(Self) XSS at Mozilla's internal Phonebook

This is a short summary about a goofy XSS/CSRF exploit on an internal web page at Mozilla.

A few weeks ago I discovered that our "phonebook" supports a limited wiki-syntax in the profile descriptions (i.e. [link text http://example.com]). Despite proper sanitizing to forbid all markup injections into HTML tags and attributes it allowed linking to javascript URLs. Liking the rich capabilities that come with JavaScript, I naturally had to insert some script into my profile. So I started with a small script that changes the background color randomly. Like this:

Click me, I dare you



What it does? Well, let's start simple. setInterval(…,500) runs the code supplied in the first argument every 500ms. The code itself is an assignment to the document's background color. So we change it quite often. The rest is a quick way to generate a random six digits hex string, i.e. an RGB color value: Math.random() gives a float with a lot of random digits after the floating point. Multiplying by 10e16 (that is a 10 followed by 16 zeros) makes it a long integer without decimals. toString(16) transforms it into a hex string and substr cuts after six digits. Et violà! void makes sure the code doesn't have a return value, which means that the browser stays on the current web page. I have manually shortened the code to work without spaces, so it does not break the wiki-syntax.

Well that was the first revision. I left it for a while and I guess people clicked on it. I don't know how many, but surely somebody found it. That was part one. I came back to this a few weeks later and had to realize that changing the background color is quite boring. What else can we do? Right. We can try to change other people's profiles.

This is where the fun starts:1


Again, this link came with the same "I dare you" text. Let's dissect the source code. First we create an invisible iframe that loads the "Edit Profile" page.


Now let's fill out the form and submit. The frame takes some time to load, so we will prepare a piece of source code in a string and defer execution with setTimeout: The code finds the form in the iframe, DOM navigation is quite easy since the input fields have proper name attributes, which exposes them to JavaScript as attributes of the form element. The source code uses atob, \x5D and \x20 so we don't break wiki-syntax. The escape sequence \uD83D\uDC35 is the unicode monkey face 🐵.

payload = 'f=(i.contentDocument.forms[0\x5D);f[atob("bmFtZVtd")\x5D.value+="\x20\uD83D\uDC35";f.submit()'

Once the payload is prepared we just execute it after a certain delay:


So to recap: If you click this link you will add a monkey face to your name in the background, but it's quite subtle.

So let's search for the monkey face in the phonebook: Search results

At least 12 people have a monkey in their profile 🐵2

This isn't strictly a self-xss as discussed in these three very interesting blog posts (1, 2, 3) but I'd argue that the average Mozillian should be careful enough not to click links that start with javascript: ;)

  1. I have modified a few bits so the exploit doesn't properly apply to the phonebook. Script kiddie protection. 

  2. I swear it was only 11 when I started writing this blog post. Also, modulo a false positive: One person had a monkey in their description before I started this. 

Other posts

  1. Help Test Firefox's built-in HTML Sanitizer to protect against UXSS bugs
  2. Remote Code Execution in Firefox beyond memory corruptions
  3. XSS in The Digital #ClimateStrike Widget
  4. Chrome switching the XSSAuditor to filter mode re-enables old attack
  5. Challenge Write-up: Subresource Integrity in Service Workers
  6. Finding the SqueezeBox Radio Default SSH Passwort
  7. New CSP directive to make Subresource Integrity mandatory (`require-sri-for`)
  8. Firefox OS apps and beyond
  9. Teacher's Pinboard Write-up
  10. A CDN that can not XSS you: Using Subresource Integrity
  11. The Twitter Gazebo
  12. German Firefox 1.0 ad (OCR)
  13. My thoughts on Tor appliances
  14. Subresource Integrity
  15. Revoke App Permissions on Firefox OS
  16. (Self) XSS at Mozilla's internal Phonebook
  17. Tales of Python's Encoding
  18. On the X-Frame-Options Security Header
  19. html2dom
  20. Security Review: HTML sanitizer in Thunderbird
  21. Week 29 2013
  22. The First Post