CVE-2026-41305: Postcss Cross-Site Scripting Vulnerability

MODERATE
Advisory Identifier: GHSA-qx2v-qp2m-jg93
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CVSS 6.1
CWE-79
Published
Advisory Summary

CVE-2026-41305 is a MODERATE (CVSS 6.1) cross-site scripting vulnerability (CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')) affecting Postcss libraries in the npm ecosystem. The issue involves postcss has xss via unescaped </style> in its css stringify output. Remediation is available by upgrading affected packages to version 8.5.10.

CVE-2026-41305 at a Glance

CVE Identifier CVE-2026-41305
GHSA Identifier GHSA-qx2v-qp2m-jg93
Severity MODERATE
CVSS v3.1 Base Score 6.1
CWE Classification CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Affected Product Postcss
Package Ecosystem npm
Published Date
Last Updated

Security Impact

CVSS: 6.1

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction Required (UI:R)
Scope Changed (S:C)
Confidentiality Low (C:L)
Integrity Low (I:L)
Availability None (A:N)

How to Fix CVE-2026-41305

To remediate this vulnerability, upgrade all affected Postcss dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

postcss fixed: 8.5.10 
npm install postcss@8.5.10
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

npm ls postcss

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-41305, inspect your project dependencies using npm:

npm ls postcss

Analyze the terminal output to locate any version of Postcss in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 8.5.10), your application is affected.

Note: The npm ls postcss command inspects dependencies declared in your package.json and lockfile; it does not perform automated runtime dynamic scanning.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting'), where the product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. In Postcss, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

# 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 ```javascript const postcss = require('postcss'); // Parse user CSS and re-stringify for page embedding const userCSS = 'body { content: "</style><script>alert(1)</script><style>"; }'; const ast = postcss.parse(userCSS); const output = ast.toResult().css; const html = `<style>${output}</style>`; console.log(html); // <style>body { content: "</style><script>alert(1)</script><style>"; }</style> // // Browser: </style> closes the style tag, <script> executes ``` **Tested output** (Node.js v22, postcss v8.5.5): ``` Input: body { content: "</style><script>alert(1)</script><style>"; } Output: body { content: "</style><script>alert(1)</script><style>"; } Contains </style>: true ``` ## 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: ```javascript output = output.replace(/<\/(style)/gi, '<\\/$1'); ``` ## Credits Discovered and reported by [Sunil Kumar](https://tharvid.in) ([@TharVid](https://github.com/TharVid))

CVE-2026-41305 Affected Postcss Versions

1 component(s)
Fixed Versions:
8.5.10
Product / Package Ecosystem Affected Versions Fixed Version
postcss
npm/postcss
npm
< 8.5.10
8.5.10

References

GitHub Advisory / Wiki ADVISORY
https://github.com/postcss/postcss/security/advisories/GHSA-qx2v-qp2m-jg93
NVD — CVE-2026-41305 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-41305
Postcss Source Repository REPOSITORY
https://github.com/postcss/postcss
Postcss Release RELEASE
https://github.com/postcss/postcss/releases/tag/8.5.10

Frequently Asked Questions

What is CVE-2026-41305?
CVE-2026-41305 is a security vulnerability in Postcss involving PostCSS has XSS via Unescaped </style> in its CSS Stringify Output under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting').
Which Postcss versions are affected?
Affected versions include releases prior to 8.5.10 across affected packages: postcss.
What version fixes CVE-2026-41305?
Upgrading to 8.5.10 (or the latest patched release for your respective branch) resolves the vulnerability.
Is postcss affected?
Yes, postcss is affected in releases prior to 8.5.10. Upgrade your dependency to 8.5.10 or later.
What is the CVSS score of CVE-2026-41305?
CVE-2026-41305 has a CVSS v3.1 base score of 6.1 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.
What CWE is associated with CVE-2026-41305?
CVE-2026-41305 is classified under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.).
How can I check npm dependencies for CVE-2026-41305?
Run the dependency tree command: 'npm ls postcss'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-qx2v-qp2m-jg93",
  "modified" : "2026-04-24T15:31:42Z",
  "published" : "2026-04-24T15:31:42Z",
  "aliases" : [ "CVE-2026-41305" ],
  "summary" : "PostCSS has XSS via Unescaped </style> in its CSS Stringify Output",
  "details" : "# PostCSS: XSS via Unescaped `</style>` in CSS Stringify Output\n\n## Summary\n\nPostCSS 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.\n\n## Proof of Concept\n\n```javascript\nconst postcss = require('postcss');\n\n// Parse user CSS and re-stringify for page embedding\nconst userCSS = 'body { content: \"</style><script>alert(1)</script><style>\"; }';\nconst ast = postcss.parse(userCSS);\nconst output = ast.toResult().css;\nconst html = `<style>${output}</style>`;\n\nconsole.log(html);\n// <style>body { content: \"</style><script>alert(1)</script><style>\"; }</style>\n//\n// Browser: </style> closes the style tag, <script> executes\n```\n\n**Tested output** (Node.js v22, postcss v8.5.5):\n```\nInput: body { content: \"</style><script>alert(1)</script><style>\"; }\nOutput: body { content: \"</style><script>alert(1)</script><style>\"; }\nContains </style>: true\n```\n\n## Impact\n\nImpact 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.\n\n## Suggested Fix\n\nEscape `</style` in all stringified output values:\n```javascript\noutput = output.replace(/<\\/(style)/gi, '<\\\\/$1');\n```\n\n## Credits\nDiscovered and reported by [Sunil Kumar](https://tharvid.in) ([@TharVid](https://github.com/TharVid))",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "postcss"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "8.5.10"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/postcss/postcss/security/advisories/GHSA-qx2v-qp2m-jg93"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-41305"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/postcss/postcss"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/postcss/postcss/releases/tag/8.5.10"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-24T15:31:42Z",
    "nvd_published_at" : "2026-04-24T03:16:11Z"
  }
}