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Black Box Meteor - Triple Brace XSS

Written by Pete Corey on Apr 3, 2015.

Meteor is an incredibly interesting framework from a security perspective. Due to the unique way in which it deals with it’s client/server separation, most of the information and functionality passed to the client is presented in a very uniform, organized way. From a black box security testing perspective, this can make life much easier!

We’ll start our black box adventure by searching for potential Cross Site Scripting (XSS) and/or Stored XSS attack vectors. One potential attack vector is the use of triple brace tags. Allowing un-sanitized user input into these raw tags can potentially let malicious users execute javascript on other clients’ machines. This is bad!

Finding Triple Braces

Thanks to the Spacebars compiler, we have all of the information we need to find templates that make use of triple braces.

Let’s take a tour of the template information given to each client. All of the templates used in the application live in the Template object. We can get a list of template names (along with some other Template specific object keys, such as instance, etc…) by grabbing the keys of the object:


If you’re interested in a particular template, you can drill into its renderFunction and access it directly, or turn it into a string and peruse the source:


You’ll notice that the renderFunction holds the result of the Spacebars compilation process. The DOM is represented as HTMLjs objects, interspersed with Spacebars objects. In our case, the triple braces we’re looking for are transformed into calls to Spacebars.makeRaw.

Using this knowledge, we can easily find all of the templates that use triple braces throughout the application:

    return Template[key] &&
           Template[key].renderFunction &&
           Template[key].renderFunction.toString().indexOf('Spacebars.makeRaw') > -1;

We could take this a step further by doing some monkey patching. Let’s replace the Spacebars.makeRaw method with a method that adds a data attribute to the first element passed into it:

Spacebars._makeRaw = Spacebars.makeRaw;
Spacebars.makeRaw = function (value) {
    function injectDataAttr() {
        var idx = value.indexOf('>');
        if (idx == -1) {
            return value;
        return value.substr(0, idx) +
               ' data-make-raw' +
    if (value == null)
        return null;
    else if (value instanceof HTML.Raw) {
        return HTML.Raw(injectDataAttr(value.value));
    else {
        return HTML.Raw(injectDataAttr(value));

You can then add a CSS rule to outline each raw element:

[data-make-raw] {
    outline: 1px solid tomato;

Or do it from the console:

var style = document.createElement("style");
style.sheet.insertRule('[data-make-raw] { outline: 1px solid tomato; }',0);

Here’s an example running against Telescope:

Telescope example

In this case I would make absolutely sure that the post title and each of the comments are being properly sanitized before being stored in the database and rendered on the client. If we miss either of these, we’re looking at a Stored XSS vulnerability!

What Does It All Mean?

This is just a tool to point you in the direction of potential vulnerabilities. It will show you all uses of triple braces throughout your application, but not all triple braces are bad! There are many valid uses of these raw tags, but you need to ensure that the content being rendered is truly safe. Always be sure to sanitize any user input that may find its way into a triple brace tag.

Also keep in mind that there are other XSS attack vectors such as the use of Spacebars.SafeString, dynamic attributes, and even within attribute values on certain attributes like href and onclick, to name a few.

Always be vigilant! Never trust user input!

Recursive Components with Meteor and Polymer

Written by Pete Corey on Mar 30, 2015.

I spent some time last week playing with Meteor’s Blaze templates and Polymer elements to build recursive components. The final result is a neat DOM based Cantor set. Before we dig into the source, take a look at a polished version of the Polymer implementation on Codepen:

See the Pen Polymer Cantor Set by Pete Corey (@pcorey) on CodePen.

Building Our Cantor Set

The general idea is to build a component that renders one iteration of the Cantor set construction. By one iteration, I mean a single line (or block), and below it another line divided equally with another instance of the component, and blank space and another instance of the component.

I’m using flexbox to simplify the spacing between my components.

To get a better idea of the construction I’m describing, try inspecting the DOM generated by either, or both, of the implementations.

Meteor Implementation

I started by experimenting with recursive custom block helpers in Meteor. After some trial and error, I came up with a cantor template that renders the Template.contentBlock for the set pieces, and Template.elseBlock for the empty pieces. Take a look at the template and the template’s helpers:

<template name="cantor">
    <div class="row">
        {{> Template.contentBlock}}
        {{#if more}}
            <div class="flex">
                {{#cantor max=max i=next}}
                    {{> Template.contentBlock}}
                    {{> Template.elseBlock}}

                {{> Template.elseBlock}}

                {{#cantor max=max i=next}}
                    {{> Template.contentBlock}}
                    {{> Template.elseBlock}}
    more: function () {
        return (this.i || 0) < this.max-1;
    next: function() {
        return (this.i || 0) + 1;

There are a few key take-aways from this layout. The row element defaults to display:block, which forces its content onto a new line. The flex element is set to display:flex. All of its children have their widths set to 100%, which effectively evenly distributes them within the flex element.

The recursion is where the magic happens. You can see what we’re including the cantor custom block helper inside of itself! We’re passing in an incremented value of i to the next iteration of the component. The component guards itself against infinite repetition with the {{#if more}} check before recursing.

The Meteor implementation can be used like this:

{{#cantor max=6}}
    <section> </section>
    <div class="E"> </div>

Polymer Implementation

The Polymer implementation is very similar, if a little cleaner. Check out the source below:

<polymer-element name="flex-cantor" attributes="max i">
            .e { border: 1px solid rgba(0,0,0,1); border-bottom: none; }
            .c { height: {{100/max}}vh; background-color: tomato; }
            .f { display: flex; }
            *  { width: 100%; }
        <div class="row">
            <section class="c"> </section>
            <template if="{{i < max - 1}}">
                <div class="f">
                    <flex-cantor max="{{max}}" i="{{i + 1}}"></flex-cantor>
                    <section class="e"> </section>
                    <flex-cantor max="{{max}}" i="{{i + 1}}"></flex-cantor>
            i: 0,
            max: 0

Polymer allows us to declare the style for the component inline with the component declaration. Additionally, due to the power of Polymer interpolation, we can include all of our increment and guard logic inline in the markup, rather than offloading these to helper methods like we were forced to do in the Meteor implementation.

The Polymer implementation can be used like this:

<flex-cantor max="6"></flex-cantor>

Final Thoughts

I set up a Github repository with branches for both the Meteor implementation and the Polymer implementation. Check it out!

This was very much an experiment, but I’m very interested in further developing this idea. I can already think of other fun implementations of this idea, like a recursive version of my golden spiral layout, but I’m more interested in practical applications for this type of component composition.

Let me know if you have any ideas for recursive components!

Materialize Highs and Lows

Written by Pete Corey on Mar 25, 2015.

After reading Nick Wientge’s great post on using the Materialize framework to add Material Design ideals to your Meteor project, I was eager to jump on board.

The CSS and static component side of the Materialize framework is fantastic! After a few hours, I had converted a Bootstrap project over to a more Material Design inspired aesthetic, complete with schnazzy animations and transitions.

Unfortunately, I began to hit a few roadblocks when I started combining Materialize’s javascript components with reactive data from Meteor.

Form Select

The application I was converting to Materialize made heavy use of reactively populated select elements. I figured the transition to Materialize would be as simple as calling material_select when the select was rendered: = function() {

But, since Materialize mirrors the options data in a hidden list in the DOM, we’ll need to re-initialize the select plugin every time the data changes. No problem: = function() {
    this.autorun(function() {; //autorun trigger goes here

And, it doesn’t work! At least with the most current Materialize release at the time of this post (v0.95.3). In v0.95.3, re-running the material_select plugin will not re-generate the options list, even if new options have been added to the select. Thankfully, this has been reported as a bug and subsequently fixed, but you’ll need to grab to latest code for yourself to make use of the fix.

These issues can also be entirely avoided by adding the browser-default class to your select. This will cause Materialize to not mirror your select’s options in the DOM and use a styled version of the native select instead. Reactivity will work out of the box, as it would for any other select element.

Collapsible Elements

Collapsible elements also have issues with dynamic content. Collapsible elements are initialized by calling the collapsible plugin on the collapsible list. This will turn all of the child list items into collapsible containers:

<template name="collapsible">
    <ul class="collapsible">
        {{#each items}}
                <div class="collapsible-header">{{header}}</div>
                <div class="collapsible-body">{{body}}</div>
Template.collapsible.rendered = function() {

But what happens when another item is added to items? We’ll need to re-initialize the collapsible plugin: = function() {
    this.autorun(function() {; //autorun trigger goes here

Unfortunately, this doesn’t work exactly as we expected. While the new item is collapsible, re-initializing the plugin also closes all currently open items. If we dig into the source, we can see why this happens.

Let’s take a look at the “expandable” data path. When the plugin is initialized, it loops over each collapsible-header and checks its active status. If it is active, it calls expandableOpen. Take a look.

expandableOpen toggles the active class on the collapsible-header’s parent (li), and then checks its value. If it is active, it expands the container, otherwise it collapses it. Check it out. The re-initialize issue happens because the parent li already has the active class for previously initialized items. When we toggle the class, we inadvertently close the container.

The accordion data path is a little more complicated, but the same general issue exists.

I’ve created an issue and a pull request to fix this issue with the collapsible plugin. Go open source!

Final Thoughts

Materialize is a great front-end framework. It allowed me to quickly and easily build a Material Design style application.

That being said, I don’t think it’s the best fit for a Meteor application. I’m not interested in dealing with the unnecessary complexity of managing the initialization and re-initialization of each of my components as they’re reactively added to the DOM.

At the end of the day, I believe I’m better off using Polymer as a static component library to build my Material Design applications.