Automatically applying context-sensitive output escaping to prevent XSS!
IntroductionCheck out the latest slide deck, presented in the OWASP AppSec USA 2015.
Security is of utmost importance!
Imagine a template is written like so: <a href="{{url}}">{{url}}</a>. When it is compiled with an untrusted user data like {"url": "javascript:alert(666)"}, secure-handlebars automatically applies contextual escaping and generates the HTML <a href="x-javascript:alert(666)">javascript:alert(666)</a> as a result.
Clearly, the same {{url}} must be escaped according to different output contexts to prevent malicious script executions, which otherwise would be vulnerable if the original Handlebars is used alone.
This is archived by enhancing the original Handlebars to perform the following steps:
<div>{{output}}</div> HTML Comment <!-- {{output}} --> HTML Attribute Value
<a class={{output}}>
<div class='{{output}}'>
<div class="{{output}}"> URI in Attribute Value
<a href={{output}}>
<a href='{{output}}'>
<a href="{{output}}"> CSS in Attribute Value
<div style="color:{{output}}">
<div style="backgrount:url({{output}})">
It is generally a bad idea to place an {{expression}} inside those scriptable contexts (e.g., <script>{{script}}</script> or <div onclick="{{onclick}}"). Check out the Section of Warnings and Workarounds for resolutions.
We highly recommend using the express-secure-handlebars npm for a streamlined experience of template pre-processing, compilating, context-sensitive output escaping, and data binding.
Client-side UseAutomatically apply Contextual XSS Escaping for Handlebars templates on client-side
Advanced Usage for Pre-processing Templates Only<script src="dist/secure-handlebars.min.js"></script>
Â
<script>
var html = '<a href="{{url}}">{{url}}</a>',
    data = {url: 'javascript:alert(666)'};
Â
var html = Handlebars.compile(html)(data);
</script>Â
You can perform offline pre-processing for your templates using the provided CLI utility, which rewrites the templates to insert contextual output escaping filter markups. Fully compatible with the original Handlebars, the rewritten templates can be further compiled and data-binded with secure-handlebars-helpers.
To achieve this, install the secure-handlebars npm globally, so it can be used in any project.
npm install secure-handlebars -g
Given a handlebars template file named sample.hbs like so:
<!doctype html>
<html><title>{{title}}</title></html>
Get the template with contextual escaping filters inserted:
handlebarspp sample.hbs > sample.shbs
The pre-processed template file sample.shbs that is fully-compatible with the original (runtime) Handlebars:
<!doctype html>
<html><title>{{{yd title}}}</title></html>
These rewritten templates can then go through the standard Handlebars pre-compilation process, and be used with secure-handlebars-helpers during runtime compilation. On the other hand, this utility also faciilates statistics collection. For instance, you can write a simple script to count the number of dangerous contexts (such as <script>{{script}}</script>).
<!doctype html>).<style> tags yet. See the section below for details.{{>partial}} and {{{{rawblock}}}} are always placed in the HTML Data context, and that they will result in the same Data context after data binding (hence, in-state and out-state are both of the data context).When output expressions are found inside dangerous (yet-to-be-supported) contexts, we echo warnings and gracefully fallback to apply the default Handlebars escapeExpression(). These warnings are indications of potential security exploits, and thus require closer inspections. Instead of simply abusing {{{raw_expression}}} to suppress the warnings, here are some alternative suggestions to secure your applications.
<script> tag<input type="hidden" id="strJS" value="{{strJS}}">
<script>var strJS = document.getElementById('strJS').value;</script>Â
[WARNING] SecureHandlebars: Unsafe output expression found at onclick JavaScript event attribute
Case 1. the data is trusted, or will not be used as URI/HTML output
```
uriData (a patched encodeURI()), uriComponentData (alias of encodeURIComponent()), and the xss-filters that are already registered as Handlebars helpers.``` The manually-applied filters here are to pre-escape `{{keyword}}` depending on the ultimate output contexts, while the `{{` `}}` is still needed (**NOT** `{{{ }}}`) to let `secure-handlebars` automatically applies the escaping filter for the immediate attribute value context.
```
LicenseThis software is free to use under the BSD license. See the LICENSE file for license text and copyright information.
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