Secure Contexts

1. Introduction

This section is not normative.

As the web platform is extended to enable more useful and powerful applications, it becomes increasingly important to ensure that the features which enable those applications are enabled only in contexts which meet a minimum security level. This document describes threat models for feature abuse on the web (see §4.1 Threat Models) and outlines normative requirements which should be incorporated into documents specifying new features (see §7 Implementation Considerations).

The most obvious of the requirements discussed here is that application code with access to sensitive or private data be delivered confidentially over authenticated channels that guarantee data integrity. Delivering code securely cannot ensure that an application will always meet a user’s security and privacy requirements, but it is a necessary precondition.

Less obviously, application code delivered over an authenticated and confidential channel isn’t enough in and of itself to limit the use of powerful features by non-secure contexts. As §4.2 Ancestral Risk explains, cooperative frames can be abused to bypass otherwise solid restrictions on a feature. The algorithms defined below ensure that these bypasses are difficult and user-visible.

The following examples summarize the normative text which follows:

1.1. Top-level Documents

Top-level documents are secure as long as they don’t have a non-secure opener browsing context. This is a bit convoluted, so let’s go straight to the examples:

<a href="https://example.com/" target="_blank">Link!</a>
      
<script>
  var w = window.open("https://example.com/");
</script>

<a href="https://example.com/" rel="noopener" target="_blank">Link!</a>
      
<script>
  var w = window.open("https://example.com/", "", "noopener");
</script>

1.2. Framed Documents

Framed documents can be secure contexts if they are delivered from potentially trustworthy origins, and if they’re embedded in a secure context. That is:

1.3. Web Workers

Dedicated Workers are similar in nature to framed documents. They’re secure contexts when they’re delivered from potentially trustworthy origins, only if their owner is itself a secure context:

1.4. Shared Workers

Multiple contexts may attach to a Shared Worker. If a secure context creates a Shared Worker, then it is a secure context, and may only be attached to by other secure contexts. If a non-secure context creates a Shared Worker, then it is not a secure context, and may only be attached to by other non-secure contexts.

1.5. Service Workers

Service Workers are always secure contexts. Only secure contexts may register them, and they may only have clients which are secure contexts.

2. Framework

A settings object is considered a secure context#secure-contextReferenced in:1.1. Top-level Documents (2)1.2. Framed Documents (2) (3)1.3. Web Workers (2) (3) (4)1.4. Shared Workers (2) (3) (4)1.5. Service Workers (2) (3)2. Framework (2)2.2.1. Shared Workers (2) (3)2.2.2. Feature Detection (2)3.1. Is settings object a secure context? 4.2. Ancestral Risk4.3. Risks associated with non-secure contexts (2)5.1. Incomplete Isolation6. Privacy Considerations7.3. Restricting New Features (2) (3)7.4. Restricting Legacy Features (2) (3)7.4.1. Example: Geolocation (2) if the algorithm in §3.1 Is settings object a secure context? returns "Secure". The settings object is otherwise non-secure.

Likewise, a global object is considered a secure context if its relevant settings object is a secure context.

2.1. Intergration with WebIDL

This section is non-normative.

A new <a>[SecureContext]</a> attribute is available for operators, which ensures that they will only be exposed into secure contexts. The following example should help:

interface ExampleFeature {
  // This call will succeed in all contexts.
  Promise <double> calculateNotSoSecretResult();

  // This operation will not be exposed to a non-secure context.
  [SecureContext] Promise<double> calculateSecretResult();

  // The same applies here: the operation will not be exposed to a non-secure context.
  [SecureContext] boolean getSecretBoolean();
};

[SecureContext]
interface SecureFeature {
  // This interface will not be exposed to non-secure contexts.
  Promise<any> doAmazingThing();
};

Specification authors are encouraged to use this attribute when defining new features.

2.2. Modifications to HTML

2.2.1. Shared Workers

The SharedWorker() constructor will throw a SecurityError exception if a secure context attempts to attach to an Worker which is not a secure context, and if a non-secure context attempts to attach to a Worker which is a secure context. The constructor is modified as follows:

1. As the first substep of the SharedWorker() constructor’s current step 7.7 ("If worker global scope is not null, then run these steps:"), run the following step:

   1. If the result of executing §3.1 Is settings object a secure context? on the incumbent settings object does not match the result of executing the same algorithm on worker global scope’s relevant settings object, then throw a SecurityError exception, and abort these steps.

2.2.2. Feature Detection

To determine whether a context is capable of making use of features which require secure contexts, a simple boolean attribute is added to the global object:

partial interface Window {
  readonly attribute boolean isSecureContext#dom-window-issecurecontextReferenced in:2.2.2. Feature Detection;
};

partial interface WorkerGlobalScope {
  readonly attribute boolean isSecureContext#dom-workerglobalscope-issecurecontextReferenced in:2.2.2. Feature Detection;
};

Both Window's isSecureContext and WorkerGlobalScope's isSecureContext attributes' getters return true if the relevant settings object for the getter’s global object is a secure context, and false otherwise.

3. Algorithms

3.1. Is settings object a secure context?

Given a settings object (settings object) this algorithm returns "Secure" if the object represents a context which the user agent obtained via a secure channel, and "Not Secure" otherwise.

1. If settings object’s global object is a WorkerGlobalScope, then:

   1. For each Document (document) in settings object’s global object’s list of the worker’s Documents:

      1. If document’s relevant settings object is not a secure context, return "Not Secure".

2. Otherwise, settings object’s global object is a Window. If settings object’s responsible document has a creator Document, creator:

   1. If creator’s browsing context is an ancestor browsing context or opener browsing context of settings object’s responsible document and creator’s relevant settings object is not a secure context, return "Not Secure".

      Note: Since we take account of auxiliary browsing contexts' status, a popups' status depend on how it is opened, as discussed in §1.1 Top-level Documents.

3. If settings object’s HTTPS state is "modern", return "Secure".

   Most of the time, this check will be enough to determine whether a particular context was securely delivered. Documents delivered over TLS will have their HTTPS state set, and srcdoc Documents inherit their ancestor’s HTTPS state (as do other kinds of requests which inherit their requestor’s origin: see the basic fetch algorithm for details on some of these [FETCH]).

   We only continue past this check in order to allow resources delivered from "trustworthy" but unauthenticated locations like http://127.0.0.1/ to be treated as creating a secure context.

4. Let origin be settings object’s origin.

5. If origin is an opaque identifier, set origin to the origin of settings object’s creation URL.

   Note: We use the origin of the URL here because sandboxed content that is treated as being in a unique origin (e.g. <iframe sandbox src="http://localhost/">) would be treated as non-trustworthy by §3.2 Is origin potentially trustworthy?. Since sandboxing is a strict reduction in the content’s capabilities, and therefore in the risk it poses, we look at the origin of its URL to determine whether we would have considered it trustworthy had it not been sandboxed.

6. If the result of executing the §3.2 Is origin potentially trustworthy? algorithm on origin is not Potentially Trustworthy, then return "Not Secure".

7. Return "Secure".

3.2. Is origin potentially trustworthy?

A potentially trustworthy origin#potentially-trustworthy-originReferenced in:1.2. Framed Documents1.3. Web Workers is one which a user agent can generally trust as delivering data securely.

This algorithms considers certain hosts, scheme, and origins as potentially trustworthy, even though they might not be authenticated and encrypted in the traditional sense. In particular, the user agent SHOULD treat file URLs and URLs with hostnames names equivalent to localhost as potentially trustworthy. In principle the user agent could treat local files and local web servers as untrustworthy, but, given the information that is available to the user agent at runtime, the resources appear to have been transported securely from disk to the user agent. Additionally, treating such resources as potentially trustworthy is convenient for developers building an application before deploying it to the public.

This developer-friendlyness is not without risk, however. User agents which prioritize security over such niceties MAY choose to more strictly assign trust in a way which excludes localhost. and file.

On the other hand, the user agent MAY choose to extend this trust to other, vendor-specific URL schemes like app: or chrome-extension: which it can determine a priori to be trusted (see §7.1 Packaged Applications for detail).

Given an origin (origin), the following algorithm returns "Potentially Trustworthy" or "Not Trustworthy" as appropriate.

1. If origin is an opaque identifier, return "Not Trustworthy".

2. If origin’s scheme is either "https" or "wss", return "Potentially Trustworthy".

   Note: This is meant to be analog to the a priori authenticated URL concept in [MIX].

   Note: The origin of blob: and filesystem: URLs is the origin of the context in which they were created. Therefore, blobs created in a trustworthy origin will themselves be potentially trustworthy. The origin of data: and javascript: URLs, on the other hand, is an opaque identifier, which will not be considered potentially trustworthy.

3. If origin’s host component is or falls within "localhost." [RFC6761], return "Potentially Trustworthy".

4. If origin’s host component matches one of the CIDR notations 127.0.0.0/8 or ::1/128 [RFC4632], return "Potentially Trustworthy".

5. If origin’s scheme component is file, return "Potentially Trustworthy".

6. If origin’s scheme component is one which the user agent considers to be authenticated, return "Potentially Trustworthy".

   Note: See §7.1 Packaged Applications for detail here.

7. If origin has been configured as a trustworthy origin, return Potentially Trustworthy".

   Note: See §7.2 Development Environments for detail here.

8. Return "Not Trusted".

4. Threat models and risks

This section is non-normative.

4.1. Threat Models

Granting permissions to unauthenticated origins is, in the presence of a network attacker, equivalent to granting the permissions to any origin. The state of the Internet is such that we must indeed assume that a network attacker is present. Generally, network attackers fall into 2 classes: passive and active.

4.1.1. Passive Network Attacker

A "Passive Network Attacker" is a party who is able to observe traffic flows but who lacks the ability or chooses not to modify traffic at the layers which this specification is concerned with.

Surveillance of networks in this manner "subverts the intent of communicating parties without the agreement of these parties" and one "cannot defend against the most nefarious actors while allowing monitoring by other actors no matter how benevolent some might consider them to be." [RFC7258] Therefore, the algorithms defined in this document require mechanisms that provide for the privacy of data at the application layer, not simply integrity.

4.1.2. Active Network Attacker

An "Active Network Attacker" has all the capabilities of a "Passive Network Attacker" and is additionally able to modify, block or replay any data transiting the network. These capabilities are available to potential adversaries at many levels of capability, from compromised devices offering or simply participating in public wireless networks, to Internet Service Providers indirectly introducing security and privacy vulnerabilities while manipulating traffic for financial gain ([VERIZON] and [COMCAST] are recent examples), to parties with direct intent to compromise security or privacy who are able to target individual users, organizations or even entire populations.

4.2. Ancestral Risk

The §3.1 Is settings object a secure context? algorithm walks through all the ancestors of a particular context in order to determine whether or not the context itself is secure. Why wouldn’t we consider a securely-delivered document in an iframe to be secure, in and of itself?

The short answer is that this model would enable abuse. Chrome’s implementation of [WEBCRYPTOAPI] was an early experiment in locking APIs to secure contexts, and it does not walk through a context’s ancestors. The assumption was that locking the API to a resouce which was itself delivered securely would be enough to ensure secure usage. The result, however, was that entities like Netflix built iframe- and postMessage()-based shims that exposed the API to non-secure contexts. The restriction was little more than a speed-bump, slowing down non-secure access to the API, but completely ineffective in preventing such access.

While the algorithms in this document do not perfectly isolate non-secure contexts from secure contexts (as discussed in §5.1 Incomplete Isolation), the ancestor checks provide a fairly robust protection for the guarantees of authentication, confidentiality, and integrity that such contexts ought to ptovide.

4.3. Risks associated with non-secure contexts

Certain web platform features that have a distinct impact on a user’s security or privacy should be available for use only in secure contexts in order to defend against the threats above. Features available in non-secure contexts risk exposing these capabilities to network attackers:

1. The ability to read and modify sensitive data (personally-identifying information, credentials, payment instruments, and so on). [CREDENTIAL-MANAGEMENT] is an example of an API that handles sensitive data.
2. The ability to read and modify input from sensors on a user’s device (camera, microphone, and GPS being particularly noteworthy, but certainly including less obviously dangerous sensors like the accelerometer). [GEOLOCATION-API] and [MEDIACAPTURE-STREAMS] are historical examples of features that use sensor input.
3. The ability to access information about other devices to which a user has access. [DISCOVERY] and [BLUETOOTH] are good examples.
4. The ability to track users using temporary or persistent identifiers, including identifiers which reset themselves after some period of time (e.g. sessionStorage), identifiers the user can manually reset (e.g. [ENCRYPTED-MEDIA], Cookies [RFC6265], and [IndexedDB]), as well as identifying hardware features the user can’t easily reset.
5. The ability to introduce some state for an origin which persists across browsing sessions. [SERVICE-WORKERS] is a great example.
6. The ability to manipulate a user agent’s native UI in some way which removes, obscures, or manipulates details relevant to a user’s understanding of their context. [FULLSCREEN] is a good example.
7. The ability to introduce some functionality for which user permission will be required.

This list is non-exhaustive, but should give you a feel for the types of risks we should consider when writing or implementing specifications.

Note: While restricting a feature itself to secure contexts is critical, we ought not forget that facilities that carry such information (such as new network access mechanisms, or other generic functions with access to network data) are equally sensitive.

5. Security Considerations

5.1. Incomplete Isolation

The secure context definition in this document does not completely isolate a "secure" view on an origin from a "non-secure" view on the same origin. Exfiltration will still be possible via increasingly esoteric mechanisms such as the contents of localStorage/sessionStorage, storage events, BroadcastChannel, and others.

6. Privacy Considerations

The secure context definition in this document does not in itself have any privacy impact. It does, however, enable other features which do have interesting privacy implications to lock themselves into contexts which ensures that specific guarantees can be made regarding integrity, authenticity, and confidentiality.

From a privacy perspective, specification authors are encouraged to consider requiring secure contexts for the features they define.

7. Implementation Considerations

7.1. Packaged Applications

A user agent that support packaged applications MAY whitelist specific URL schemes whose contents are authenticated by the user agent. For example, FirefoxOS application resources are referred to by a URL whose scheme component is app:. Likewise, Chrome’s extensions and apps live on chrome-extension: schemes. These could reasonably be considered trusted origins.

7.2. Development Environments

In order to support developers who run staging servers on non-loopback hosts, the user agent MAY allow users to configure specific sets of origins as trustworthy, even though §3.2 Is origin potentially trustworthy? would normally return Not Trusted.

7.3. Restricting New Features

This section is non-normative.

When writing a specification for new features, we recommend that authors and editors guard sensitive APIs with checks against secure contexts. For example, something like the following might be a good approach:

Authors should also ensure that sensitive APIs are only exposed to secure contexts by guarding them with the [SecureContext] attribute:

[SecureContext]
interface SensitiveFeature {
  Promise<double> getTheSecretDouble();
};

// Or:

interface AnotherSensitiveFeature {
  [SecureContext] void doThatPowerfulThing();
};

7.4. Restricting Legacy Features

This section is non-normative.

The list above clearly includes some existing functionality that is currently available to the web over non-secure channels. We recommend that such legacy functionality begin requiring a secure context as quickly as is reasonably possible.

1. If such a feature is not widely implemented, we recommend that the specification be immediately modified to include a restriction to secure contexts.

2. If such a feature is widely implemented, but not yet in wide use, we recommend that it be quickly restricted to secure contexts by adding a check as described in §7.3 Restricting New Features to existing implementations, and modifying the specification accordingly.

3. If such a feature is in wide use, we recommend that the existing functionality be deprecated; the specification should be modified to note that it does not conform to the restrictions outlined in this document, and a plan should be developed to both offer a conformant version of the feature and to migrate existing users into that new version.

7.4.1. Example: Geolocation

The [GEOLOCATION-API] is a good concrete example of such a feature; it is widely implemented and used on a large number of non-secure sites. A reasonable path forward might look like this:

1. Modify the specification to include checks against secure context before executing the algorithms for getCurrentPosition() and watchPosition().

   If the incumbent settings object is not a secure context, then the algorithm should be aborted, and the errorCallback invoked with a code of PERMISSION_DENIED.

2. The user agent should announce clear intentions to disable the API for non-secure contexts on a specific date, and warn developers accordingly (via console messages, for example).

3. Leading up to the flag day, the user agent should announce a deprecation schedule to ensure both that site authors recognize the need to modify their code before it simply stops working altogether, and to protect users in the meantime. Such a plan might include any or all of:

   1. Disallowing persistent permission grants to non-secure origins

   2. Coarsening the accuracy of the API for non-secure origins (perhaps consistently returning city-level data rather than high-accuracy data)

   3. UI modifications to inform users and site authors of the risk

8. Acknowledgements

This document is largely based on the Chrome Security team’s work on [POWERFUL-NEW-FEATURES]. Chris Palmer, Ryan Sleevi, and David Dorwin have been particularly engaged. Anne van Kesteren, Jonathan Watt, Boris Zbarsky, and Henri Sivonen have also provided very helpful feedback.