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Low, CVSS 1.1amended

Sveltia CMS: Stored XSS in Markdown/RichText preview via unsandboxed same-origin iframe

GHSA-h5jc-78hr-3pc9

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (@sveltia/cms 0.167.3), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
1.1 · CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N/E:U · score source
EPSS
No dated EPSS signal available.
Source description

As published by the source; long descriptions are shortened.

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 /…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-h5jc-78hr-3pc9

  2. Revision 2 · amended ·

    Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

  3. Revision 1 · created ·

    Published from osv record GHSA-h5jc-78hr-3pc9

Agent brief

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: GHSA-h5jc-78hr-3pc9

Advisory: Sveltia CMS: Stored XSS in Markdown/RichText preview via unsandboxed same-origin iframe
IDs: GHSA-h5jc-78hr-3pc9
Record: https://patchnext.com/advisories/ghsa-h5jc-78hr-3pc9 (revision 3, source checked 26 Oct 2026, 12:56 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 1.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @sveltia/cms 0.167.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @sveltia/cms: affected < 0.167.3; fixed in 0.167.3

## Suggested inspection (read-only)
- `npm ls @sveltia/cms --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on @sveltia/cms.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-h5jc-78hr-3pc9 (OSV record GHSA-h5jc-78hr-3pc9 lists affected ranges: npm @sveltia/cms >=0 <0.167.3 (fixed))
- Verified: https://github.com/advisories/GHSA-h5jc-78hr-3pc9 (GitHub-reviewed advisory GHSA-h5jc-78hr-3pc9 lists affected ranges: npm @sveltia/cms >=0 <0.167.3)

Commands are suggestions: review each one before running it.

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