Summary: A malicious website may be able to cause unauthorized file changes in a developer’s project while an affected Storybook development server is running. Under additional conditions, a changed file could be executed by a local runtime with the developer’s privileges. We are not aware of exploitation in the wild.

Who is impacted?

You are impacted if you run a Storybook dev server in the affected versions.

You are not impacted if:

- You run a dev server that you never open in a web browser (e.g. headless automation, MCP clients) - You run or visit a built version of Storybook

Recent versions of Chrome and Firefox may ask for permission before a website can connect to a localhost service. This reduces the risk, but does not remove it. Do not grant this permission to websites you don’t trust.

Recommended actions

Upgrade Storybook

Storybook has released patched versions of all affected major Storybook versions that fix all the vulnerabilities necessary for the worst-case scenario above to occur.

Upgrade to a patched version listed below before running storybook dev again. If you cannot upgrade immediately, do not run an affected Storybook dev server until you can apply the fix.

- 11.0.0-alpha.6 - 10.6.2 - 9.1.21 - 8.6.19

Review potentially breaking changes

Due to the exceptional nature of the reported vulnerabilities, the Storybook team has decided to change the default behavior of Storybook, including versions 8 and 9, for several network-related features that are not essential to most Storybook users' workflow. If your team relies on the affected features, we've added feature flags that can disable specific security updates to support your existing workflows.

Please carefully read the introduced flags and the consequences of using them.

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.

Storybook is a frontend workshop for building user interface components and pages in isolation. Prior to versions 7.6.23, 8.6.17, 9.1.19, and 10.2.10, the WebSocket functionality in Storybook's dev server, used to create and update stories, is vulnerable to WebSocket hijacking. This vulnerability only affects the Storybook dev server; production builds are not impacted. Exploitation requires a developer to visit a malicious website while their local Storybook dev server is running. Because the WebSocket connection does not validate the origin of incoming connections, a malicious site can silently send WebSocket messages to the local instance without any further user interaction. If the Storybook dev server is intentionally exposed publicly (e.g. for design reviews or stakeholder demos) the risk is higher, as no malicious site visit is required. Any unauthenticated attacker can send WebSocket messages to it directly. The vulnerability affects the WebSocket message handlers for creating and saving stories. Both are vulnerable to injection via unsanitized input in the componentFilePath field, which can be exploited to achieve persistent XSS or Remote Code Execution (RCE). Versions 7.6.23, 8.6.17, 9.1.19, and 10.2.10 contain a fix for the issue.

Summary

The WebSocket functionality in Storybook's dev server, used to create and update stories, is vulnerable to WebSocket hijacking. This vulnerability only affects the Storybook dev server; production builds are not impacted.

Details

Exploitation requires a developer to visit a malicious website while their local Storybook dev server is running. Because the WebSocket connection does not validate the origin of incoming connections, a malicious site can silently send WebSocket messages to the local instance without any further user interaction.

If a Storybook dev server is intentionally exposed publicly (e.g. for design reviews or stakeholder demos) the risk is higher, as no malicious site visit is required. Any unauthenticated attacker can send WebSocket messages to it directly.

The vulnerability affects the WebSocket message handlers for creating and saving stories, which can be exploited via unauthorized WebSocket connections to achieve persistent XSS or Remote Code Execution (RCE).

Note: recent versions of Chrome have some protections against this, but Firefox does not.

Impact This vulnerability can lead to supply chain compromise. Key risks include:

- Remote Code Execution: The vulnerability can allow attackers to execute malicious code, with the extent of impact depending on the configuration. Server-side RCE is possible in non-default configurations, such as when stories are executed via portable stories in JSDOM, potentially allowing attackers to exfiltrate credentials and environment variables, access source code and the filesystem, establish backdoors, or pivot to internal network resources. - Persistent XSS: Malicious payloads are written directly into story source files. If the malicious payload is committed to version control, it becomes part of the codebase and can propagate to deployed Storybook documentation sites, affecting developers and stakeholders who view them. - Supply Chain Propagation: If the modified source files are committed, injected code can spread to other team members via git, execute in CI/CD pipelines, and affect shared component libraries used across multiple projects.

Affected versions

8.1 and above. While the exploitable functionality was introduced in 8.1, the patch has been applied to 7.x as a precautionary measure given the underlying WebSocket behaviour.

Recommended actions Update to one of the patched versions: 7.6.23, 8.6.17, 9.1.19, 10.2.10.

On December 11th, the Storybook team received a responsible disclosure alerting them to a potential vulnerability in certain built and published Storybooks.

The vulnerability is a bug in how Storybook handles environment variables defined in a .env file, which could, in specific circumstances, lead to those variables being unexpectedly bundled into the artifacts created by the storybook build command. When a built Storybook is published to the web, the bundle’s source is viewable, thus potentially exposing those variables to anyone with access. If those variables contained secrets, they should be considered compromised.

Who is impacted?

For a project to be vulnerable to this issue, it must:

- Build the Storybook (i.e. run storybook build directly or indirectly) in a directory that contains a .env file (including variants like .env.local) - The .env file contains sensitive secrets - Use Storybook version 7.0.0 or above - Publish the built Storybook to the web

Storybooks built without a .env file at build time are not affected, including common CI-based builds where secrets are provided via platform environment variables rather than .env files.

Users' Storybook runtime environments (i.e. storybook dev) are not affected. Deployed applications that share a repo with a project's Storybook are not affected.

Storybook 6 and below are not affected.

Recommended actions

First, Storybook recommends that everyone audit for any sensitive secrets provided via .env files and rotate those keys.

Second, Storybook has released patched versions of all affected major Storybook versions that no longer have this vulnerability. Projects should upgrade their Storybook—on both local machines and CI environments—to one of these versions before publishing again.

- 10.1.10+ - 9.1.17+ - 8.6.15+ - 7.6.21+

Finally, some projects may have been relying on the undocumented behavior at the heart of this issue and will need to change how they reference environment variables after this update. If a project can no longer read necessary environmental variable values, it can either prefix the variables with STORYBOOK or use the env property in Storybook’s configuration to manually specify values. In either case, do not include sensitive secrets as they will be included in the built bundle.

Further information

Details of the vulnerability can be found on the Storybook announcement.