empty buffer
`subslice: 0, 30;`	Empty buffer	Full buffer (11 bytes)

8.11.20.4. Truncation Behavior

When the buffer has fewer bytes than offset + nbytes, the transform will either trim the resulting buffer as though offset + nbytes == buffer_length or produce an empty buffer on which bsize:0 would match. The behavior is determined by the inclusion of truncate with the keyword.

The following examples use an input buffer with the value curl/7.64.1.

Without truncate (the default), when offset + nbytes exceeds the buffer length, the transform produces an empty buffer on which bsize:0 would match:

subslice: 0, 30;

When truncate is present, nbytes + offset is reduced to equal the input buffer length. The transform produces curl/7.64.1:

subslice: 0, 30, truncate;

truncate does not require nbytes to be present. The transform produces curl/7.64.1:

subslice: 0, truncate;

8.11.20.5. Negative Offset Handling

When a negative offset's absolute value exceeds the buffer length, the behavior depends on whether truncate is present:

Without truncate, the transform produces an empty buffer. For example, with input buffer This is Suricata (16 bytes), using subslice: -17; produces an empty string and bsize:0 would match:

subslice: -17;

With truncate present, an excessive negative offset is reset to the buffer length, effectively starting at offset 0. Using the same input buffer This is Suricata (16 bytes), subslice: -17, truncate; is treated as subslice: -16, truncate; and produces the full buffer This is Suricata:

subslice: -17, truncate;

This also works with nbytes. For example, subslice: -20, 5, truncate; with input buffer This is Suricata starts at offset 0 and takes 5 bytes, producing This (with a trailing space):

subslice: -20, 5, truncate;

Similarly, when truncate is present, negative nbytes values that would place the endpoint before the beginning of the buffer are reset to the beginning of the buffer. For example, subslice: 0, -30, truncate; with input buffer This is Suricata (16 bytes) resets the endpoint to the beginning of the buffer, producing an empty buffer:

subslice: 0, -30, truncate;

However, a moderate negative nbytes works normally. For example, subslice: 0, -8, truncate; ends 8 bytes from the end (position 8), producing This is (with a trailing space):

subslice: 0, -8, truncate;

8.11.20.6. Full Rule Examples

The following rule inspects the first 4 bytes of the HTTP User-Agent header to match traffic with a User-Agent beginning with curl:

alert http any any -> any any (msg:"curl User-Agent"; http.user_agent; subslice: 0, 4, truncate; content:"curl"; bsize:4; sid:1;)

The following rule inspects the last 4 bytes of the HTTP URI to match requests whose URI ends with .php:

alert http any any -> any any (msg:"PHP URI"; http.uri; subslice: -4; content:".php"; bsize:4; sid:2;)