Tag Archives: DoS

Overriding Firefox’s Window.Alert – Part 3

Wow.  I think I got it.  I’ve got a Firefox plugin that can suppress all alert() dialogs on a page if the user checks a “suppress” box on the second alert() dialog.

The trick, was not to rely on the DOMContentLoaded event to fire to do the override.  Instead, I used the DOMWillOpenModalDialog to detect the first alert().  After detection, I overrode with an alertCheck which asked the user whether or not to “suppress more dialogs”.  If the user answers in the affirmative, alert() is simply overwritten with an empty function.

Piece of cake.

A couple of issues though…

Security

In order to override the alert() function, I have to write to document.getElementById(‘content’).contentWindow.wrappedJSObject.alert.

Remember how I mentioned the distance between the Extension JavaScript, and the inline content JavaScript?  I said it felt like a security layer.

I was totally right.

Check this out. I’ll quote:

You should be aware of XPCNativeWrappers when working with untrusted content. With XPCNativeWrappers turned on (which is the default in Firefox 1.5+), your extension can safely access the DOM of the content document, but not the content JavaScript. Bypassing XPCNativeWrapper to work with content JavaScript directly can lead to security problems.

Hrmph.  So I seem to be violating some security rules here.  So maybe my approach isn’t the greatest idea.  “Mook” from irc.mozilla.net #extdev suggested looking into commonDialog.xul…but I can’t seem to wrap my head around that just yet.

Imperfections

Not sure why yet, but while I can suppress dialog floods like this:

for (i = 0; i < 10; ++i) {
  alert(i);
}

It seems to fail on this:

for (i = 0; i < 10; ++i) {
  alert(i);
  confirm(i);
}

For some reason, regardless of whether or not I choose to suppress the dialogs, they just keep coming.  It works fine when I swap out the confirm() for a second alert().  Not exactly sure why.  Yet.

Ok, so I’m going to clean the code up, and post it soon.  I’ll also post a link to a real, brutally annoying website where you can test the alertCheck extension.  Just give me a bit.

Overriding Firefox’s Window.Alert – Part 2

Ok, so there have been some developments.

Before I go into this though, I just want to make it clear that I have very little knowledge or experience working with XUL, or writing Firefox extensions in general.  I’ve dabbled, but I’m mostly ignorant.  So all you Firefox aficionados out there…go easy on me.

So, developments:

Using this site, I created a skeleton for my extension, calling it “alertCheck”.

And then I hit a brick wall right away, trying to find the right “alert” to override.

Let me explain.  Mozilla extension development is strange for me because of all of the various layers to Firefox, and the distance between the JavaScript in the extension, and any page that is loaded in the browser.  It feels like a security layer – and it makes sense:  you really don’t want the Javascript on a website to monkey around with the internals of your browser.

Thankfully, this forum and this forum provided some help.

Now, after the appcontent is loaded, I wait for the DOMContentLoaded event to fire, and then write my new alert function to here:

document.getElementById('content').contentWindow.wrappedJSObject.alert

The problem is, if I wait for the DOMContentLoaded event to fire, the alert override happens (obviously) after all of the DOM Content is loaded into the browser.  This is only useful if the alert’s that we want to capture are fired using <body onLoad> or some other function that detects when the DOM has been loaded.  It’s no good for inline Javascript alerts, since these commands are parsed and executed line by line as the HTML is processed.

Phew.  Big mouthful.

So, where does that leave me?  Well, I was just hanging out in irc.mozilla.net in the #extdev channel.  Here’s a chunk of what was said:

<mike_conley> Hey all - I'm trying to write an extension to override window.alert, with an alertCheck that allows users to disable future alerts.
<mike_conley> So far, I'm able to override alert by writing to document.getElementById('content').contentWindow.wrappedJSObject.alert
<mike_conley> however, now it's a matter of timing - I'm doing the override after the DOMContentLoaded event is fired
<mike_conley> But this means that it doesn't catch alerts that are fired using inline javascript.
<mike_conley> So my question is, when should I do the override?
<Mook> I suspect just overriding commonDialog.xul is easier
<Mook> trying to poke the content JS's definition is going to be full of pain
<mike_conley> Cool, I'll look into that - thanks!
<Mook> (the content JS starts around http://mxr.mozilla.org/mozilla/source/dom/src/base/nsGlobalWindow.cpp#4022 and goes the the prompt service which pokes that xul)
<mike_conley> Excellent - I appreciate it.

So maybe there’s another approach that I should be taking – I’m going to look into overriding commonDialog.xul… I’ll write more when I have it.

Overriding Firefox’s Window.Alert – Part 1

Window.alert is a native function built into Firefox – but that doesn’t mean it can’t be overridden.

Check this out:

Open Firebug, and get to the console.  Then, click that little red arrow at the end of the input line so that you get the large box input on the right side of the screen.

Type this into the input box:

var alert_count = 0;
var old_alert = window.alert;
var alert_max = 5;
window.alert = function(alert_text) {
  if (alert_count < alert_max) {
    ++alert_count;
    old_alert(alert_text);
  } else {
    console.log("Reached maximum alerts");
  }
}

Now, hit “Run” at the bottom of that input window.  We’ve just overridden the window.alert function during runtime.

Hit “Clear” at the bottom of the input window, and type in:

for (i = 0; i < 10; ++i) {
  alert(i);
}

Hit “Run”.  Click “OK” for the first 5 alert windows, and watch as the rest of them are spewed out to the console.  Nice.

So, I don’t think this helps me much in creating my plug-in, but it’s interesting to see how window.alert is malliable at run-time.

This seems to be a more relevant discovery – Mozilla’s Chrome lets me create an alert popup with a checkbox using alertCheck.  I think this is exactly what I’m looking for.

I’ll tinker with it over the next few days, and post some code.

Summer Project: Firefox Plugin to Override Window.Alert

When I don’t have work to do, I get antsy.

And right now, I’ve got no work to do.

So I’ve come up with a project for myself:  remember how I created a Firefox Plugin a few months back?  I’d like to make another one – but this one will actually serve a useful purpose.

Have you ever been to a page that suddenly started spewing window.alert boxes at you?

If you haven’t, open up Firebug, and paste this into the console:

for(i = 0; i < 10; ++i){ alert(i); }

Now imagine if instead of 10 alert boxes, it spewed hundreds…or thousands….or god forbid, it uses a while(true) loop, and throws infinity alert boxes at you.

It totally cripples Firefox. It’s a super simple browser DoS attack.

Mozilla knows this, but so far, no solution except for killing the Firefox process, or disabling Javascript manually, or with NoScript (a plugin that I highly recommend).

Google Chrome has solved this problem by providing a checkbox on alert dialogs that allow a user to disable future popups from the current site.

Cool.  I want Firefox to have the same feature.

So, this summer, I’m going to try to build a Firefox Plugin that will override the standard window.alert function, with one that provides a checkbox, letting the user disable future alerts.

I don’t even know if this is possible, but I’m looking into it.

I’ll blog my research and progress as I go along, and share my code / final plugin when it’s all finished (or when I abandon it…hey, it happens).

So stay tuned.