PostCSS has XSS via Unescaped in its CSS Stringify Output

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

6.1 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

EPSS

• CVE-2026-41305: 0.21% estimated exploitation activity in the next 30 days; percentile 10.51 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

PostCSS: XSS via Unescaped </style in CSS Stringify Output Summary PostCSS v8.5.5 (latest) does not escape </style sequences when stringifying CSS ASTs. When user-submitted CSS is parsed and re-stringified for embedding in HTML <style tags, </style in CSS values breaks out of the style context, enabling XSS. Proof of Concept Tested output (Node.js v22, postcss v8.5.5): Impact Impact non-bundler use cases since bundlers for XSS on their own. Requires some PostCSS plugin to have malware code, which can inject XSS to website. Suggested Fix Escape </style in all stringified output values: Credits Discovered and reported by Sunil Kumar (@TharVid)

Published by the source

24 Apr 2026, 15:31 UTC

Last modified by the source

10 Sep 2026, 03:51 UTC

Source last checked

26 Oct 2026, 12:48 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-qx2v-qp2m-jg93

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-qx2v-qp2m-jg93

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# Patch Next advisory brief: CVE-2026-41305

Advisory: PostCSS has XSS via Unescaped in its CSS Stringify Output
IDs: CVE-2026-41305, GHSA-qx2v-qp2m-jg93
Record: https://patchnext.com/advisories/cve-2026-41305 (revision 2, source checked 26 Oct 2026, 12:48 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: postcss 8.5.10. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm postcss: affected < 8.5.10; fixed in 8.5.10

## Suggested inspection (read-only)
- `npm ls postcss --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 postcss.
- 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-qx2v-qp2m-jg93 (OSV record GHSA-qx2v-qp2m-jg93 lists affected ranges: npm postcss >=0 <8.5.10 (fixed))
- Verified: https://github.com/advisories/GHSA-qx2v-qp2m-jg93 (GitHub-reviewed advisory GHSA-qx2v-qp2m-jg93 lists affected ranges: npm postcss >=0 <8.5.10)

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