Iframe embedding over the network is disabled by default
Storybook no longer allows the manager or preview to be embedded as an iframe on a website with a different host name. Built versions of Storybook are unaffected and can be freely embedded.
If you rely on embedding a Storybook dev server through a separate web app, run that separate app on localhost. You may also use the security.insecureRemoteIframeEmbed feature flag to re-enable iframe embedding from any origin.
The preview iframe rejects messages sent by an unknown host
Both the Storybook preview and manager now restrict the origin of iframes and parent windows that they can send messages to and receive messages from using the postMessage API.
By default, Storybook in dev mode only accepts communication with its own origin and other localhost services. Built Storybooks continue to accept communication from unknown origins.
A Storybook addon or integration could hypothetically rely on postMessage to update a live story's args. If you do that, reach out to the Storybook team to explore safer alternatives.
Storybook composition also relies on postMessage, so a composed Storybook on a different origin now fails to load unless it has been built. The security.insecureIframeMessaging feature flag disables this protection, but we recommend you build composed Storybooks instead.
The highlight event signature has changed
Storybook’s own accessibility addon uses the highlight event to inject highlights into the preview, which a user can click. Clicking the highlight opens a menu with a link to a documentation page.
The API for that event was too permissive and could be used to trigger arbitrary events, including events processed by the user’s dev server. We reworked the API to prevent abuse. Besides the accessibility addon, Chromatic's Visual Tests addon uses this API to link to documentation from highlighted ignored elements. Update the Visual Tests addon to version 5.5.1 or later to keep that link working. Because no other addons use this API as far as we know, we have not added a feature flag to restore the old API.
Disclosure timeline
On September 30th, the Storybook team received a responsible disclosure showing how a malicious website can embed a Storybook dev server's preview iframe and send events to the dev server on the developer's machine.
They combined this information with findings from an ongoing security audit to come up with a worst-case-scenario exploitation of the disclosed vulnerability, and the ones we independently discovered. They then worked to address all layers necessary for this worst-case scenario to happen. We have not received evidence of exploitation in real Storybook projects. The attack outlined in this advisory relies on correctly exploiting a sequence of many vulnerabilities, some of which were never disclosed to the Storybook team.
Worst-case attack scenario
An attacker-controlled website can interact with the preview of a running Storybook dev server. With user interaction, it may cause the dev server to make unauthorized changes to story files. If a malicious file is subsequently executed by a local runtime, the attack could lead to arbitrary code execution with the developer’s permissions.