browser page 2 - Dave's Blog


The decline of the mobile web...

2014 Apr 8, 6:49

The decline of the mobile web

PermalinkCommentstechnical web browser mobile phone

FitBit and WebOC Application Compatibility Errors

2013 Aug 29, 7:17
I just got a FitBit One from my wife. Unfortunately I had issues running their app on my Windows 8.1 Preview machine. But I recognized the errors as IE compatibility issues, for instance an IE dialog popup from the FitBit app telling me about an error in the app's JavaScript. Given my previous post on WebOC versioning you may guess what I tried next. I went into the registry and tried out different browser mode and document mode versions until I got the FitBit software running without error. Ultimately I found the following registry value to work well ('FitBit connect.exe' set to DWORD decimal 8888).
Windows Registry Editor Version 5.00

"Fitbit Connect.exe"=dword:000022b8

For those familiar with the Windows registry the above should be enough. For those not familiar, copy and paste the above into notepad, save as a file named "fitbit.reg", and then double click the reg file and say 'Yes' to the prompt. Hopefully in the final release of Windows 8.1 this won't be an issue.

Subtleties of postMessage

2013 Jul 15, 1:00

In IE10 and other new browsers one may create MessageChannel objects that have two MessagePorts each connected (w3c spec calls it entangled) to one another such that postMessage on one port results in the message event firing on the other. You can pass an array of ports as the last parameter to postMessage and they show up in the ports property of the message event arg.


The postMessage here is like the worker postMessage and unlike the window and iframe postMessage in that it applies no origin checking:

  1. No origin postMessage in workers and MessagePorts: postMessage(messageData, ports)
  2. Origin postMessage in windows and iframes: postMessage(messageData, targetOrigin, ports)

Unfortunately the origin isn't an optional parameter at the end to make the two postMessages have the same signature.

On the event handler side, the event arg always has an origin property. But in the no origin case it is always the empty string.


There is also a source property on the message event arg which if set is an object that has a postMessage property allowing you to post back to your caller. It is set for the origin case, however, in the no origin case this property is null. This is somewhat reasonable because in the case of MessagePort and Workers there are only two endpoints so you always know the source of a message implicitly. Unlike the origin case in which any iframe or window can be calling postMessage on any other iframe or window and the caller is unknown. So not unreasonable but it would be nice if the source property was always set for consistency.

MessageChannel start

When a MessageChannel is created it has two MessagePorts, but until those ports are started they will queue up any messages they receive. Once started they will dispatch all queued messages. Ports don't have to be started to send messages.

A port may be started in two ways, either by explicitly calling the start method on the port, or by setting the onmessage callback property on the port. However, adding an event listener via addEventListener("message", does not start the port. It works this way in IE and Chrome and the spec states this as well.

The justification is that since you can have only one callback via onmessage that once set you must implicitly be ready to receive messages and its fine to start the port. As opposed to the addEventListener in which case the user agent cannot start implicitly because it doesn't know how many event listeners will be added.  I found Hixie stating this justification in geoloc meeting notes.


W3C Spec

Opera introduction

PermalinkCommentsDOM html javascript postMessage technical web-worker worker

Stripe CTF - XSS, CSRF (Levels 4 & 6)

2012 Sep 10, 4:43

Level 4 and level 6 of the Stripe CTF had solutions around XSS.

Level 4


> Registered Users 

  • <% @registered_users.each do |user| %>
    <% last_active = user[:last_active].strftime('%H:%M:%S UTC') %>
    <% if @trusts_me.include?(user[:username]) %>

  • <%= user[:username] %>
    (password: <%= user[:password] %>, last active <%= last_active %>)
  • Issue

    The level 4 web application lets you transfer karma to another user and in doing so you are also forced to expose your password to that user. The main user page displays a list of users who have transfered karma to you along with their password. The password is not HTML encoded so we can inject HTML into that user's browser. For instance, we could create an account with the following HTML as the password which will result in XSS with that HTML:

    This HTML runs script that uses jQuery to post to the transfer URI resulting in a transfer of karma from the attacked user to the attacker user, and also the attacked user's password.


    Code review red flags in this case included lack of encoding when using user controlled content to create HTML content, storing passwords in plain text in the database, and displaying passwords generally. By design the web app shows users passwords which is a very bad idea.

    Level 6



    def self.safe_insert(table, key_values)
    key_values.each do |key, value|
    # Just in case people try to exfiltrate
    # level07-password-holder's password
    if value.kind_of?(String) &&
    (value.include?('"') || value.include?("'"))
    raise "Value has unsafe characters"



    This web app does a much better job than the level 4 app with HTML injection. They use encoding whenever creating HTML using user controlled data, however they don't use encoding when injecting JSON data into script (see post_data initialization above). This JSON data is the last five most recent messages sent on the app so we get to inject script directly. However, the system also ensures that no strings we write contains single or double quotes so we can't get out of the string in the JSON data directly. As it turns out, HTML lets you jump out of a script block using no matter where you are in script. For instance, in the middle of a value in some JSON data we can jump out of script. But we still want to run script, so we can jump right back in. So the frame so far for the message we're going to post is the following:

PermalinkCommentscsrf encoding html internet javascript percent-encoding script security stripe-ctf technical web xss

Stripe CTF - Input validation (Levels 1 & 2)

2012 Sep 6, 5:00

Stripe's web security CTF's Level 1 and level 2 of the Stripe CTF had issues with missing input validation solutions described below.

Level 1


          $filename = 'secret-combination.txt';
if (isset($attempt)) {
$combination = trim(file_get_contents($filename));
if ($attempt === $combination) {


The issue here is the usage of the extract php method which extracts name value pairs from the map input parameter and creates corresponding local variables. However this code uses $_GET which contains a map of name value pairs passed in the query of the URI. The expected behavior is to get an attempt variable out, but since no input validation is done I can provide a filename variable and overwrite the value of $filename. Providing an empty string gives an empty string $combination which I can match with an empty string $attempt. So without knowing the combination I can get past the combination check.


Code review red flag in this case was the direct use of $_GET with no validation. Instead of using extract the developer could try to extract specifically the attempt variable manually without using extract.

Level 2


    $dest_dir = "uploads/";
$dest = $dest_dir . basename($_FILES["dispic"]["name"]);
$src = $_FILES["dispic"]["tmp_name"];
if (move_uploaded_file($src, $dest)) {
$_SESSION["dispic_url"] = $dest;
chmod($dest, 0644);
echo "

Successfully uploaded your display picture.



This code accepts POST uploads of images but with no validation to ensure it is not an arbitrary file. And even though it uses chmod to ensure the file is not executable, things like PHP don't require a file to be executable in order to run them. Accordingly, one can upload a PHP script, then navigate to that script to run it. My PHP script dumped out the contents of the file we're interested in for this level:


Code review red flags include manual file management, chmod, and use of file and filename inputs without any kind of validation. If this code controlled the filename and ensured that the extension was one of a set of image extensions, this would solve this issue. Due to browser mime sniffing its additionally a good idea to serve a content-type that starts with "image/" for these uploads to ensure browsers treat these as images and not sniff for script or HTML.

PermalinkCommentsinput-validation php security technical

Web Security Contest - Stripe CTF

2012 Aug 27, 4:18

Stripe is running a web security capture the flag - a series of increasingly difficult web security exploit challenges. I've finished it and had a lot of fun. Working on a web browser I knew the theory of these various web based attacks, but this was my first chance to put theory into practice with:

  • No adverse consequences
  • Knowledge that there is a fun security exploit to find
  • Access to the server side source code

Here's a blog post on the CTF behind the scenes setup which has many impressive features including phantom users that can be XSS/CSRF'ed.

I'll have another post on my difficulties and answers for the CTF levels after the contest is over on Wed, but if you're looking for hints, try out the CTF chatroom or the level specific CTF chatroom.

PermalinkCommentscontest security technical

Everybody hates Firefox updates - Evil Brain Jono's Natural Log

2012 Jul 16, 1:59

Former FireFox developer on the switch to their continuous update cycle. 

Oh no, Chrome is doing such-and-such; we’d better do something equivalent or we’ll fall behind! We thought we needed a rapid update process like Chrome. We were jealous of their rapid update capability, which let them deploy improvements to users continuously. We had to “catch up” with Chrome’s updating capability.

Dealing with servicing on IE for years had led me to some of the same thoughts when I heard FireFox was switching to continuous updates.

PermalinkCommentsfirefox via:ericlaw web-browser technical web browser servicing update software

HTTP Compression Documentation Reference

2012 Jun 13, 3:08
There's a lot of name reuse in HTTP compression so I've made the following to help myself keep it straight.
HTTP Content Coding Token gzip deflate compress
An encoding format produced by the file compression program "gzip" (GNU zip) The "zlib" format as described in RFC 1950. The encoding format produced by the common UNIX file compression program "compress".
Data Format GZIP file format ZLIB Compressed Data Format The compress program's file format
Compression Method Deflate compression method LZW
Deflate consists of LZ77 and Huffman coding

Compress doesn't seem to be supported by popular current browsers, possibly due to its past with patents.

Deflate isn't done correctly all the time. Some servers would send the deflate data format instead of the zlib data format and at least some versions of Internet Explorer expect deflate data format instead of zlib data format.

PermalinkCommentscompress compression deflate gzip http http-header technical zlib

A Tale Of Two Pwnies (Part 2)

2012 Jun 11, 6:39

Summary of one of the Chrome security exploits from pwn2own.  Basically XSS into the chrome URI scheme which gives access to special APIs.

PermalinkCommentstechnical browser web-browser security xss

Try Chrome in Metro mode

2012 Jun 7, 3:34

Early Chrome for Win8 Metro!

PermalinkCommentschrome google web-browser web metro win8 windows technical

Working with files in JavaScript, Part 5: Blobs

2012 Jun 5, 4:13

How to create Blobs directly and via BlobBuilder.

PermalinkCommentsblob javascript programming technical web web-browser

Code: Flickr Developer Blog » Parsing Exif client-side using JavaScript

2012 Jun 1, 2:51

Flickr parses the exif out of images using Web Workers, and Blob (File API)!

PermalinkCommentsjavascript blob exif image technical programming dom webworker web-browser

Working with files in JavaScript, Part 4: Object URLs

2012 Jun 1, 2:50

On the topic of blobs and createObjectURL.  Woo blobs!

PermalinkCommentsjavascript blob dom web-browser technical programming

The Lazy Man's URL Parsing (

2012 May 7, 12:41

Web apps really make obvious the lack of URI APIs in the DOM or JavaScript.  This blog post goes over using DOM API side effects to resolve relative URIs and parse URIs.  An additonal benefit of this mechanism is that you avoid security issues caused by mismatched behavior between the browser’s URI parsing and your app’s URI parsing.

PermalinkCommentstechnical uri api dom browser hack url web-browser

Google Chrome - Fuzzing for Security

2012 Apr 26, 3:09

Overview of Google’s fuzzing security practices for Chrome.

PermalinkCommentstechnical security fuzz fuzzing chrome web-browser browser google

Understanding Enhanced Protected Mode

2012 Mar 23, 1:18

Eric Lawrence describes the new security features of IE10.

PermalinkCommentstechnical ie10 security ie web-browser eric-lawrence

HTML5 Table Flipper Experiment

2012 Mar 2, 1:02

The goal of this experiment was to combine the flipping tables emoticons with the Threw It On The Ground video using shiny new HTML5-ish features and the end result is the table flipper flipping the Threw It On the Ground video.

The table flipper emoticon is CSS before content that changes on hover. Additionally on hover a CSS transform is applied to flip the video upside down several times and move it to the right and there's a CSS transition to animate the flipping. The only issue I ran into is that (at least on Windows) Flash doesn't like to have CSS transform rotations applied to it. So to get the most out of the flip experiment you must opt-in to HTML5 video on YouTube. And of course you must use a browser that supports the various things I just mentioned, like the latest Chrome (or not yet released IE10).

PermalinkCommentscss-transform flipping-tables css-transition html5-video technical threw-it-on-the-ground

Client Side Cross Domain Data YQL Hack

2012 Feb 27, 2:28

One of the more limiting issues of writing client side script in the browser is the same origin limitations of XMLHttpRequest. The latest version of all browsers support a subset of CORS to allow servers to opt-in particular resources for cross-domain access. Since IE8 there's XDomainRequest and in all other browsers (including IE10) there's XHR L2's cross-origin request features. But the vast majority of resources out on the web do not opt-in using CORS headers and so client side only web apps like a podcast player or a feed reader aren't doable.

One hack-y way around this I've found is to use YQL as a CORS proxy. YQL applies the CORS header to all its responses and among its features it allows a caller to request an arbitrary XML, HTML, or JSON resource. So my network helper script first attempts to access a URI directly using XDomainRequest if that exists and XMLHttpRequest otherwise. If that fails it then tries to use XDR or XHR to access the URI via YQL. I wrap my URIs in the following manner, where type is either "html", "xml", or "json":

        yqlRequest = function(uri, method, type, onComplete, onError) {
var yqlUri = "" +
encodeURIComponent("SELECT * FROM " + type + ' where url="' + encodeURIComponent(uri) + '"');

if (type == "html") {
yqlUri += encodeURIComponent(" and xpath='/*'");
else if (type == "json") {
yqlUri += "&callback=&format=json";

This also means I can get JSON data itself without having to go through JSONP.
PermalinkCommentsxhr javascript yql client-side technical yahoo xdr cors

Efficient Script Yielding

2012 Feb 3, 12:14

The setImmediate DOM method is like window.setTimeout(callback, 0) but better.

PermalinkCommentstechnical dom setimmediate perf web web-browser

"If there’s a way for a site to take dependency on a browser quirk, and break if that quirk is..."

2012 Feb 1, 5:10
“If there’s a way for a site to take dependency on a browser quirk, and break if that quirk is removed, it will happen.”

- -Eric Lawrence, Web Browser Legend
PermalinkCommentstechnical eric-lawrence the-eric-lawrence browser web-browser compat
Older EntriesNewer Entries Creative Commons License Some rights reserved.