CVE-2026-106444

Handlebars provides the power necessary to let users build semantic templates. From 4.0.0 until 4.7.10, Handlebars.precompile() uses quotedString() in lib/handlebars/compiler/code-gen.js to emit static template text into generated JavaScript without escaping sequences that terminate an enclosing HTML script element. When an application precompiles attacker-controlled template text and embeds the generated source directly in an inline script element, a closing script delimiter can end the element and cause following attacker-controlled markup to be parsed and executed. Ordinary server-side rendering and precompiled templates served as external JavaScript files are not affected. This issue is fixed in version 4.7.10.

Handlebars provides the power necessary to let users build semantic templates. From 4.0.0 until 4.7.10, Handlebars.precompile() uses quotedString() in lib/handlebars/compiler/code-gen.js to emit static template text into generated JavaScript without escaping sequences that terminate an enclosing HTML script element. When an application precompiles attacker-controlled template text and embeds the generated source directly in an inline script element, a closing script delimiter can end the element and cause following attacker-controlled markup to be parsed and executed. Ordinary server-side rendering and precompiled templates served as external JavaScript files are not affected. This issue is fixed in version 4.7.10.

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

Handlebars provides the power necessary to let users build semantic templates. From 4.0.0 until 4.7.10, Handlebars.precompile() uses quotedString() in lib/handlebars/compiler/code-gen.js to emit static template text into generated JavaScript without escaping sequences that terminate an enclosing HTML script element. When an application precompiles attacker-controlled template text and embeds the generated source directly in an inline script element, a closing script delimiter can end the element and cause following attacker-controlled markup to be parsed and executed. Ordinary server-side rendering and precompiled templates served as external JavaScript files are not affected. This issue is fixed in version 4.7.10.

Handlebars provides the power necessary to let users build semantic templates. From 4.0.0 until 4.7.10, Handlebars.precompile() uses quotedString() in lib/handlebars/compiler/code-gen.js to emit static template text into generated JavaScript without escaping sequences that terminate an enclosing HTML script element. When an application precompiles attacker-controlled template text and embeds the generated source directly in an inline script element, a closing script delimiter can end the element and cause following attacker-controlled markup to be parsed and executed. Ordinary server-side rendering and precompiled templates served as external JavaScript files are not affected. This issue is fixed in version 4.7.10.

Handlebars provides the power necessary to let users build semantic templates. From 4.0.0 until 4.7.10, Handlebars.precompile() uses quotedString() in lib/handlebars/compiler/code-gen.js to emit static template text into generated JavaScript without escaping sequences that terminate an enclosing HTML script element. When an application precompiles attacker-controlled template text and embeds the generated source directly in an inline script element, a closing script delimiter can end the element and cause following attacker-controlled markup to be parsed and executed. Ordinary server-side rendering and precompiled templates served as external JavaScript files are not affected. This issue is fixed in version 4.7.10.