Origins, Sites and other Terminologies

In order to fully discuss security issues, their common root causes and useful prevention or mitigation techniques, you will need some common ground on the security model of the web. This, in turn, relies on various terms and techniques that will be presented in the next sections.

Feel free to skip ahead, if you are familiar with some of the following concepts.

The Same-Origin Policy

The most important notion of scope on the web is an Origin. An origin is usually a tuple of a scheme, a host and a port of a URL. Generally, documents can only interact with each other when hosted on the same origin. In essence, this means that two communicating documents' URLs should have the same scheme, host and port.

However, an origin can also be a so-called an opaque origin, which is considered a restricted context that is always cross-origin to everything else. This is used in e.g., <iframe> elements with the sandbox attribute.

Cross-origin resources that are loaded into the current document (e.g., scripts, images) can be used (e.g., executed or displayed) but not properly read: A cross-origin image can be drawn onto a <canvas>, but its pixels can not be read. A cross-origin's script can be executed but the actual source text is unreadable. However, the script's side effects are observable when modifying the global scope (e.g., defining a new global variable). This can lead to unintended leaks, called Cross-Site Script Inclusions.

The Same-Origin Policy can not always apply purely based on comparing the origins of two URLs, as we will likely see in another post. For now, a quick link to my 2012 diploma thesis "Origin Policy Enforcement in Modern Browsers" will have to suffice.

Interactions that are subject to the Same-Origin Policy can be roughly grouped in two scenarios:

1) DOM Access

Synchronous access in JavaScript across origin boundaries can happen through a window or document object. These are typically obtained through an <iframe> or a popup created with and are always gated by the Same-Origin Policy - except for some interesting corner cases:

  1. Reading the window.length property, which reveals the number of frames.
  2. Reading closed attribute (used for popups) and calling close().
  3. Invoking focus() and blur().
  4. Calling postMessage allows sending data, which triggers a MessageEvent on the receiving window asynchronously.
  5. A cross-origin window can be navigated away by assigning to window.location or invoking location.replace().

Interestingly, this already allows for some tricks and attacks where a cross-origin window is suddenly replaced with a similar-looking spoof.

2) HTTP Requests

Websites can perform HTTP requests, but will not be able to read the responses, unless the request's URL is same-origin. Requests are typically issued with APIs like XMLHttpRequest and fetch().

However, there are some exceptions and techniques towards relaxing same-origin checks, like Cross Origin Resource Sharing (CORS). Many of these exceptions grew organically based on some specific need. We will go through them in a later post.

Site, Registerable Domain and Public Suffix

Some APIs are governed by the notion of a Site, instead of an Origin. A Site is a combination of a scheme and a host's registerable domain.

Looking up the registerable domain of a hostname, is a quite literally a check which domain had to be registered (e.g., or is the registerable domain for as well as for This lookup is useful to include an entity and all of its subdomains, but nothing above.

The idea of a Site is used in a variety of specs that want to allow related web pages to collaborate for convenience or legacy support reasons. Among them are Storage Access API (3rd Party Cookie Access), WebAuthn and Federated Credential Management.

Historical context: Previously, people used the terminology of a top-level domain, where the top was presumed to be exactly one level of nesting and not more. This has been long-since incorrect and impractical - given the existence of "top" levels with additional nesting like Therefore, a variety of other terms have emerged, like eTLD+1 (the effective top-level domain, plus another level of nesting). Web standards have converged on registerable domain.
What this means is that the boundary where a "top" level begins has to be defined otherwise: In practice, this happens in a manually maintained text file otherwise known as the Public Suffix List.

Aside: Even before that, people used to refer to same-site by just comparing two registerable domains, without the scheme. This has been renamed to schemelessly same-site. Let's pretend that never happened in the first place.

Getting a domain of yours added to the aforementioned Public Suffix List, allows to enforce privilege separation along the lines of the Same-Origin Policy: If your domain is considered a public suffix, then every name below that becomes its own origin. It is generally recommended to do that in order to assign user-generated content into separated namespaces. A great example is, where each user is getting a namespace underneath the public suffix and can therefore not affect cookies or site-specific settings with other users.

Secure Contexts

Secure Context is a generalized notion of whether a website was served over HTTPS. A simple look at the protocol scheme does not fully work and the generalization is necessary to allow for situations where a document does not have a HTTP(S) URL but is inheriting its context (e.g., <iframe srcdoc>, about:blank documents). Additionally, pages hosted on *.localhost or via the local filesystem (file://) are also considered secure. Whether the current page was delivered securely is exposed in JavaScript via the window.isSecureContext property.

Being in a Secure Context is often times required for newer, powerful APIs which want some level of assurance that a web page has not been intercepted or modified by a network attacker. A typical API that requires it is ServiceWorkers, because persistently installed background code should not be perpetuated into future sessions when coming from an insecure connection.

Note: A document delivered over HTTPS is not a secure context, if it has been embedded from an insecure context (e.g. HTTP site contains <iframe> of HTTPS site).

Cross-Origin Isolation

Cross-Origin Isolation is a newer security boundary, that was created as a reaction to two noteworthy attack groups. The first attack group being so-called cross-site leaks (henceforth called xsleaks) and microarchitectural side-channel attacks against modern CPUs (Spectre, Meltdown, and its various successors).

Xsleaks attacks rely on abusing some pre-existing global state (e.g. browser cache, a resource limit for maximum allowed opened socket) to infer or leaking cross-site application state. This is a generalization upon previously known methods for e.g., redirection-detections to identify whether a user is logged in with a third-party site, but also history stealing attacks. The xsleaks wiki has many examples and suggested countermeasures.

Microarchitectural attacks (like Spectre and Meltdown) rely on specific CPU behavior in order to infer or leak information from the processor itself. Roughly speaking, exploitation allows to read arbitrary memory within the same operating system (OS) process. Carrying out a Spectre attack, for example, is believed to be possible using shared memory access and high-precision timers. Both exist as APIs provided by the web platform with SharedArrayBuffer and the Performance API.

As a result, an attacker would have been able to violate the Same-Origin Policy by embedding a cross-origin resource (using e.g., an <img> element) and reading process memory. As a reaction, browser engines have originally disabled access to those APIs or reduced timing granularity.

Despite blocking Web APIs, major browsers like Chrome and Firefox have also gone through significant re-architecturing efforts to create and assign separate OS-processes to websites of different Sites. These long-term engineering projects (called Site-Isolation) were mostly without side effects for web developers. However, this is not enough, given the embedding attack example above.

In order to reenable access to these coveted APIs, a website now has to ensure that is not including any cross-origin content (using Cross-Origin-Embedder-Policy or COEP) or only content that has been explicitly listed as public across process boundaries by its author (using Cross-Origin-Resource-Policy). Furthermore, the web page needs to disallow synchronous window handle access using the Cross-Origin-Opener-Policy (also known as COOP).

When used in combination, COEP and COOP lead to the window.crossOriginIsolated property becoming true. This give access to the SharedArrayBuffer constructor and high-precision timing in the Performance API.

To summarize, Cross-Origin Isolation and is a mechanism to retrofit the web security model in light of new attacks: A website can only get access by being assigned its very own, unique browser process that is also free of potentially sensitive cross-origin content.

What else?

Do you feel like something is unclear or missing?

If you find a mistake in this article, you can submit a pull request on GitHub.

Other posts

  1. The Mozilla Monument in San Francisco (Fri 05 July 2024)
  2. What is mixed content? (Sat 15 June 2024)
  3. How I got a new domain name (Sat 15 June 2024)
  4. How Firefox gives special permissions to some domains (Fri 02 February 2024)
  5. Examine Firefox Inter-Process Communication using JavaScript in 2023 (Mon 17 April 2023)
  6. Origins, Sites and other Terminologies (Sat 14 January 2023)
  7. Finding and Fixing DOM-based XSS with Static Analysis (Mon 02 January 2023)
  8. DOM Clobbering (Mon 12 December 2022)
  9. Neue Methoden für Cross-Origin Isolation: Resource, Opener & Embedding Policies mit COOP, COEP, CORP und CORB (Thu 10 November 2022)
  10. Reference Sheet for Principals in Mozilla Code (Mon 03 August 2020)
  11. Hardening Firefox against Injection Attacks – The Technical Details (Tue 07 July 2020)
  12. Understanding Web Security Checks in Firefox (Part 1) (Wed 10 June 2020)
  13. Help Test Firefox's built-in HTML Sanitizer to protect against UXSS bugs (Fri 06 December 2019)
  14. Remote Code Execution in Firefox beyond memory corruptions (Sun 29 September 2019)
  15. XSS in The Digital #ClimateStrike Widget (Mon 23 September 2019)
  16. Chrome switching the XSSAuditor to filter mode re-enables old attack (Fri 10 May 2019)
  17. Challenge Write-up: Subresource Integrity in Service Workers (Sat 25 March 2017)
  18. Finding the SqueezeBox Radio Default SSH Password (Fri 02 September 2016)
  19. New CSP directive to make Subresource Integrity mandatory (`require-sri-for`) (Thu 02 June 2016)
  20. Firefox OS apps and beyond (Tue 12 April 2016)
  21. Teacher's Pinboard Write-up (Wed 02 December 2015)
  22. A CDN that can not XSS you: Using Subresource Integrity (Sun 19 July 2015)
  23. The Twitter Gazebo (Sat 18 July 2015)
  24. German Firefox 1.0 ad (OCR) (Sun 09 November 2014)
  25. My thoughts on Tor appliances (Tue 14 October 2014)
  26. Subresource Integrity (Sun 05 October 2014)
  27. Revoke App Permissions on Firefox OS (Sun 24 August 2014)
  28. (Self) XSS at Mozilla's internal Phonebook (Fri 23 May 2014)
  29. Tales of Python's Encoding (Mon 17 March 2014)
  30. On the X-Frame-Options Security Header (Thu 12 December 2013)
  31. html2dom (Tue 24 September 2013)
  32. Security Review: HTML sanitizer in Thunderbird (Mon 22 July 2013)
  33. Week 29 2013 (Sun 21 July 2013)
  34. The First Post (Tue 16 July 2013)