postMessage API
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| postMessage API | |
|---|---|
| Name | postMessage API |
| Type | Web API |
| Developer | World Wide Web Consortium; WHATWG |
| First release | 2009 |
| Stable release | Living standard |
| License | Open web standards |
postMessage API
The postMessage API enables controlled cross-origin communication between browsing contexts such as iframes, windows, Web Workers, and Service Workers. Designed to address same-origin policy constraints introduced after incidents like the Morris worm and security debates in the early web, it provides an event-driven, serialized messaging channel that complements other web platform features such as Cross-Origin Resource Sharing and Content Security Policy.
Overview
postMessage provides a mechanism for one execution context to asynchronously deliver a message to another context identified by a target origin or global object reference. It operates within the browser runtime alongside APIs implemented by browser vendors including Google Chrome, Mozilla Firefox, Microsoft Edge, Apple Safari, and was coordinated through standards bodies such as the World Wide Web Consortium and the WHATWG. The API supports structured clone serialization compatible with JSON (JavaScript Object Notation) and transferable objects used in contexts like WebGL texture sharing and IndexedDB operations.
Syntax and Parameters
Typical invocation uses a method on a target global, taking a payload and an origin string or options object. Implementations accept primitive values, objects, and transferable objects like ArrayBuffer and MessagePort. Parameters commonly include: - target: a Window reference returned by methods such as Window.open or frame element contentWindow. - message: a serializable value following the structured clone algorithm originating from Brendan Eich's work on JavaScript environments and Netscape Communications history. - targetOrigin: a string specifying an exact origin such as an Uniform Resource Identifier derived origin like https://example.com, or the wildcard "*" where permitted by application policy. Many browsers extend the core signature with options enabling transfer of MessagePort objects from Channel Messaging API and optimization flags used in Chromium-based engines.
Security Considerations
Because messages cross origin boundaries, message sources and payloads must be validated to mitigate risks associated with Cross-site scripting, Cross-site request forgery, and origin spoofing techniques explored in security research by teams at Google Project Zero, Mozilla Security Engineering, and CERT Coordination Center. Implementers are encouraged to check the event.origin against a whitelist of trusted origins including entities such as GitHub, Facebook, Twitter, and private cloud providers like Amazon Web Services and Microsoft Azure. Use of Content Security Policy and strict targetOrigin values reduces attack surface; additional defenses include message signing patterns influenced by cryptography research from Ronald Rivest and Whitfield Diffie and tokenization schemes similar to OAuth 2.0 flows used by Google APIs and Facebook Platform. Historical vulnerabilities disclosed in audits by Cisco Talos and academic conferences like USENIX Security Symposium highlight the necessity of treating received data as untrusted, performing origin checks, and avoiding execution of code from messages.
Use Cases and Examples
Common use cases include embedding third-party widgets hosted by providers such as YouTube, Google Maps, PayPal, and Stripe inside host pages while maintaining origin separation; cooperating between Single-page application frames built with libraries like React (JavaScript library), Angular (web framework), Vue.js, or Svelte; delegating computation to Web Workers; and coordinating state between a Service Worker and multiple Window clients in progressive web apps leveraging APIs like Cache API and Background Sync API. Example patterns appear in integrations for Salesforce, Zendesk, and Microsoft 365 add-ins where messages carry serialized application events validated against schemas influenced by JSON Schema conventions. Transferable object examples include offloading image processing using WebGL textures in Mozilla's Servo experimental work and passing ArrayBuffers for binary protocols such as WebSocket framing.
Browser and Platform Support
postMessage is implemented across major browsers including Google Chrome, Mozilla Firefox, Microsoft Edge, Apple Safari, and mobile browsers from Android (operating system) and iOS WebKit-based engines. Server-side JavaScript environments like Node.js expose similar message-passing primitives for worker threads and cluster modules. Related platform integrations appear in hybrid frameworks such as Electron (software framework), Cordova (software), and React Native WebViews which map native messaging interfaces to the web postMessage pattern.
History and Standardization
Origins trace to work on secure cross-document messaging proposals advanced in the late 2000s by browser vendors and standards groups following debates at W3C Technical Architecture Group meetings and public drafts circulated on WHATWG mailing lists. The API stabilized through contributions from engineers at Google, Mozilla Corporation, Apple Inc., and Microsoft Corporation and was codified in living standards maintained by WHATWG and referenced in HTML5 documentation. Security incidents and academic analyses presented at conferences such as ACM Conference on Computer and Communications Security and IEEE Symposium on Security and Privacy influenced iterative refinements to guidance and best practices.