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Securer Contexts

Mike West, Feb 2020 (©2020, Google)

A Problem

In 2015, we worked together to define a minimum bar for websites' security. At the time, the threats we were most concerned about mitigating required limiting the capabilities of active and passive network attackers. To that end, we defined a "secure context" that enforced security at the transport layer by ensuring that a given document and its ancestors were securely delivered. We've very successfully parlayed that definition into [SecureContext] requirements for a broad swath of important APIs, ensuring that they're available only when they're safely out of the hands of everyone on the network between the user and that site.

Focusing the threat model on the transport layer made a good deal of sense several years ago, while we were in the midst of a large-scale migration from plaintext HTTP to encrypted HTTPS. That migration has been wildly successful, with the vast majority of page views and time spent on sites delivered securely.

Today, it seems clear that encryption is no longer our primary focus. While we can't quite declare victory, encryption has become something we can simply assume for modern applications, rather than something for which we must actively advocate. It seems reasonable, then, to step back and revisit our conception of what makes a site secure-enough, with an eye toward the next set of things that we'd like web developers to begin doing in the service of protecting their users and themselves.

A Proposal

Secure Contexts' threat model should extend beyond encrypting the transport layer, and bring attention to application layer threats that rely on either injection or insufficient isolation. Concretely, the model ought to include attackers who can:

  1. Passively observe network traffic flows on the one hand; or actively modify, block, or replay traffic flows on the other. The risks these attackers pose is well-understood, and forms the core of the existing Secure Context restrictions.

  2. Cause a server to "reflect" unexpected content directly into the body of any given response, or manipulate the inputs to client-side code (DOM APIs and otherwise), potentially leading to unexpected script execution.

  3. Obtain references to a victim's window, which provides a postMessage() channel, and insight into the victim's state (via window.frames and friends).

  4. Include a victim's resources in an attacker-controlled context, which creates opportunities to read the victim's data via clever exploitation of side-channels.

Mitigations for these threats seem to break down into threeish categories: encrypted transport, new isolation primitives (COOP, COEP, CORP, etc) and defenses against unintentional script execution (CSP, Trusted Types, etc.). I'd suggest that we support selective application of these diverse mitigations by adding arguments to the declaration, e.g. [SecureContext=(Transport,Isolation,Injection)].


Parameterizing [SecureContext] is strange.

That's not a question.

Fine. Parameterizing [SecureContext] is strange, isn't it?

Adding parameters to [SecureContext] is not my first choice. It would be ideal to simply upgrade the restrictions we apply when [SecureContext] is present, and call it a day. Unfortunately, this would break existing applications that rely upon APIs gated on that attribute, which is probably not something we ought to do. I think we need something that will allow us to apply these new restrictions incrementally.

An approach seems appealing is to rename the existing attribute to something like [SecureTransport]. Running sed -i '' -e 's/SecureContext/SecureTransport/g' over all the specifications would thereby make room for a redefinition of [SecureContext], and we can decide together which APIs require that higher standard, giving spec authors and developers a clear story about the bar they need to meet.

Setting that bar, however, is a challenge, because I see several bars that might be relevant.

I expect us to be able to agree relatively easily on the set of APIs that require isolation defense, as user agents are starting to align on a post-Spectre threat model and a set of mitigations that address it. I'm hopeful that we'll be working towards defaulting the web experience to requiring COOP/COEP/CORP assertions; attaching those restrictions to specific APIs as stepping-stones along the way seems unobjectionable. Injection mitigation is unfortunately trickier.

I'm quite interested in requiring developers to do some work to defend themselves against injection attacks, especially in the presence of APIs that grant access to things that are hard to reason about in the web's general origin-based security model (here, I'm thinking of device-bound capabilities, like WebUSB, clipboard, etc). This is a real line-drawing problem that I think we'll need to work out together, but I expect that we'll collectively decide that this set of APIs is as extensive as the set of APIs that should require isolation, which itself isn't as extensive as the set of APIs that should require secure transport.

Given these distinctions, and given the apparent difficulty of deploying robust defenses against injection attacks, I'm not sure it makes sense to have a single definition that we aspire to for everything. Today, at least, more nuance seems valuable, and a distinction between injection and isolation seems reasonable to encode into our understanding of secure context restrictions.

Assuming we agree that that's reasonable, I see two broad approaches to supporting that nuance:

  1. Separating the mitigation categories into distinct attributes (e.g. [X-Bikeshed-Transport], [X-Bikeshed-Isolation], [X-Bikeshed-Injection]).

  2. Binding the categories to a single attribute, as in [SecureContext=(Transport,Isolation,Injection)].

The differences between these spellings seem largely aesthetic. I like that the latter binds all the categories to the single [SecureContext] attribute, which makes it clear to me that they all are taken into account when deciding whether their subject is exposed. I like that the former would make it possible for us to change the spelling of the transport restriction to an opt-out rather than an opt-in (e.g. we'd mark everything that currently lacks a [SecureContext] attribute with the inverted [UnsafelyAllowedViaNonsecureTransport]), which could have some interesting impacts on where we set the burden of proof in the conversation around API exposure.

Currently, I prefer the latter. I look forward to suggestions for alternatives, as I'm sure the approaches I've outlined above are not exhaustive.

What defenses would [SecureContext=Isolation] require?

Pages would need to assert Cross-Origin-Opener-Policy: same-origin and Cross-Origin-Embedder-Policy: require-corp. This would have the effect of requiring their dependencies to assert an appropriate Cross-Origin-Resource-Policy.

What APIs would require [SecureContext=Isolation]?

Certainly SharedArrayBuffer and performance.{measureMemory(),now()}. Probably future paint timing APIs, and others we'll come up with over time. Ideally we'd make these the default behavior, but we need some implementation experience before pushing for that seems reasonable.

In the short term, we could just call it a day by marking window.performance as [SecureContext=Transport,Isolation] to avoid line-drawing arguments and appeals to the status quo, but I suspect that will draw some pitchforks... so, let's decide how much we want to be yelled at, shall we?

What defenses would [SecureContext=Injection] require?

An excellent question! I think the requirements we'd start with would be along the lines of asserting a Content-Security-Policy header (note: not via <meta>) that restricts object-src, base-uri, and script-src in ways that more or less align with StrictCSP. This would boil down to a policy like that tracked by Chromium's kCSPWithReasonableRestrictions.

It would be nice to pull things like Trusted Types in here as well. Let's see how that looks as we agree upon the final details of an MVP, and get it out the door.

TODO(mkwst): Write down something about an opt-out along the lines of the proposals in, and/or an alternative baseline of "You have a CSP, and it has a script-src." (And note that I prefer the former, as the latter will break if we ever want to raise the bar.)

What APIs would require [SecureContext=Injection]?

Another excellent question! As noted above, I'm thinking about APIs that grant access to things that are hard to reason about in the web's general origin-based security model. Device-bound capabilities seem reasonable to consider, like WebUSB, clipboard, etc. Perhaps this could extend to "Anything behind a user permission (e.g. geolocation, sensors, camera/microphone, etc)."

What about document.domain and other web weirdnesses?

document.domain itself is neutered by [SecureContext=Isolation] via the same-origin restrictions in the COOP/COEP mechanisms that enforce isolation more generally. This is instructive, as I'd like to make sure that we're imposing requirements that tie back directly to the threat model discussed above. There are a number of other things that I would love to deprecate, and this might be a reasonable way of doing so, but it will be simpler to roll out if folks don't have to squint too hard to understand why a given restriction is in place. [SecureContext] is something of a sledgehammer, and it's not going to be appropriate for all deprecations (even ones that really annoy me personally, like MIME sniffing!).


Secure Contexts, but with _more_ secureness!






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