The JavaScript protocol was added to Nuclei v3 to allow you to write checks and detections for exploits in JavaScript and to bridge the gap between network protocols.
  • Internally any content written using the JavaScript protocol is executed in Golang.
  • The JavaScript protocol is not intended to fit into or be imported with any existing JavaScript libraries or frameworks outside of the Nuclei ecosystem.
  • Nuclei provides a set of functions, libraries that are tailor-made for writing exploits and checks and only adds required/necessary functionality to complement existing YAML-based DSL.
  • The JavaScript protocol is not intended to be used as a general purpose JavaScript runtime and does not replace matchers, extractors, or any existing functionality of Nuclei.
  • Nuclei v3.0.0 ships with 15+ libraries (ssh, ftp, RDP, Kerberos, and Redis) tailored for writing exploits and checks in JavaScript and will be continuously expanded in the future.

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Simple Example

Here is a basic example of a JavaScript protocol template: 
id: ssh-server-fingerprint

info:
  name: Fingerprint SSH Server Software
  author: Ice3man543,tarunKoyalwar
  severity: info
  

javascript:
  - code: |
      var m = require("nuclei/ssh");
      var c = m.SSHClient();
      var response = c.ConnectSSHInfoMode(Host, Port);
      to_json(response);
    args:
      Host: "{{Host}}"
      Port: "22"

    extractors:
      - type: json
        json:
          - '.ServerID.Raw'
In the Nuclei template example above, we are fingerprinting SSH server software by connecting in non-auth mode and extracting the server banner. Let’s break down the template.

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Code Section

The code: contains actual JavaScript code that is executed by Nuclei at runtime. In the above template, we are:
  • Importing nuclei/ssh module/library
  • Creating a new instance of SSHClient object
  • Connecting to SSH server in Info mode
  • Converting response to json

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Args Section

The args: section can be simply understood as variables in JavaScript that are passed at runtime and support DSL usage.

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Output Section

The value of the last expression is returned as the output of JavaScript protocol template and can be used in matchers and extractors. If the server returns an error instead, then the error variable is exposed in the matcher or extractor with an error message.

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SSH Bruteforce Example

SSH Password Bruteforce Template 
id: ssh-brute

info:
  name: SSH Credential Stuffing
  author: tarunKoyalwar
  severity: critical
  

javascript:
  - pre-condition: |
      var m = require("nuclei/ssh");
      var c = m.SSHClient();
      var response = c.ConnectSSHInfoMode(Host, Port);
      // only bruteforce if ssh server allows password based authentication
      response["UserAuth"].includes("password")

    code: |
      var m = require("nuclei/ssh");
      var c = m.SSHClient();
      c.Connect(Host,Port,Username,Password);

    args:
      Host: "{{Host}}"
      Port: "22"
      Username: "{{usernames}}"
      Password: "{{passwords}}"

    threads: 10
    attack: clusterbomb
    payloads:
      usernames: helpers/wordlists/wp-users.txt
      passwords: helpers/wordlists/wp-passwords.txt

    stop-at-first-match: true
    matchers:
      - type: dsl
        dsl:
          - "response == true"
          - "success == true"
        condition: and
In the example template above, we are bruteforcing ssh server with a list of usernames and passwords. We can tell that this might not have been possible to achieve with the network template. Let’s break down the template.

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Pre-Condition

pre-condition is an optional section of JavaScript code that is executed before running “code” and acts as a pre-condition to exploit. In the above template, before attempting brute force, we check if:
  • The address is actually an SSH server.
  • The ssh server is configured to allow password-based authentication.
Further explanation
  • If pre-condition returns true only then code is executed; otherwise, it is skipped.
  • In the code section, we import nuclei/ssh module and create a new instance of SSHClient object.
  • Then we attempt to connect to the ssh server with a username and password. This template uses payloads to launch a clusterbomb attack with 10 threads and exits on the first match.
Looking at this template now, we can tell that JavaScript templates are powerful for writing multistep and protocol/vendor-specific exploits, which is a primary goal of the JavaScript protocol.

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Init

init is an optional JavaScript section that can be used to initialize the template, and it is executed just after compiling the template and before running it on any target. Although it is rarely needed, it can be used to load and preprocess data before running a template on any target. For example, in the below code block, we are loading all ssh private keys from nuclei-templates/helpers directory and storing them as a variable in payloads with the name keys. If we were loading private keys from the “pre-condition” code block, then it would have been loaded for every target, which is not ideal. 
variables:
  keysDir: "helpers/"  # load all private keys from this directory

javascript:
    # init field can be used to make any preperations before the actual exploit
    # here we are reading all private keys from helpers folder and storing them in a list
  - init: |
      let m = require('nuclei/fs');
      let privatekeys = m.ReadFilesFromDir(keysDir)
      updatePayload('keys',privatekeys)

    payloads:
      # 'keys' will be updated by actual private keys after init is executed
      keys: 
        - key1
        - key2
Two special functions that are available in the init block are
FunctionDescription
updatePayload(key,value)updates payload with given key and value
set(key,value)sets a variable with given key and value
A collection of JavaScript protocol templates can be found here.