yara-parser

yara-parser is a Go library for manipulating YARA rulesets. Its key feature is that it uses the same grammar and lexer files as the original libyara to ensure that lexing and parsing work exactly like YARA. The grammar and lexer files have been modified to fill Go data structures for ruleset manipulation instead of compiling rulesets for data matching.

Using yara-parser, one will be able to read YARA rulesets to programatically change metadata, rule names, rule modifiers, tags, strings, and more.

The ability to serialize rulesets to JSON for rule manipulation in other languages is provided with the y2j tool.

Installation

For the following go get commands, if you experience any issues, they are likely due to outdated versions of Go. The project uses features introduced in Go 1.10. Installation should proceed normally after an update.

y2j: YARA to JSON

Use the following command to install the y2j command for converting YARA rulesets to JSON.

go get -u github.com/Northern-Lights/yara-parser/cmd/y2j

Of course, this will install y2j to $GOPATH/bin, so ensure that the latter is in your $PATH.

The grammar and lexer files are frozen so that building them with goyacc and flexgo are not necessary.

Grammar Library

Use the following command to install the grammar library for deserializing YARA rulesets without installing y2j.

go get -u github.com/Northern-Lights/yara-parser/grammar

y2j Usage

Command line usage for y2j looks like the following:

$ y2j --help            
Usage of y2j: y2j [options] file.yar

options:
  -indent int
        Set number of indent spaces (default 2)
  -o string               
        JSON output file

In action, y2j would convert the following ruleset:

import "pe"
import "cuckoo"

include "other.yar"

global rule demo : tag1 {
meta:
    description = "This is a demo rule"
    version = 1
    production = false
    description = "because we can"
strings:
    $string = "this is a string" nocase wide
    $regex = /this is a regex/i ascii fullword
    $hex = { 01 23 45 67 89 ab cd ef [0-5] ?1 ?2 ?3 }
condition:
    $string or $regex or $hex
}

to this JSON output:

{
   "file": "sample.yar",
   "imports": [
      "pe",
      "cuckoo"
   ],
   "includes": [
      "other.yar"
   ],
   "rules": [
      {
         "modifiers": {
            "global": true,
            "private": false
         },
         "identifier": "demo",
         "tags": [
            "tag1"
         ],
         "meta": [
            {
               "Key": "description",
               "Val": "This is a demo rule"
            },
            {
               "Key": "version",
               "Val": 1
            },
            {
               "Key": "production",
               "Val": false
            },
            {
               "Key": "description",
               "Val": "because we can"
            }
         ],
         "strings": [
            {
               "id": "$string",
               "type": 0,
               "text": "this is a string",
               "modifiers": {
                  "nocase": true,
                  "ascii": false,
                  "wide": true,
                  "fullword": false,
                  "i": false,
                  "s": false
               }
            },
            {
               "id": "$regex",
               "type": 2,
               "text": "this is a regex",
               "modifiers": {
                  "nocase": false,
                  "ascii": true,
                  "wide": false,
                  "fullword": true,
                  "i": true,
                  "s": false
               }
            },
            {
               "id": "$hex",
               "type": 1,
               "text": " 01 23 45 67 89 ab cd ef [0-5] ?1 ?2 ?3 ",
               "modifiers": {
                  "nocase": false,
                  "ascii": false,
                  "wide": false,
                  "fullword": false,
                  "i": false,
                  "s": false
               }
            }
         ],
         "condition": "$string or $regex or $hex"
      }
   ]
}

Note that the string types are as follows:

String type int code	Designation
0	string
1	hex pair bytes
2	regex

Go Usage

Sample usage for working with rulesets in Go looks like the following:

package main

import (
	"fmt"
	"log"
	"os"

	"github.com/Northern-Lights/yara-parser/grammar"
)

func main() {
	input, err := os.Open(os.Args[1])   // Single argument: path to your file
	if err != nil {
		log.Fatalf("Error: %s\n", err)
	}

	ruleset, err := grammar.Parse(input, os.Stdout)
	if err != nil {
		log.Fatalf(`Parsing failed: "%s"`, err)
	}

    fmt.Printf("Ruleset:\n%v\n", ruleset)
    
    // Manipulate the first rule
    rule := ruleset.Rules[0]
    rule.Identifier = "new_rule_name"
    rule.Modifiers.Global = true
    rule.Modifiers.Private = false
}

Development

Currently, there is a Makefile which will build the parser, lexer, and main application. For this to work, the following are needed:

Command	Source (go get)
goyacc	golang.org/x/tools/cmd/goyacc
flexgo	github.com/pebbe/flexgo (Tool must be built manually)

Limitations

Currently, there are no guarantees with the library that modified rules will serialize back into a valid YARA ruleset. For example, you can set rule.Identifier = "123", but this would be invalid YARA. Additionally, adding or removing strings may cause a condition to become invalid, and conditions are currently treated only as text.