Privileged 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 bar. This document outlines threat models for feature abuse on the Web and outline normative requirements which should be incorporated into documents specifying new features.

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

2. Key Concepts and Terminology

privileged context

A settings object is considered a privileged context if the algorithm defined in §5.1 Is settings object a privileged context? returns Privileged when executed upon it. Moreover:

• A Document is considered a privileged document if the algorithm defined in §5.1 Is settings object a privileged context? returns Privileged when executed upon its incumbent settings object.
• A Worker, SharedWorker, or ServiceWorker is considered a privileged worker if the algorithm defined in §5.1 Is settings object a privileged context? returns Privileged when executed upon its incumbent settings object.

embedding document

Given a Document A, the embedding document of A is the Document through which A’s browsing context is nested.

3. Should [insert feature here] require a privileged context?

This section is non-normative.

Certain web platform features that have a distinct impact on a user’s security or privacy should be available for use only in privileged contexts.

Broadly speaking, we consider a feature powerful enough to restrict when it fits into one or more of the following categories:

1. The feature provides access to sensitive data (personally-identifying information, credentials, payment instruments, and so on). [CREDENTIAL-MANAGEMENT] is an example of such an API.
2. The feature provides access to sensor data 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.
3. The feature provides access to or information about other devices a user has access to. [DISCOVERY] and [BLUETOOTH] are good examples.
4. The feature exposes 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 feature introduces some state for an origin which persists across browsing sessions. [SERVICE-WORKERS] is a great example.
6. The feature manipulates a user agent’s native UI in some way which removes, obscures, or manipulates details relevant to a user’s understanding of her context. [FULLSCREEN], for instance.
7. The feature introduces some functionality for which user permission will be required.

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

Note: While restricting the feature itself to privileged 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.

3.1. 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.

3.1.1. 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 mechansims that provide for the privacy of data at the application layer, not simply integrity.

3.1.2. 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. Restricting Features

4.1. New Features

When writing a specification for new features, we recommend that authors and editors guard sensitive APIs with checks against §5.1 Is settings object a privileged context? . For example, something like the following would be a good approach:

4.2. Legacy Features

The list above clearly includes some existing functionality that is currently available to the web over insecure channels. We recommend that such legacy functionality begin requiring a privileged 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 privileged contexts.
2. If such a feature is widely implemented, but not yet in wide use, we recommend that it be quickly restricted to privileged contexts by adding a check as described in §4.1 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.

4.2.1. Example: Geolocation

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

1. Modify the specification to include checks against [[#settings-privileged] before executing the algorithms for getCurrentPosition() and watchPosition().

   If §5.1 Is settings object a privileged context? returns Not Privileged, then the algorithms should be aborted, and the errorCallback invoked with a code of PERMISSION_DENIED.

2. User agents should announce clear intentions to disable the API for unprivileged contexts on a specific date, and warn developers accordingly (via console messages, for example).
3. Leading up to the flag day, user agents 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:
   • Disallowing persistent permission grants to insecure origins
   • Coarsening the accuracy of the API for insecure origins (perhaps consistently returning city-level data rather than high-accuracy data)
   • UI modifications to inform users and site authors of the risk

5. Algorithms

6. Implementation Considerations

6.1. Packaged Applications

User agents 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.

6.2. Development Environments

In order to support developers who run staging servers on non-loopback hosts, user agents MAY allow users to configure specific sets of origins as trustworthy, even though §5.2 Is origin potentially trustworthy? would normally return Not Trusted.

7. 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 and Henri Sivonen have also provided very helpful feedback.