Polymer Resin

XSS mitigation for Polymer webcomponents.

This document explains what polymer-resin is. See "Getting Started" if you're interested in how to use it.

Relevant Concepts & Specs

• Webcomponents
• Auto-sanitization
• Safe HTML Types

Relevant Code

• PolyGerrit UI on GoogleSource
• Polymer value hooks (V1, V2)
• JavaScript Safe HTML APIs

Summary

Polymer-resin hooks into Polymer and checks values from data binding expressions just before they reach browser internals. It applies configurable policies with type-safe exceptions so that developers can write data binding expressions without worrying about untrusted inputs abusing web APIs.

For example, if a <bar-element url={{url}}> is backed by an <a href={{url}}> and an attacker can cause url to be javascript:alert(1), then, without Polymer-resin, Polymer will assign that string to <a href> at which point the browser takes over, and routes alert(1) to the JavaScript engine, which unpacks and executes the attacker's payload: alert(1).

Background

In most template languages, one defines templates, but in Polymer one defines custom elements. This means that there is not a closed class of HTML elements and attributes about which we can reason as there is for Closure Templates auto-sanitizer.

Goal

Make it easy for security auditors to quickly check whether a project's custom element definitions fall into a known-safe subset of Polymer.

Security Assumptions

Existing auto-escaping template systems (CTemplates, Closure Templates, Go's html/template) assume that:

1. There is a large community of non-malicious application developers who author templates.

   This community is large enough that we cannot assume that they uniformly apply rigorous secure coding and review practices.

   A system worked on by N developers is vulnerable if just one of them mistakenly introduces a vulnerability. Thus, large groups of individuals who individually rarely make mistakes are nevertheless likely to produce a system that has vulnerabilities.

2. There is a much smaller group of expert browser implementors who define the semantics of tags and attributes. They are aware of the security consequences of their work, and the code they produce is heavily reviewed, but they produce blunt instruments.

For Polymer, we assume that

1. There is a large community of non-malicious custom element authors.
2. There is a large community of non-malicious application developers.
3. These two communities overlap to a great degree and are (similar to above) large enough that we cannot assume uniform rigor in applying secure practices.

4. The novel security consequences of webcomponents arise because they expand the ways in which unchecked values can reach builtin sinks.

   A builtin sink is a property or attribute that can be set by user code and is handled specially by the browser in a manner that may have security implications, in particular, attacker-controlled script execution. Builtin sinks tend to correspond to IDL attributes that are annotated with Reflect, CustomElementCallbacks, or CEReactions.

   There is a hazard whenever an unchecked value reaches a builtin sink. An unchecked value is one that could be controlled by an attacker; it might originate from outside an origin controlled by the application author. For instance, the href attribute of an HTMLAnchorElement is a security-relevant sink; if it can be reached by a value entirely under an attacker's control, that attacker can execute arbitrary script in the context of the user's browser session through injection of a javascript: URL.

   There are many ways that JavaScript can manipulate builtin sinks, and we will use JSConformance policies to guide developers towards safe patterns, and focus instead on web-component specific hazards including

   • an unchecked value reaches a builtin sink on a normal HTML element, e.g. <a href="https://github.com/Polymer/polymer-resin/blob/master/[[...]]"> where [[...]] can be controlled by an attacker;
   • an unchecked value reaches a builtin sink inherited by a custom element, e.g. <my-custom-element onclick="[[...]]">;
   • an unchecked value reaches a custom property on a custom element that is then forwarded to a builtin-sink by element or framework code, e.g. <my-custom-element my-url="[[...]]"> and the custom element's shadow DOM contains a builtin sink <template><a href="https://github.com/Polymer/polymer-resin/blob/master/[[myUrl]]"></template>;
   • an unchecked value reaches a builtin sink on a customized-builtin element either directly (<a is="my-custom-link" href="https://github.com/Polymer/polymer-resin/blob/master/[[...]]">) or indirectly (<a is="my-custom-link" href="https://github.com/Polymer/polymer-resin/blob/master/[[my-url]]" my-url="[[...]]">)
   • an unchecked value specifies a property name <a [[...]]="some-value"> or can specify the type of custom element <a is="[[...]]"> or <[[...]] src="https://github.com/Polymer/polymer-resin/raw/master/some-value">.

Our security goal is to allow element authors to write code that receives unchecked values, and routes them to builtin sinks without the risk of XSS, redirection attacks, etc. We do this by taking the burden of avoiding these attacks off the authors and reviewers of large amounts of application code and move it into a small amount of vetted infrastructure code.

It is not a goal to address direct access to builtin sinks by JavaScript (e.g. HTMLElement.setAttribute(...) or HTMLElement.innerHTML = ...) as those are well handled by existing JSConformance policies.

High-Level Design

We could do this by tweaking Polymer to attach data provenance to properties and attributes but this would be deeply backward incompatible.

We could do this by static analysis of custom element definition, but this requires analysis of JavaScript, and our existing JS type systems are unsound, so any sound analysis would require a lot of duplication of effort or would produce many false positives.

Instead we propose to use existing property value hooks (links at top) provided by Polymer. A security auditor can then check that a Polymer project is properly configured to load these hooks, and check that the project uses JSConformance to prevent forgery of safe string values.

We provide a standalone importable HTML file polymer-resin.html that implements Polymer.sanitizeDOMValue to intercept assignments to builtin sinks given values that originate from expressions specified in Polymer HTML. We also provide a Polymer v1 shim that checks Polymer.version to see if it needs to patch Polymer.Base._computeFinalAnnotationValue to call the same sanitizer.

A security auditor should check that polymer-resin is running early in the page render process. It should load and initialize before the applications main element is instantiated so that it can intercept reflected XSS. Putting the HTML import or script load immediately after the load of framework code suffices.

Text interpolation

When text is interpolated

<script>
before;
[[interpolation]]
after;
</script>

Polymer denormalizes the DOM so that "before;", "[[interpolation]]" and "after;" are three different text nodes, and control reaches the sanitizer with a TextNode as the node, and a null property name.

We use this to intercept text interpolation and allow it only when the content is human-readable HTML.

Life of a Polymer+Resin page

Normally, when an HTML page is parsed, the browser knows that, for an <A> element, it creates an HTMLAElement instance. The custom elements draft specification explicitly allows parsing of <my-custom> before the JavaScript that will eventually define and register HTMLMyCustomElement.

<head>
<script src="https://github.com/Polymer/polymer-resin/raw/master/webcomponents-lite.js"></script>
<link rel="import" href="https://github.com/Polymer/polymer-resin/blob/master/custom-element.html">
<body>
<custom-element id="app"></custom-element>

The life-cycle of a polymer app often looks like

1. Synchronously load webcomponents-lite.js
2. Start parsing HTML imported page custom-element.html
3. Indirectly HTML import load polymer.html which provides a framework for custom elements and has the hooks we need to intercept bound data values.
4. Finish processing custom-element.html which registers the custom element definition.
5. Instantiate <custom-element>

window.addEventListener('WebComponentsReady', …)

seems like it might run at the right time, and the 'HTMLImportsLoaded' event is another good candidate.

Both run after the HTML element definitions have been loaded, and before the app has been provisioned with state loaded from the server, but state that is initialized based on location or query parameters will already have reached custom elements, meaning reflected XSS is still possible.

To prevent reflected XSS, we need to initialize after Polymer is loaded, and before the first custom element definition is registered (except for those defined by Polymer internally).

polygerrit-ui/app/index.html is a good example of a polymer app. The column on the left shows the app before resin is added, and the column on the right shows how we want it to work with Resin. Note that polymer.html is not explicitly loaded by the index.html page; it's loaded via a transitive HTML import.

We provide a polymer-resin.html, an importable HTML file that does two things.

1. HTML import polymer.html so that we have a place to install the hooks
2. synchronously load a script to install the hooks

Bound data handler

The handler receives

1. node - A DOM element
2. property - the property or attribute name
3. info.type (polymer v1) or type (polymer v2) - one of "attribute" or "property"
4. value - the untrusted value

and returns a safe value.

Sanitize DOM Value Algorithm

When sanitizing a property or attribute value we

1. Allow all falsey values. This allows resetting, initializing to blank/nullish. This has the side effect of also allowing 0, NaN, false which we deem low-risk.
2. Classify the containing element as customized or not-customized.
3. Find a clean (no non-default attributes or JS muckery) proxy for the element.
   • For custom elements, this is a vanilla HTMLElement instance.
   • For a builtin or customized-builtin element, it is a vanilla document.createElement(builtinElementName).
   • For legacy elements, treat as builtins.
   • For customizable elements (those which meet the naming convention but for which no custom element constructor has yet been registered), treat as a custom element. Our analysis is dynamic, so we need not assume the worst.
4. If the proxy does not have the named property in it, then allow any value without unwrapping or checking typed string values.
5. Otherwise, if the value is whitelisted according to the element/attribute curated contract tables (JS namespace security.html.contracts), then unwrap any typed string values and allow.
6. Otherwise, log as appropriate, and return a known-safe value.

We could break from the loop if the prototype has an own property with the given name. We could memoize the fact that we found a result with the original key if we’re willing to assume that no properties are deleted from prototypes during program execution.

Table of Security-Relevant Properties and Attributes

The security.html.contracts module captures builtin HTML element and attribute relationships, and we apply the following filters.

The definitions of privileged types assume polymer-resin is installed with the closure bridge. Other safe type bridges may define privileged types differently.

No processing is applied to custom properties.

Values that are of the privileged type are unwrapped and allowed to reach a builtin attribute alias.

Values that are of other type string types are unwrapped before being filtered.

Rejected values are replaced with an innocuous value.

Testing

There are two main failure modes:

1. False negatives -- a failure to apply the appropriate handler to a payload.
2. False positives -- trustworthy code in a custom element definition constructs an attribute value that triggers a filter but does not wrap it in an appropriate safe string type. For example myElement.href = “javascript:myVeryOwnFunction()”

We can check for false negatives by writing custom elements

    <dom-module id=”xss-me”>
      <template>
         <a href="https://github.com/Polymer/polymer-resin/blob/master/{{myhref}}">

that use the relevant properties, and instantiating them with variables bound to known payloads like { “myhref”: “javascript:shouldNotBeCalled()” }.

We will also write regression tests for polygerrit that programmatically creates an author, changelist, and review comment with common payloads, and uses selenium to view the pages and check for breaches.

We will try to get a handle on the kinds of false positives and their frequency by running polygerrit/app/*_test.sh, and looking for regressions.

Running both an instrumented version, and an uninstrumented version side by side in two browser windows should make changes in behavior apparent.

There are a few minor failure modes:

1. Failure to load early enough or at all. Manual inspection of the JS debugger when running polygerrit should suffice. poly-resin.js could also set a property after load that the app could assert.
2. Failure to recognize and reject unsafe values in a handler. Since we’re reusing Soy sanitizers which have a long history of use in production by large projects, I consider this low risk.

Deployment

Gerrit builds via bazel but loads most of its scripts via bower. Polymer resin is available as a bower component.

There are three deployment options.

1. polymer-resin.html which is best for closure-friendly polymer apps.
2. standalone/polymer-resin.html which includes a single JS bundle that includes pre-compiled JS.
3. standalone/polymer-resin-debug.html which is like the previous file but the JS is not obfuscated and it logs to the console whenever a property value is rejected.

To deploy in the custom element example the head changes from

<script src="https://github.com/Polymer/polymer-resin/raw/master/webcomponents-lite.js"></script>
<link rel="import" href="https://github.com/Polymer/polymer-resin/blob/master/custom-element.html">

to

<script src="https://github.com/Polymer/polymer-resin/raw/master/webcomponents-lite.js"></script>
<link rel="import" href="https://github.com/Polymer/polymer-resin/blob/master/polymer-resin/polymer-resin.html">
<script>
// This step is essential to the security of this project.
security.polymer_resin.install({ /* config */ });
</script>
<link rel="import" href="https://github.com/Polymer/polymer-resin/blob/master/custom-element.html">

or with one of the standalone variants above. The <script> is required because it forces the imports above it to be handled before custom-element.html. The install call is explained in configuring.

Running tests from the command line

Per https://github.com/Polymer/web-component-tester make sure that you have bower installed and have run bower update. Then use the test script.

$ ./run_tests.sh

Running tests in the browser

From the project root

$ ./run_tests.sh -p -l chrome

causes it to keep the server open. See the log output for the localhost URL to browse to.