How severe is GHSA-h5jc-78hr-3pc9?

No CVSS score has been assigned to GHSA-h5jc-78hr-3pc9 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by GHSA-h5jc-78hr-3pc9?

GHSA-h5jc-78hr-3pc9 affects the following packages: @sveltia/cms (npm). Ecosystems affected: npm.

How do I fix GHSA-h5jc-78hr-3pc9?

Update @sveltia/cms to 0.167.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-h5jc-78hr-3pc9 is resolved across your whole dependency graph.

How do I detect GHSA-h5jc-78hr-3pc9 in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @sveltia/cms, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate GHSA-h5jc-78hr-3pc9 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is GHSA-h5jc-78hr-3pc9 actively exploited in the wild?

No public exploit code has been indexed for GHSA-h5jc-78hr-3pc9 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

When was GHSA-h5jc-78hr-3pc9 published, and has it been updated?

GHSA-h5jc-78hr-3pc9 was published on June 19, 2026 and was last updated on September 10, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦 npm

Not in CISA KEV

Sveltia CMS: Stored XSS in Markdown/RichText preview via unsandboxed same-origin iframeGHSA-h5jc-78hr-3pc9

Fix: sveltia/sveltia-cms@8c46294

GHSA-h5jc-78hr-3pc9 is a security vulnerability in @sveltia/cms. A fix is available for @sveltia/cms — see the affected versions and patch details below.

Published

Jun 19, 2026

Updated

Sep 10, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Sep 10, 2026 · OSV.dev, FIRST.org (EPSS)

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

6other npm packages depend on this — each one inherits the vulnerability until it's patched upstream

@sveltia/cmsnpm

24Kdownloads / week

Description

Impact

A stored cross-site scripting (XSS) vulnerability affected the Markdown/RichText field preview renderer in Sveltia CMS.

The DOMPurify sanitization configuration used for Markdown previews explicitly permitted iframe elements without enforcing a sandbox attribute or restricting iframe sources. Sanitized Markdown output was then inserted into the CMS preview DOM as raw HTML. Because no sandboxing or source validation was applied, a Markdown field containing an iframe whose src pointed to a same-origin uploaded or publicly accessible HTML asset would render an unsandboxed iframe. JavaScript in the framed document could then access the parent CMS window via the browser’s same-origin policy, allowing it to read and write the parent window’s state, DOM, and browser storage, and to trigger actions in the CMS context.

The practical impact is limited in currently supported Sveltia CMS usage because the CMS is intended for a single developer or a small trusted team, and open authoring / untrusted multi-user authoring is not currently implemented. Exploitation requires the ability to place malicious content into the repository or content source that the CMS loads.

Patches

The issue has been patched by replacing the direct DOMPurify call in the Markdown preview renderer with a two-pass sanitization pipeline:

DOMPurify processes the raw HTML as before.
A post-processing step validates every iframe element in the sanitized output. Any iframe whose src does not use an https:// URL, uses a same-origin URL, uses a relative path, or uses any other non-HTTPS scheme (javascript:, data:, blob:, , etc.) is removed from the output. Surviving iframes (HTTPS cross-origin only) have a sandbox attribute enforced with at minimum allow-scripts allow-same-origin, which confines the framed document to its own origin and prevents it from accessing the parent CMS window.

Users should upgrade to Sveltia CMS v0.167.3 or later.

Workarounds

If upgrading is not immediately possible, avoid loading CMS content from untrusted authors and review Markdown field content for iframe elements pointing to same-origin or relative paths.

Administrators can also reduce exposure by limiting repository or content source write access to trusted users only.

References

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`@sveltia/cms`	all versions	0.167.3`npm install @sveltia/cms@0.167.3`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @sveltia/cms, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update @sveltia/cms to 0.167.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-h5jc-78hr-3pc9 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

### Impact A stored cross-site scripting (XSS) vulnerability affected the Markdown/RichText field preview renderer in Sveltia CMS. The DOMPurify sanitization configuration used for Markdown previews explicitly permitted `iframe` elements without enforcing a `sandbox` attribute or restricting iframe sources. Sanitized Markdown output was then inserted into the CMS preview DOM as raw HTML. Because no sandboxing or source validation was applied, a Markdown field containing an `iframe` whose src pointed to a same-origin uploaded or publicly accessible HTML asset would render an unsandboxed ifram

O3 Security · Impact-Aware SCA

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