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. As an extension of the TAG’s recommendations in [SECURING-WEB], 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

An environment settings object is considered a secure context if the algorithm in §3.1 Is the environment settings object settings a secure context? returns "Secure", and a non-secure context otherwise.

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

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();
};

2.2.3. 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 WindowOrWorkerGlobalScope {
  readonly attribute boolean isSecureContext;
};

interface GlobalSecureContext {
  readonly attribute boolean isSecureContext;
};
Window implements GlobalSecureContext;
WorkerGlobalScope implements GlobalSecureContext;

The isSecureContext attribute’s getter returns true if §3.1 Is the environment settings object settings a secure context? returns "Secure" when executed upon this global object’s relevant settings object, and false otherwise.

3. Algorithms

3.1. Is the environment settings object settings a secure context?

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

1. Let global be settings’s global object.

2. If global is a WorkerGlobalScope, then:

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

      1. Assert: Workers must be same-origin with the context that created them, so document’s relevant settings object’s origin and HTTPS state is the same as global’s relevant settings object’s origin and HTTPS state.

      2. If §3.1 Is the environment settings object settings a secure context? returns "Not Secure" when executed upon document’s relevant settings object, return "Not Secure".

   2. Return "Secure".

      Note: Given the assertion above, if we’ve reached this step, the worker must have been created from a secure context, and therefore must itself be a secure context.

3. Assert: global is a Window.

4. Let document be settings’s responsible document.

5. Return "Not Secure" if any of the following are true:

   1. document’s active sandboxing flag set contains the sandboxed secure browsing context flag.

      Note: This check is "at risk". See §2.2.1 Sandboxing for details.

   2. document has an creator browsing context (context), and context’s creator context security is "Not Secure".

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

      The 'creator context security' concept landed in WHATWG’s HTML in whatwg/html#1561, but doesn’t yet exist in W3C’s HTML. <https://github.com/w3c/html/issues/524>

      This exclusion is "at risk", as implementation is lagging, and there’s some discussion as to whether or not it can be softened while maintaining the mitigations against direct communication channels. <https://github.com/w3c/webappsec-secure-contexts/issues/42>

   3. settings’s HTTPS state is "deprecated".

   4. document’s active sandboxing flag set includes the sandboxed origin browsing context flag, and §3.3 Is url potentially trustworthy? returns "Not Trustworthy" when executed upon settings’s creation URL.

      Note: We check the creation URL here because sandboxed content that is treated as being in an opaque origin (e.g. <iframe sandbox="allow-secure-context" src="http://127.0.0.1/">) would otherwise 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.

   5. document’s active sandboxing flag set does not include the sandboxed origin browsing context flag, and §3.2 Is origin potentially trustworthy? returns "Not Trustworthy" when executed upon settings’s origin.

6. Return "Secure".

3.2. Is origin potentially trustworthy?

A potentially trustworthy origin 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 as potentially trustworthy. In principle the user agent could treat local files 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 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 origin, return "Not Trustworthy".

2. Assert: origin is a tuple origin.

3. 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].

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 Trustworthy".

Note: Neither origin’s domain nor port has any effect on whether or not it is considered to be a secure context.

3.3. Is url potentially trustworthy?

A potentially trustworthy URL is one which either inherits context from it’s creator (about:blank, about:srcdoc) or one whose origin is a potentially trustworthy origin. Given a URL (url), the following algorithm returns "Potentially Trustworthy" or "Not Trustworthy" as appropriate:

1. If url’s scheme is "data", return "Not Trustworthy".

   Note: This aligns the definition of a secure context with the de facto "data: URL as opaque origin" behavior that a majority of today’s browsers have agreed upon, rather than the de jure "data: URL inherits origin" behavior defined in HTML.

2. If url is "about:blank" or "about:srcdoc", return "Potentially Trustworthy".

3. Return the result of executing §3.2 Is origin potentially trustworthy? on url’s origin.

   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.

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 the environment settings object settings 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-1] 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-API] and [WEB-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. window.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.

5.2. localhost

Section 6.3 of [RFC6761] lays out the resolution of localhost. and names falling within .localhost. as special, and suggests that local resolvers SHOULD/MAY treat them specially. For better or worse, resolvers often ignore these suggestions, and will send localhost to the network for resolution in a number of circumstances. Given that uncertainty, this document errs on the conservative side by special-casing 127.0.0.1, but not localhost.

This carveout is "at risk", as there’s currently only one implementation, and a parallel suggestion that we resolve this in the other direction, perhaps by mandating new behavior for conforming user agents' DNS resolvers to treat the SHOULDs and MAYs of the RFCs as MUSTs. <https://github.com/w3c/webappsec-secure-contexts/issues/43>

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 Trustworthy".

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 could alternatively 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();
};

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.