Box-js – Tool For Studying JavaScript Malware

Box-js - Tool For Studying JavaScript Malware xploitlab

Javascript code is not just programming language because it can be a code to hijack your browser. We know that almost any website use javascript to build the stuctures of the website. But in hacker hand javascript can be used for harmful things like stealing data, silent mining cryptocurency, spy your computer and more.


A utility to analyze malicious JavaScript. Tool to help you study and learn about Javascript malware, because javascript is very dangerous programming languange if someone use that for harm other’s. Hacker can build a Botnet or Malware with just javascript.


Simply install box-js from npm:

npm install box-js –global


Looking to use box-js with Cuckoo? Use as an analysis package.

Let’s say you have a sample called sample.js: to analyze it, simply run

box-js sample.js

Chances are you will also want to download any payloads; use the flag --download to enable downloading. Otherwise, the engine will simulate a 404 error, so that the script will be tricked into thinking the distribution site is down and contacting any fallback sites.

Box.js will emulate a Windows JScript environment, print a summary of the emulation to the console, and create a folder called sample.js.results (if it already exists, it will create sample.js.1.results and so on). This folder will contain:

  • analysis.log, a log of the analysis as it was printed on screen;
  • a series of files identified by UUIDs;
  • snippets.json, a list of pieces of code executed by the sample (JavaScript, shell commands, etc.);
  • urls.json, a list of URLs contacted;
  • active_urls.json, a list of URLs that seem to drop active malware;
  • resources.json, the ADODB streams (i.e. the files that the script wrote to disk) with file types and hashes;
  • IOC.json, a list of behaviours identified as IOCs (Indicators of Compromise). These include registry accesses, written files, HTTP requests and so on.

You can analyze these by yourself, or you can automatically submit them to Malwr, VirusTotal or a Cuckoo sandbox: for more information, run box-export --help.

For further isolation, it is recommended to run the analysis in a temporary Docker container. Consult integrations/ for more information.

If you wish to automate the analysis, you can use the return codes – documented in integrations/ – to distinguish between different types of errors.

Batch usage

While box.js is typically used on single files, it can also run batch analyses. You can simply pass a list of files or folders to analyse:

box-js sample1.js sample2.js /var/data/mySamples …

By default box.js will process samples in parallel, running one analysis per core. You can use a different setting by specifying a value for --threads: in particular, 0 will remove the limit, making box-js spawn as many analysis threads as possible and resulting in very fast analysis but possibly overloading the system (note that analyses are usually CPU-bound, not RAM-bound).

You can use --loglevel=warn to silence analysis-related messages and only display progress info.

After the analysis is finished, you can extract the active URLs like this:

cat ./*.results/active_urls.json | sort | uniq


NAME                   DESCRIPTION                                                                     
 -h, --help             Show the help text and quit                                                     
 -v, --version          Show the package version and quit                                               
 --license              Show the license and quit                                                       
 --debug                Die when an emulation error occurs, even in "batch mode", and pass on the exit  
 --loglevel             Logging level (debug, verbose, info, warning, error - default "info")           
 --threads              When running in batch mode, how many analyses to run at the same time (0 =      
                        unlimited, default: as many as the number of CPU cores)                         
 --download             Actually download the payloads                                                  
 --encoding             Encoding of the input sample (will be automatically detected by default)        
 --timeout              The script will timeout after this many seconds (default 10)                    
 --output-dir           The location on disk to write the results files and folders to (defaults to the 
                        current directory)                                                              
 --preprocess           Preprocess the original source code (makes reverse engineering easier, but takes
                        a few seconds)                                                                  
 --unsafe-preprocess    More aggressive preprocessing. Often results in better code, but can break on   
                        some edge cases (eg. redefining prototypes)                                     
 --no-kill              Do not kill the application when runtime errors occur                           
 --no-echo              When the script prints data, do not print it to the console                     
 --no-rewrite           Do not rewrite the source code at all, other than for `@cc_on` support          
 --no-catch-rewrite     Do not rewrite try..catch clauses to make the exception global-scoped           
 --no-cc_on-rewrite     Do not rewrite `/*@cc_on <...>@*/` to `<...>`                                   
 --no-eval-rewrite      Do not rewrite `eval` so that its argument is rewritten                         
 --no-file-exists       Return `false` for Scripting.FileSystemObject.FileExists(x)                     
 --no-folder-exists     Return `false` for Scripting.FileSystemObject.FileExists(x)                     
 --function-rewrite     Rewrite function calls in order to catch eval calls                             
 --no-rewrite-prototype Do not rewrite expressions like `function A.prototype.B()` as `A.prototype.B =  
 --no-hoist-prototype   Do not hoist expressions like `function A.prototype.B()` (implied by            
 --no-shell-error       Do not throw a fake error when executing `WScriptShell.Run` (it throws a fake   
                        error by default to pretend that the distribution sites are down, so that the   
                        script will attempt to poll every site)                                         
 --no-typeof-rewrite    Do not rewrite `typeof` (e.g. `typeof ActiveXObject`, which must return         
                        'unknown' in the JScript standard and not 'object')                             
 --proxy                [experimental] Use the specified proxy for downloads. This is not relevant if   
                        the --download flag is not present.                                             
 --windows-xp           Emulate Windows XP (influences the value of environment variables)              
 --dangerous-vm         Use the `vm` module, rather than `vm2`. This sandbox can be broken, so **don't  
                        use this** unless you're 100% sure of what you're doing. Helps with debugging by
                        giving correct stack traces. 

Analyzing the output

Console output

The first source of information is the console output. On a succesful analysis, it will typically print something like this:

Using a 10 seconds timeout, pass --timeout to specify another timeout in seconds
Analyzing sample.js
Header set for User-Agent Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0)
Emulating a GET request to
Downloaded 301054 bytes.
Saved sample.js.results/a0af1253-597c-4eed-9e8f-5b633ff5f66a (301054 bytes)
sample.js.results/a0af1253-597c-4eed-9e8f-5b633ff5f66a has been detected as data.
Saved sample.js.results/f8df7228-7e0a-4241-9dae-c4e1664dc5d8 (303128 bytes)
sample.js.results/f8df7228-7e0a-4241-9dae-c4e1664dc5d8 has been detected as PE32 executable (GUI) Intel 80386, for MS Windows. is an active URL.
Executing sample.js.results/d241e130-346f-4c0c-a698-f925dbd68f0c in the WScript shell
Header set for User-Agent Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0)
Emulating a GET request to

In this case, we are seeing a dropper that downloads a file from, setting the HTTP header User-Agent to Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0). Then, it proceeds to decode it, and write the result to disk (a PE32 executable). Finally, it runs some command in the Windows shell.

  • sample.js.results/a0af1253-597c-4eed-9e8f-5b633ff5f66a will contain the payload as it was downloaded from;
  • sample.js.results/f8df7228-7e0a-4241-9dae-c4e1664dc5d8 will contain the actual payload (PE executable);
  • sample.js.results/d241e130-346f-4c0c-a698-f925dbd68f0c will contain the command that was run in the Windows shell.

JSON logs

Every HTTP request is both printed on the terminal and logged in urls.json. Duplicate URLs aren’t inserted (i.e. requesting the same URL twice will result in only one line in urls.json).

active_urls.json contains the list of URLs that eventually resulted in an executable payload. This file is the most interesting, if you’re looking to take down distribution sites.

snippets.json contains every piece of code that box-js came across, either JavaScript, a cmd.exe command or a PowerShell script.

resources.json contains every file written to disk by the sample. For instance, if the application tried to save Hello world! to $PATH/foo.txt, the content of resources.json would be:

    "9a24...": {
         "path": "(path)\\foo.txt",
         "type": "ASCII text, with no line terminators",
         "md5": "86fb269d190d2c85f6e0468ceca42a20",
         "sha1": "d3486ae9136e7856bc42212385ea797094475802",
         "sha256": "c0535e4be2b79ffd93291305436bf889314e4a3faec05ecffcbb7df31ad9e51a"

The resources.json file is also important: watch out for any executable resource (eg. with "type": "PE32 executable (GUI) Intel 80386, for MS Windows").


Some scripts in the wild have been observed to use new Date().getYear() where new Date().getFullYear(). If a sample isn’t showing any suspicious behaviour, watch out for Date checks.

If you run into .JSE files, compile the decoder and run it like this:

cc decoder.c -o decoder
./decoder foo.jse bar.js
node run bar.js 


You may occasionally run into unsupported components. In this case, you can file an issue on GitHub, or emulate the component yourself if you know JavaScript.

The error will typically look like this (line numbers may be different):

1 Jan 00:00:00 - Unknown ActiveXObject WinHttp.WinHttpRequest.5.1
    at kill (/home/CapacitorSet/box-js/run.js:24:10)
    at Proxy.ActiveXObject (/home/CapacitorSet/box-js/run.js:75:4)
    at evalmachine.<anonymous>:1:6471
    at ContextifyScript.Script.runInNewContext (vm.js:18:15)
    at ... 

You can see that the exception was raised in Proxy.ActiveXObject, which looks like this:

function ActiveXObject(name) {
         name = name.toLowerCase();
         /* ... */
         switch (name) {
                 case "":
                          return require("./emulator/WScriptShell");
                 /* ... */
                          kill(`Unknown ActiveXObject ${name}`);

Add a new case "winhttp.winhttprequest.5.1" (note the lowercase!), and have it return an ES6 Proxy object (eg. ProxiedWinHttpRequest). This is used to catch unimplemented features as soon as they’re requested by the malicious sample:

/* emulator/WinHttpRequest.exe */
const lib = require("../lib");
module.exports = function ProxiedWinHttpRequest() {
         return new Proxy(new WinHttpRequest(), {
                 get: function(target, name, receiver) {
                          switch (name) {
                                   /* Add here "special" traps with case statements */
                                            if (name in target) return target[name];
                                            else lib.kill(`WinHttpRequest.${name} not implemented!`)
function WinHttpRequest() {

Rerun the analysis: it will fail again, telling you what exactly was not implemented.

1 Jan 00:00:00 - not implemented!
    at kill (/home/CapacitorSet/box-js/run.js:24:10)
    at Object.ProxiedWinHttpRequest.Proxy.get (/home/CapacitorSet/box-js/run.js:89:7) 

Emulate as needed:

function WinHttpRequest() { = function(method, url) {
                 URLLogger(method, url);
                 this.url = url;

and iterate until the code emulates without errors.


@CapacitorSet: Main developer


  • npm packaging
  • command-line help
  • --output-directory
  • bugfixes


  • support for non-UTF8 encodings
  • bug reporting


  • advice on integrating UglifyJS in box-js
  • improving the features of UglifyJS used in deobfuscation


  • bugfixes


  • bugfixes

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