When third parties stop being polite... and start getting real

Charles Vazac
@vazac
Nic Jansma
@nicj
github.com/SOASTA/boomerang
soasta.com/mpulse

boomerang
Free, Open-Source RUM github.com/SOASTA/boomerang
(not this RUM) (powers this RUM)

Why are we here today?
3rd parties are a necessary component of most
websites.
We depend on 3rd party software to help us
build websites with blazing speed.
They (theoretically) add value to our sites.
But do they come at a cost?
We’ll cover:
● Performance
● Compatibility
● Privacy & Security
● How to:
○ Evaluate
○ Monitor
○ Protect

Examples of 3rd parties:
● Frameworks
● Social media widgets
● Analytics
● A/B testing
● RUM
● Utility libraries
● Polyfills
● Ads
● Chat
● Marketing
● Fonts
● CSS
A 3rd Party library is any library you
didn’t write.
They might be packaged in your
application’s JavaScript bundle,
included via a cross-origin <script>
tag, or injected via a tag manager.
What is a 3rd Party?

This is the true story... of 14 strangers...This is the true story... of 14 third parties...

ads-1.js
ads-2.js
ads-3.js polyfill.js
spa.js
errors.js
remarketing.js
no-one-remembers.js
analytics.js
rum.js
konami-code.js
chat-bot.js
something-
bob-added.js
comments.js

Marketing has a
new tag they’d
like on the site.
I’m going to
need you to add
it over the
weekend,
mm’k?
You Bossman

That one little line can:
● Cause your page to stop loading
● Slow down other components
● Create incompatibilities with other libraries
● Change from underneath you
● Take total control of your site
What can go wrong?

But it says async!
async allows the browser to continue parsing the HTML
instead of waiting for that script to load.
The browser still waits to fire the load event until all
async scripts are loaded. SPOF!
A slow async script will make your visitors think your site
is still loading (slowly!)

“Everything should have a value, because everything has a cost” - @tkadlec
How can we judge the cost of a script?
$ ls -al modernizr.js*
-rw-r--r--@ 1 nicjansma staff 92,475 May 30 20:20 modernizr.js
-rw-r--r-- 1 nicjansma staff 32,599 May 30 20:21 modernizr.js.gz
… it’s... cheap???
Evaluating the Cost of a 3rd Party

A third-party’s size (bytes) contributes to the overall Page Weight.
Page Weight is important - it has an effect on how long the page takes to load,
especially on lower-end devices or slower connections.
Lowering the Page Weight can improve load times, so you want to factor the
byte cost of a third-party into your overall Performance Budget.
… but while it’s the easiest way to judge a third party, it’s just one aspect of the
overall cost.
Resource Weight

A 3rd-Party Script’s Lifecycle
1. Loader Snippet / <script>
2. Download
3. Parse + Compile
4. Initialize
5. Runtime / event handlers

A task is work the browser is doing to build the page, such as parsing HTML,
executing JavaScript, or performing layout. This happens on the main thread.
The browser cannot respond to user input (clicking, scrolling, etc) while
executing a task.
Long Tasks are due to complex work that requires more than 50ms of
execution time. i.e. parsing or executing complex JavaScript.
Long Tasks will delay Time to Interactive - the point at which your app is
responsive.
Long Tasks and Time to Interactive

LongTasks and Time to Interactive
Navigate First Paint Page Load
Time to
Interactive
Framework
Parse / Init
Task
Long Task
(>50ms) Click (delayed)
Onload
Handlers
busy idle busy idle
network
main thread
Click
(responsive!)
Click
(responsive!)

2. Download
3. Parse + Compile
4. Initialize
5. Runtime / events
Critical path!
Script tag itself has no cost: <script src="..."></script>
Snippets have a cost (2-10ms on desktop Chrome):
<script type="text/javascript">
(function() {
var po = document.createElement('script');
po.type = 'text/javascript'; po.async = true;
po.src = 'https://.../foo.js';
var s = document.getElementsByTagName('script')[0];
s.parentNode.insertBefore(po, s);
})();
</script>

Tag Manager
● Extra connection (parallel
loads)
● Extra connection (DNS / TCP
time)
● Manager has to load first
● Manager is a third-party script
too!
● A good way to get the script
into your app without a code
review
External <script src="...">
● Extra connection (parallel
loads)
● Extra connection (DNS / TCP
time)
● You don’t control the version
● You don’t control load order
● Can cause SPOF
App Bundle
● Loaded with the rest of your
libraries
● You control of the version
● You control the load order
● Will slow down the loading of
anything behind it in the bundle
Many ways to load

2. Download
3. Parse + Compile
4. Initialize
Every byte affects overall page weight.
Critical path?
● External <script> / tag: no (unless sharing domain)
● Bundled with other components: yes?
Load from a CDN!
The script may load additional resources.

2. Download
3. Parse + Compile
4. Initialize
//requestmap.webperf.tools

2. Download
3. Parse + Compile
4. Initialize
● underscore.js 7 KB
● Google Analytics 14 KB
● moment 16 KB
● jQuery 29 KB
● React 32 KB
● Twitter 34 KB
● Boomerang 54 KB
● Angular 59 KB
● D3 71 KB

2. Download
3. Parse + Compile
4. Initialize
Critical path!
After being fetched, the browser must parse / compile the
(decompressed) JavaScript before it’s executed.
Less bytes = less parse / compile.
● Moment 5 ms 143 KB
● Boomerang 10 ms 188 KB
● Twitter Widget 10 ms 227 KB
● jQuery 11 ms 265 KB
● Angular 22 ms 1291 KB

2. Download
3. Parse + Compile
4. Initialize
Critical path!
Many scripts will initialize (do some work) at startup - create
structures, globals, hook events, etc.
● moment 2 ms
● jQuery 9 ms
● Boomerang 10 ms
● Angular 12 ms
● Twitter Widget 20 ms

Critical path!
The library should be there for a reason.
This reason will do work periodically or based on user
interactions.
● SPA framework updating the view after a route change
● Analytics scripts sending beacons at onload
● Charting library responding to user interactions
All will be done on the main thread can can cause Long Tasks.
2. Download
3. Parse + Compile
4. Initialize

Boomerang’s Performance Audit
https://nicj.net/an-audit-of-boomerangs-performance/
TL;DR boomerang’s cost (high-end to low-end devices):
1. Loader Snippet 2 - 40 ms
2. Download 164 KB raw / 47 KB gzip (non-blocking)
3. Parse 6 - 47 ms
4. Initialize 10 - 80 ms
5. @onload 10 - 300 ms
6. Beacon 2 - 20 KB
7. Runtime minimal
Tracking improvements @ https://github.com/SOASTA/boomerang/issues

Chrome Lighthouse
developers.google.com
/web/tools/lighthouse/
Evaluating for Performance
RequestMap
requestmap.webperf.tools
WebPagetest
webpagetest.org
3rdParty.io
3rdparty.io

What 3rd Party Scripts Should be Doing...
They should:
● Use a CDN
● Compress resources
● Set caching headers
● Set Timing-Allow-Origin
● Set Access-Control-Allow-
Origin
● Support HTTPS
● Support HTTP/2
● Minify
● Have ~100% uptime
Minimal:
● JavaScript size
● Work without yielding
● Network latency
● CPU
● Requests
● Cookies
● DOM changes / additions
● Event hooks
● Global variables
● Patching
● Changes without your permission
No:
● document.write()
● alert() or prompt()
● eval()
● debugger;
● Console messages
● JavaScript errors
● Including other libs
● Redirects
● Known vulnerabilities

Monitoring Performance
RUM (mPulse)
soasta.com
/performance-monitoring/
SpeedCurve
speedcurve.com
Calibre
calibreapp.com
LongTasks
www.w3.org/TR/longtasks/
var obs = new PerformanceObserver(function(list) {
var perfEntries = list.getEntries();
for (var i = 0; i < perfEntries.length; i++) {
// Process long task notifications
}
});
obs.observe({entryTypes: ["longtask"]});

● Resource Hints
● 3rd-Party <script> tag vs. self-hosting (bundling) vs. tag manager
● Lazy loading, and only load tags when they’re needed, not globally
● ServiceWorker
Every third-party should have an owner or “internal champion”.
Protecting your Performance

I’m not sure one
analytics script
is enough.
I’m going to
need you to add
two more over
the weekend,
mm’k?
You Bossman

WHO?
Compatibility
TL;DR - When bringing third-party code or integrating widgets into your
page, you need to pay special attention to how scripts can unknowingly
affect each other. The two major friction points are polyfills and built-
in patching.
WHAT? WHEN?
Very hard 🐞🐞🐞 to triage!

WHO?
Compatibility
● Rarely tested together
● Loading order is not always guaranteed
● Opaque release cycle
● Patched browser APIs ☠
● Polyfills ☠
WHAT? WHEN?
Very hard 🐞🐞🐞 to triage!

Source: https://www.tooled-up.com/artwork/prodzoom/PLCQDP1KG.jpg?w=500&h=500&404=default

// lazy loading package
window.requestAnimationFrame = window.requestAnimationFrame || setTimeout
Polyfills - Script Errors

// in-page feature detection
if (typeof requestAnimationFrame === 'function') {
var requestId = requestAnimationFrame(function() {
/* fancy animation codez */
})
}

// in-page feature detection
if (typeof requestAnimationFrame === 'function') {
var requestId = requestAnimationFrame(function() {
/* fancy animation codez */
})
// sometime later
cancelAnimationFrame(requestId) // throws Uncaught ReferenceError: cancelAnimationFrame is not defined
}

Polyfills - Loading Strategy
On developers wanting to use the new hotness while maintaining backwards
compat:
“The problem with this approach is it prioritizes developer
convenience over user experience, and it unnecessarily
penalizes users on modern browsers by forcing them to
download a lot of code they don’t need.” - @philwalton

Source: https://image.shutterstock.com/image-vector/funny-monkey-260nw-294497702.jpg

Patched built-ins - broken listeners
var addEventName = window.addEventListener ? 'addEventListener' : 'attachEvent'
var addEvent = window[addEventName]
var handlers = {hashchange: [], popstate: []}
window[addEventName] = function(type, listener, options) {
if (handlers[type]) {
handlers[type].push(listener)
return
}
// delegate to built-in
addEvent(type, listener, options)
}

var addEventName = window.addEventListener ? 'addEventListener' : 'attachEvent'
return
}
}
un-binds the method from window

top.addEventListener = (function(_addEventListener) {
return function() {
// run some code here
return _addEventListener.apply(this, arguments)
}
})(top.addEventListener)
var addEventName = window.addEventListener
? 'addEventListener'
: 'attachEvent'
return
}
}
boomerang.jsthird-party.js

top.addEventListener = (function(_addEventListener) {
return function() {
// run some code here
return _addEventListener.apply(this, arguments)
}
})(top.addEventListener)
var addEventName = window.addEventListener
? 'addEventListener'
: 'attachEvent'
return
}
}
boomerang.jsthird-party.js
this !== top

Patched built-ins - prototype chain

EventTarget.prototype.addEventListener =
function() { /* ... */ }
XMLHttpRequestEventTarget.prototype.addEventListener =
function() { /* ... */ }
XMLHttpRequest.prototype.addEventListener =
function() { /* ... */ }
// instance-level
(new XMLHttpRequest).addEventListener =
function() { /* ... */ }

var _addEventListener1 = XMLHttpRequest.prototype.addEventListener
XMLHttpRequest.prototype.addEventListener = function() {
// Angular interloping code
_addEventListener1.apply(this, arguments)
}

}
var _addEventListener2 = EventTarget.prototype.addEventListener
EventTarget.prototype.addEventListener = function() {
// Boomerang interloping code
}

}
var _addEventListener2 = EventTarget.prototype.addEventListener
EventTarget.prototype.addEventListener = function() {
// Boomerang interloping code
}
xhrInstance.addEventListener(...) // never delegates to Boomerang’s patch

Evaluate
● Prefer self-hosting or bundling over <script> tag
● Identify and understand patched built-ins - https://github.com/cvazac/detect-native-overrides
● Manually audit polyfills
○ Ensure spec compliance
○ Don’t let them rot!
Monitor/Protect
● Hold third-parties accountable for script errors
● ESLint - no-extend-native rule
Best Practices

You Bossman
Is it normal for
the fan in my
computer to go
into overdrive
every time I
open up our
webapp?

TL;DR - New browser APIs like Content Security Policy (CSP) and
Subresource Integrity (SRI) are great, but they are not the silver bullet.
Because of code obfuscation and other tactics, you also need code-
level monitoring.
Privacy & Security

Source: https://randywestergren.com/widespread-xss-vulnerabilities-ad-network-code-affecting-top-tier-publishers-retailers/
URL: http://www.example.com/#1'-alert(1)-'"-alert(1)-"

Source: https://twitter.com/Scott_Helme/status/962693995590766592

Content Security Policy
CSPs allow you to whitelist the domains that you permit to:
● Execute script (script-src)
● Include images (image-src)
● Make network requests (connect-src)
● Be form targets (form-action)
● Include IFRAMEs (frame-ancestors)
CSP is hard to get perfect, so take advantage of these tools.:
● CSP policy generator wizard - https://report-uri.com/home/generate
● CSP evaluator - https://csp-evaluator.withgoogle.com/

Subresource Integrity
Subresource Integrity allows you to make sure that the script you pull down
from a third-party is EXACTLY what you are expecting - or it will not execute.
<script
src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"
integrity="sha384-tsQFqpEReu7ZLhBV2VZlAu7zcOV+rXbYlF2cqB8txI/8aZajjp4Bqd+V6D5IgvKT"
crossorigin="anonymous">
</script>
Downside: it does complicate upgrading libraries.

Source: https://freedom-to-tinker.com/.../no-boundaries-for-credentials-password-leaks-to-mixpanel-and-session-replay-companies/

● atob()/btoa(), escape()/unescape()
● navigator.hardwareConcurrency reads
● setTimeout(...) calls with loooooooong delays
● addEventListener(...) calls: submit, mousemove, keypress
● document.createElement(...): IFRAME, OBJECT, SCRIPT, EMBED, etc
● document.cookie reads, window.localStorage read/writes
● window.clipboardData reads
● navigator.userAgent reads
● XHR.send instrumentation
● Execution context creation: eval(...), document.write(...)
● window.location.reload() | replace() calls
● (new Image).src listeners
Why would you even?

Evaluate
● Support HTTPS
● Vulnerability Scan
○ snyk.io
○ npm audit
Best Practices

Evaluate
● Support HTTPS
● Vulnerability Scan
○ snyk.io
○ npm audit
Best Practices
Monitor/Protect
● Report on long-tasks
● Sandbox in cross-origin IFRAMEs
● Content Security Policy
● Subresource Integrity
● Freeze sensitive APIs -
https://github.com/cvazac/freeze.js
● Code-level monitoring - jscrambler.com

● http://3rdparty.io/
● https://github.com/cvazac/detect-native-overrides
● https://github.com/cvazac/freeze.js
● https://nicj.net/an-audit-of-boomerangs-performance/
● https://www.webpagetest.org/
● https://developers.google.com/web/tools/lighthouse/
● http://requestmap.webperf.tools/
● http://jsmanners.com/
● https://www.ghostery.com/
● https://zoompf.com/
● https://jscrambler.com/
● https://snyk.io/
Links

References
● https://philipwalton.com/articles/loading-polyfills-only-when-needed/
● https://scotthelme.co.uk/protect-site-from-cryptojacking-csp-sri/
● https://freedom-to-tinker.com/2018/02/26/no-boundaries-for-credentials-password-leaks-to-
mixpanel-and-session-replay-companies/
● https://css-tricks.com/potential-dangers-of-third-party-javascript/
● https://randywestergren.com/widespread-xss-vulnerabilities-ad-network-code-affecting-top-tier-
publishers-retailers/

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