There is standardization work ongoing that enables developers to create
SharedArrayBuffer objects again, but changes are needed in order to be use these across threads (i.e.,
SharedArrayBuffer objects throws by default). These changes provide further isolation between sites and help reduce the impact of attacks with high-resolution timers, which can be created with shared memory.
Firefox's Nightly release from 73 onward has an experimental implementation of the features described in this document enabled by default.
Chrome intends to implement similar restrictions.
New HTTP header bonanza
As a baseline requirement, documents will need to be in a secure context.
For top-level documents, two headers will need to be set:
same-originas value (protects your origin from attackers)
require-corpas value (protects victims from your origin)
With these two headers set,
postMessage() will no longer throw for
SharedArrayBuffer objects and shared memory across threads is therefore available.
Nested documents and dedicated workers will need to set the
Cross-Origin-Embedder-Policy header as well with the same value. No further changes are needed for same-origin nested documents and subresources. Same-site (but cross-origin) nested documents and subresources will need to set the
Cross-Origin-Resource-Policy header with
same-site as value. And their cross-origin (and cross-site) counterparts need to set the same header with
cross-origin as value. Note that setting the
Cross-Origin-Resource-Policy header to any other value than
same-origin opens up the resource to potential attacks, such as Spectre.
Note that the
Cross-Origin-Opener-Policy header limits your ability to retain a reference to popups. Direct access between two top-level window contexts will essentially only work if they are same-origin and carry the same two headers with the same two values.
As a result of this newly required environment, there are a couple API implications:
Atomicsobject is always available.
SharedArrayBufferobjects are in principle always available, but unfortunately the constructor on the global object is hidden, unless the two headers mentioned above are set, for compatibility with web content. There is hope that this restriction can be removed in the future.
WebAssembly.Memorycan still be used to get an instance.
- Unless the two headers mentioned above are set, the various
postMessage()APIs will throw for
SharedArrayBufferobjects. If they are set,
Windowobjects and dedicated workers will function and allow for memory sharing.
- To avoid having to check whether
self.crossOriginIsolatedis being standardized (a getter that returns a boolean;
trueif the headers are set), available in window and worker contexts.
WebAssembly Shared Memory
The WebAssembly Threads proposal allows
WebAssembly.Memory objects to be created with a new
shared constructor flag. When this flag is set to
true, the constructed
Memory object can be shared between workers via
postMessage(), just like
SharedArrayBuffer, and the backing
buffer of the
Memory object is a
SharedArrayBuffer. Therefore, the requirements listed above for sharing a
SharedArrayBuffer between workers also apply to sharing a
The WebAssembly Threads proposal also defines a new set of atomic instructions. Just as
SharedArrayBuffer and its methods are unconditionally enabled (and only sharing between threads is gated on the new headers), the WebAssembly atomic instructions are also unconditionally allowed.
- COOP and COEP explained.
Cross-Origin-Opener-Policy: whatwg/html issue #3740, draft specification.
Cross-Origin-Embedder-Policy: whatwg/html issue #4175, draft specification.
Cross-Origin-Resource-Policy: standardized in Fetch, new
cross-originvalue is part of the
self.crossOriginIsolated: whatwg/html issue #4732, whatwg/html issue #4872, draft specification.