Handlebars: JavaScript Injection via Unsafe Inline Embedding of Precompiled Templates
Summary
Handlebars.precompile() generates JavaScript source that is commonly embedded in browser <script> elements. Before the fix, static template text containing </script> was emitted unchanged. HTML parsers recognize </script> even inside a JavaScript string literal, closing the surrounding script element and allowing following attacker-controlled markup to be parsed and executed.
This affects applications that precompile attacker-controlled templates and embed the generated source directly in an HTML <script> element. It does not affect ordinary server-side rendering or precompiled templates delivered as external JavaScript files.
Details
Static text is serialized by quotedString() in lib/handlebars/compiler/code-gen.js. The generated JavaScript is valid, but JavaScript quoting alone does not make it safe to embed in HTML. In HTML script data, the sequence </script> terminates the element regardless of JavaScript string context.
On affected releases, this template:
safe</script><script>alert("XSS")</script><script>could produce generated source containing:
return 'safe</script><script>alert("XSS")</script><script>';When included inline in an HTML document, the first </script> closes the script containing the precompiled template. The next <script> element is then parsed as HTML and executes.
Proof of Concept
const Handlebars = require('handlebars');
const template = 'safe</script><script>alert("XSS")</script><script>';
const output = Handlebars.precompile(template);
console.log(output.includes('</script>'));Affected versions print true. Embedding output directly in an inline <script> element allows the injected script tag to be parsed by the browser.
Workarounds
- Do not inline precompiled output from untrusted templates into HTML documents.
- Serve generated precompiled templates as external JavaScript files where practical.
Credits
Reported by Curly-Haired-Baboon Aka Laplas
Czego dotyczy
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| Platforma | Paczka | Podatne wersje | Poprawka w wersji |
|---|---|---|---|
| Paczka npm | handlebars | >= 4.0.0, < 4.7.10 | 4.7.10 |
Odnośniki
- https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-xw65-4hp5-5hc7
- https://nvd.nist.gov/vuln/detail/CVE-2026-106444
- https://github.com/handlebars-lang/handlebars.js/pull/2185
- https://github.com/handlebars-lang/handlebars.js/commit/609d1b11c833c9a3e00f56f2f34d22f425446725
- https://github.com/handlebars-lang/handlebars.js
- https://github.com/handlebars-lang/handlebars.js/releases/tag/v4.7.10
- Wektor CVSS
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N- Słabość (CWE)
CWE-116- Platforma
- npm
- Paczka
handlebars- Ujawniono
- 08.10.2026
- Aktualizacja
- 11.10.2026 10:22
Źródła i licencje
OSV / GitHub Advisory Database - GHSA-xw65-4hp5-5hc7 - CC BY 4.0
Historia podatności: handlebars
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