Node.js third-party modules: [sexstatic] HTML injection in directory name(s) leads to Stored XSS when malicious file is embed with <iframe> element used in directory name

ID H1:328210
Type hackerone
Reporter bl4de
Modified 2018-05-29T10:46:52


I would like to report HTML Injection vulnerability in sexstatic module. It is possible to use HTML in directory names, which might lead to run arbitrary JavaScript code in the browser.


module name: sexstatic version: 0.6.2 npm page:

Module Description

A simple static file server middleware. Use it with a raw http server, express/connect, or flatiron/union!

Also adds the ability to arbitrarily modify output HTML for whatever reason. And adding additional: simple url -> string handlers simple url -> function handlers

see opts.extras if you wanna get what i'm talking about

I may or may not extend this with further addons in the future to suit my needs.

Module Stats

Stats 4 downloads in the last day 29 downloads in the last week 158 downloads in the last month

Vulnerability Description

sexstatic can be used as static file server and presents directory index in the browser. Output is correctly sanitized against any attempts of HTML or JavaScript injection, due to sanitization done in sexstatic/lib/sexstatic/showdir.js:

```javascript // sexstatic/lib/sexstatic/showdir.js, line 132

var displayName = he.encode(file[0]) + ((isDir)? '/':'');

// TODO: use stylessheets?
html += '<tr>' +
    '<td><code>(' + permsToString(file[1]) + ')</code></td>' +
    '<td style="text-align: right; padding-left: 1em"><code>' + sizeToString(file[1], humanReadable, si) + '</code></td>' +
    '<td style="padding-left: 1em"><a href="' + href + '">' + displayName + '</a></td>' +

``` However, the same sanitization is missing from this part:

```javascript // sexstatic/lib/sexstatic/showdir.js, line 106

    var html = [
    '<!doctype html>',
    '  <head>',
    '    <meta charset="utf-8">',
    '    <title>Index of ' + pathname +'</title>',
    '  </head>',
    '  <body>',
    '<h1>Index of ' + pathname + '</h1>'
    ].join('\n') + '\n';


pathname variable is used without sanitization. This allows malicious user to use valid HTML code as directory name. Below I present complete attack scenario with this vulnerability.

Steps To Reproduce:

  • install sexstatic module:

$ npm install sexstatic

  • in the directory which will be used as root for sexstatic, create directory with following name: "><iframe src="malware_frame.html">/
  • in created directory, create file malware_frame.html with following content:

```html <!-- malware_frame.html --> <html>

<head> <meta charset="utf8" /> <title>Frame embeded with malware downloader :P</title> </head>

<body> <p>iframe element with malicious code</p> <script> alert('Uh oh, I am bad, bad malware!!!') </script> </body>

</html> ```

  • run sexstatic:

``` $ ./node_modules/sexstatic/lib/sexstatic.js -p 8080 sexstatic serving /home/rafal.janicki/playground/hackerone/Node at


  • go to http://localhost:8080 to see directory index:


  • now, click on "&gt;&lt;iframe src="malware_frame.html"&gt;/ directory name on the files list

  • malicious JavaScript code from malware_frame.html file is executed immediately:



pathname used in HTML output can be sanitized with the same he.encode() function, as it is already done for every filename.

Supporting Material/References:

  • Operating system: Ubuntu 16.04
  • Node.js 8.9.4
  • npm v. 5.6.0
  • Chromium 67.0.3367.0 (Developer Build) (64-bit)

Wrap up

I hope my report will help keeping Node.js Ecosystem users safe :)

  • I contacted the maintainer to let him know: [N]
  • I opened an issue in the related repository: [N]


Rafal 'bl4de' Janicki


Malicious user is able to inject iframe element with malicious JavaScript code via crafted directory name and trick users to open this directory in the browser.