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Per-page Suborigins

Objective

Our objective is to provide a new mechanism for allowing sites to easily separate their content into separate, flexible synthetic origins while serving content from a single physical origin. Furthermore, the synthetic origins should be predictable and convey the full physical origin so that compartmentalized content can easily use current browser technologies, such as postMessage, to interact with each other.

Background

This document is based on (and much text lifted from) an original proposal written by Michal Zalewski


Currently, web applications are almost always compartmentalized by using separate host names to establish separate web origins. This is useful for helping to prevent XSS and other cross-origin attacks, but has many unintended consequences. For example, it causes latency due to additional DNS lookups, removes the ability to use single-origin features (such as the history.pushState API), and creates cryptic host name changes in the user experience. Perhaps most importantly, it results in an extremely inflexible architecture that, once rolled out, can’t be easily and transparently changed later on.


There are several mechanisms for reducing the attack surface for XSS without creating separate host-name based origins, but each pose their own problems. Per-page Suborigins is an attempt to fill some of those gaps. Two of the most notable mechanisms are Sandboxed Frames and Content Security Policy (CSP). Both are powerful but have shortcomings and there are many external developers building legacy applications that find they cannot use those tools.


Sandboxed frames can be used to completely separate untrusted content, but they pose a large problem for containing trusted but potentially buggy code because it is very difficult, by design, for them to communicate with other frames. The synthetic origins assigned in a sandboxed frame are random and unpredictable, making the use of postMessage and CORS difficult.


Content Security Policy is also promising but is generally incompatible with current website design. Many notable companies found it impractical to retrofit most of their applications with it. On top of this, until all applications hosted within a single origin are simultaneously put behind CSP, the mechanism offers limited incremental benefits, which is especially problematic for companies with large portfolios of disparate products all under the same domain.

Overview

We want to create a new browser primitive to provide isolation within a single origin between disparate components. Often, there are many web applications that share a single real origin. One example of this is www.google.com, which many functionally separate applications (in the hundreds) share for reasons related to branding, DNS latency, and seamless interoperability, despite being very different properties. The consequence of this is that one compromised web application from an XSS at the www.google.com origin means that all properties at www.google.com are compromised. Many other popular web destinations such as Facebook, Twitter, Dropbox, etc. face similar issues.


This is an accident of the Same Origin Policy (SOP) in the sense that the SOP assumes everything at a given real origin is part of the same application. A Per-page Suborigin primitive would allow developers to apply the SOP at a finer grained granularity and specify that different applications within the same real origin should, in fact, be treated as different origins under the SOP. The primitive should fill a space between Sandboxed Frames and CSP by allowing consumers to separate trusted components into separated origins while still allowing efficient cross-origin communication via postMessage and CORS, but also without significant retrofitting limitations to legacy applications.


Per-page Suborigins would provide a way for applications at the same origin to request to be placed in separate, synthetic suborigins. These suborigins would be in a predictable namespace, so utilizing postMessage and CORS would be possible just as normal. However, it would also provide the strong isolation properties of regular SOP protection; despite being in the same real origin, code from different suborigins would not be able to access the DOM of a different suborigin.


Furthermore, Per-page Suborigins could actually help with a rollout of CSP. As noted above, when many different properties are hosted on the same domain, if only some have CSP applied, many of the benefits are lost because an XSS on one of the non-CSP enabled pages poses an immediate threat to CSP-enabled content. With Per-page Suborigins, by compartmentalizing different applications within the same domain, an exploit of one application that is not CSP enabled would not necessarily compromise an application in a different Per-page Suborigin, thus increasing the value of gradually deploying CSP for high-value components.


We see effectively three different use cases for Per-page Suborigins:

  1. Separating distinct applications that happen to be served from the same domain, but do not need to extensively interact with other content. Examples include marketing campaigns, simple search UIs, and so on. This use requires very little engineering effort and faces very few constraints; the applications may use XMLHttpRequest and postMessage to communicate with their host domain as required.

  2. Allowing for modularity within a larger web application by splitting the functional components into different suborigins. For example, Gmail might put the contacts widget, settings tab, and HTML message views in separate Per-page Suborigins. Such deployments may require relatively modest refactorings to switch to postMessage and CORS where direct DOM access and same-origin XMLHttpRequest are currently used, but we believe doing so is considerably easier than retrofitting CSP onto arbitrary code bases and can be done very incrementally.

  3. Similar to (2), applications with many users can split information relating to different users into their own suborigin. For example, Twitter might put each user profile into a unique suborigin so that an XSS within one profile cannot be used to immediately infect other users or read their personal messages stored within the account.

Detailed Design

Definition of Suborigins

Suborigins will be a synthetic origin, similar to the already implemented synthetic suborigins for HTML5 sandboxed frames. However, suborigins will have the important property of being predictable, well-defined, and hierarchical, allowing postMessage and CORS communication between sandboxed content.


A suborigin may be specified by a HTTP header such as:

Suborigin: <name>

where <name> is defined by the server or developer and should be predictable.


We will treat suborigins as a separate field from origins that must be checked whenever the SOP is being enforced. Thus, the above suborigin will result in the following SOP origin for the frame of:

origin: <protocol>://<host>:<port>

suborigin: <name>

where <protocol>, <host>, and <port> remain as they always have, and <name> is a name defined by the server of alpha-numeric characters only. In the case that no suborigin is specified, the value of suborigin should be treated as the empty string.


A potentially preferable option would be to implement this as a CSP directive. For example:

Content-Security-Policy: suborigin <name>;

which would result in the same full origin as above.


An important point to note here is that, because of security considerations, there is no way to “surrender” or “escape” a suborigin. That is, once assigned a suborigin, a page cannot return within that instance to the real origin; the ability to do so would completely defeat the sandboxing purpose of the mechanism.


We have considered alternative approaches to encoding suborigins but concluded that including it in the header is the best approach. For example, one alternative would be to encode the suborigin as part of the host or protocol. Two ways you might do this are:

origin: <protocol>+<name>://<host>:<port>

or origin: <protocol>://<name>@<host>:<port>


These have the advantage that they (a) encode the suborigin as part of the current origin, thus no new suborigin field is required, and (b) current enforcement mechanisms would require relatively few changes since it would effectively be encoding the suborigin as part of either the protocol or host, and the current SOP already enforces these components.


However, suborigins are not intended to be user understandable or interactive, and encoding suborigins in the scheme/host/port would risk users interacting with them, such as in the URL bar or having them accidentally displayed. Additionally, suborigins, as we will discuss, cannot be used in a variety of contexts that origins are valid, such as resource identifiers in <iframe> or <img> tags because we require that the server be the authority on what entities are place in suborigins. Thus, we create suborigins as a unique field alongside the origin field.


Permissions that are normally associated with origins should not be applied to Suborigins. For example, geolocation permissions and fullscreen permissions should not be propagated to execution contexts in a new Suborigin. Moreover, there should be no way for Suborigins to obtain such permissions. We apply these restrictions because there should be no way for users to see or interact with Suborigins, and any special permissions might require user confirmation that would require knowledge about the implementation level details of suborigins.


Cross-origin and cross-suborigin communication via postMessage is one of the most important features of per-page suborigins. Per-page suborigins with postMessage is a mostly straightforward extension of the current implementation. The main concern is on the receiving end of a message listener since we need both backwards compatibility and do not want to let legacy implementations accidentally accept messages from incorrect origins. Thus, the receiving event object will have event.origin set to null so that legacy applications will not confuse a message from the foo suborigin at bar.com for a message from the top level suborigin at bar.com. Instead, we propose a new property on the event object, event.finerorigin, that will contain two properties, origin and suborigin, containing the host and suborigin values respectively.


In the case of pages with a non-empty string suborigin fields, all XMLHttpRequest requests to any URL should be treated as cross-origin, thus triggering CORS logic. This contains a similar concern to the postMessage issue outlined above. In order to prevent legacy applications from accidentally accepting CORS requests from suborigins, CORS headers originating from a suborigin execution context must specify a Suborigin: <name> header and a Finer-Origin: <origin> header, while explicitly not setting an Origin header. Similarly, responses should be checked for matching new Access-Control-Allow-Finer-Origin and Access-Control-Allow-Finer-Suborigin headers, while Access-Control-Allow-Origin headers should be ignored.


Execution contexts with a Suborigin should be tagged with a flag is_suborigin that is inherited by all child contexts that normally inherit the origin of the creator. The SOP should apply as it currently does, using the suborigin in addition to the traditional origin for making policy decisions. Thus, containment should nearly be “for free,” that is it should work using the current SOP mechanisms with an additional check for the Suborigin of an execution context.


However, in order to compartmentalize, there may be things that may require limited access. As mentioned before, various permissions, such as geolocation, must be disallowed in Suborigins. Additional restrictions also apply.


Notably, a suborigin frame may not access document.cookie. While cookies will still be sent with HTTP requests based on the real origin, to simplify compartmentalization and security, suborigin contexts will not be able to access the cookie object, similar to sandboxed contexts. This will prevent information leak and limit the damage a compromised frame can do. If information from a cookie is needed, it will need to communicate with a non-suborigin frame via postMessage or another cross-origin mode of communication.


Additionally, plugins pose a problem because they use questionable methods to determine the origin of the embedding page, and may bestow some privileges associated with this origin to the plugin-executed code. In light of this, <embed>, <object>, or <applet> content within documents where is_suborign is set will not be allowed. As opposed to sandboxed frames, however, is_suborign will not unconditionally propagate to all subframes - and therefore, many forms of rich ads or Flash videos placed on sandboxed pages should continue to work as-is or with only minimal tweaks.


Checking Origins and Suborigins

As mentioned before, the SOP will now be applied to the new composite orign, that includes both an origin and suborigin field. SOP checks should remain functionally the same, however they should now include an additional check of the suborigin field for lexical case-insensitive equality.

Concerns

There are many potential unintended consequences of establishing finer- or coarser-grained origins, and there is a long history of security issues and surprising corner cases caused by many of the previously proposed mechanisms (including sandboxed frames and CSP).


Nevertheless, we think that this proposal avoids most of the pitfalls evident in previous efforts - in large part because it does not try to enforce any new, complex security rules for the sandboxed content, and does not routinely expose the new boundaries to the user.


One concern, however, is with phishing attacks. As mentioned earlier, suborigins are purposefully create as a new field, rather than being fit into the current scheme/host/port scheme. This is because we don’t want users to see these implementation details. However, this has the consequence that phishing attacks are possible by suborigin content where content pretends to be from another suborigin. However, this is already the case with CSP.

Security Considerations

When implementing the feature, care should be taken to provide several relatively straightforward guarantees:


  • The origin needs to be correctly communicated and matched by the postMessage API and in the Origin header in CORS requests (this should require no code changes, but should be confirmed by the implementor). All XMLHttpRequest initiated from an isolated origin would be treated as CORS.

  • The document.cookie API must be inaccessible from contexts marked as is_suborigin, similar to how sandboxed contexts are currently treated.

  • Password manager and the “content permissions” (geolocation, full-screen access, microphone & webcam APIs) generally should be disallowed in Suborigin execution contexts. This is generally because of UI considerations and the desire to not display anything about Suborigins to users.

Future Work and Extensions

Allow some limited access to document.cookie. Although we suggest using postMessage for transmitting and cookie related information to a suborigin, it might be useful to allow some limited access to cookies. One future approach might be to create suborigin-specific sections of cookies that only the named suborigin can access.
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