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Medium

Handlebars: JavaScript Injection via Unsafe Inline Embedding of Precompiled Templates

  • Disclosed: 08.10.2026
  • Updated: 11.10.2026 10:22

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:

handlebars
safe</script><script>alert("XSS")</script><script>

could produce generated source containing:

js
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

js
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

Affected software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
npm package handlebars >= 4.0.0, < 4.7.10 4.7.10

References

CVSS score 4.7/10
CVSS vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N
Weakness (CWE)
CWE-116
Platform
npm
Package
handlebars
Disclosed
08.10.2026
Updated
11.10.2026 10:22

Sources and licenses

OSV / GitHub Advisory Database - GHSA-xw65-4hp5-5hc7 - CC BY 4.0

Vulnerability history: handlebars

All published vulnerabilities of this package (npm package), newest first. Shows whether this is a one-off slip or a package that needs watching.

All (22) in the archive
Total entries
22
First → latest
10.2018 10.2026
Without a fix
0
Exploited in attacks
0

Severity breakdown

  • Critical 6
  • High 10
  • Medium 5
  • Low 1

Disclosures per year

Severity:
  1. Handlebars: JavaScript Injection via Unsafe Inline Embedding of Precompiled Templates this entry
    Medium · 4.7 CVE-2026-106444 >= 4.0.0, < 4.7.10 4.7.10
  2. Handlebars: JavaScript Injection via AST Type Confusion in compile (bypass of CVE-2026-33937)
    Critical · 9.8 CVE-2026-106446 >= 4.0.0, < 4.7.10 4.7.10
  3. Handlebars: JavaScript Injection via Own Property Check Bypass
    Critical CVE-2026-106445 >= 3.0.0, < 4.7.10 4.7.10
  4. Handlebars.js has JavaScript Injection in CLI Precompiler via Unescaped Names and Options
    High · 8.2 CVE-2026-33941 >= 4.0.0, < 4.7.9 4.7.9
  5. Handlebars.js has JavaScript Injection via AST Type Confusion when passing an object as dynamic partial
    High · 8.1 CVE-2026-33940 >= 4.0.0, < 4.7.9 4.7.9
  6. Handlebars.js has Denial of Service via Malformed Decorator Syntax in Template Compilation
    High · 7.5 CVE-2026-33939 >= 4.0.0, < 4.7.9 4.7.9
  7. Handlebars.js has JavaScript Injection via AST Type Confusion by tampering @partial-block
    High · 8.1 CVE-2026-33938 >= 4.0.0, < 4.7.9 4.7.9
  8. Handlebars.js has JavaScript Injection via AST Type Confusion
    Critical · 9.8 CVE-2026-33937 >= 4.0.0, < 4.7.9 4.7.9
  9. Prototype Pollution in handlebars
    Critical · 9.8 CVE-2021-23383 < 4.7.7 4.7.7
  10. Regular Expression Denial of Service in Handlebars
    High · 7.5 CVE-2019-20922 >= 4.0.0, < 4.4.5 4.4.5
  11. Arbitrary Code Execution in Handlebars
    High · 8.1 CVE-2019-20920 < 3.0.8, >= 4.0.0, < 4.5.3 3.0.8, 4.5.3
  12. Remote code execution in handlebars when compiling templates
    Critical · 9.8 CVE-2021-23369 < 4.7.7 4.7.7
  13. Arbitrary Code Execution in handlebars
    High ghsa-q2c6-c6pm-g3gh < 3.0.8, >= 4.0.0, < 4.5.3 3.0.8, 4.5.3
  14. Prototype Pollution in handlebars
    High ghsa-g9r4-xpmj-mj65 < 3.0.8, >= 4.0.0, < 4.5.3 3.0.8, 4.5.3
  15. Arbitrary Code Execution in handlebars
    High · 7.3 ghsa-2cf5-4w76-r9qv < 3.0.8, >= 4.0.0, < 4.5.2 3.0.8, 4.5.2
  16. Prototype Pollution in handlebars
    Critical · 9.8 CVE-2019-19919 >= 4.0.0, < 4.3.0, < 3.0.8 4.3.0, 3.0.8
  17. Prototype Pollution in handlebars
    High · 7.3 ghsa-q42p-pg8m-cqh6 >= 4.1.0, < 4.1.2, >= 4.0.0, < 4.0.14, < 3.0.7 4.1.2, 4.0.14, 3.0.7

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