Handlebars.js has Denial of Service via Malformed Decorator Syntax in Template Compilation
Summary
When a Handlebars template contains decorator syntax referencing an unregistered decorator (e.g. {{*n}}), the compiled template calls lookupProperty(decorators, "n"), which returns undefined. The runtime then immediately invokes the result as a function, causing an unhandled TypeError: ... is not a function that crashes the Node.js process. Any application that compiles user-supplied templates without wrapping the call in a try/catch is vulnerable to a single-request Denial of Service.
Description
In lib/handlebars/compiler/javascript-compiler.js, the code generated for a decorator invocation looks like:
fn = lookupProperty(decorators, "n")(fn, props, container, options) || fn;When "n" is not a registered decorator, lookupProperty(decorators, "n") returns undefined. The expression immediately attempts to call undefined as a function, producing:
TypeError: lookupProperty(...) is not a functionBecause the error is thrown inside the compiled template function and is not caught by the runtime, it propagates up as an unhandled exception and — when not caught by the application — crashes the Node.js process.
This inconsistency is notable: references to unregistered helpers produce a clean "Missing helper: ..." error, while references to unregistered decorators cause a hard crash.
Attack scenario: An attacker submits {{*n}} as template content to any endpoint that calls Handlebars.compile(userInput)(). Each request crashes the server process; with process managers that auto-restart (PM2, systemd), repeated submissions create a persistent DoS.
Proof of Concept
const Handlebars = require('handlebars'); // Handlebars 4.7.8, Node.js v22.x
// Any of these payloads crash the process
Handlebars.compile('{{*n}}')({});
Handlebars.compile('{{*decorator}}')({});
Handlebars.compile('{{*constructor}}')({});Expected crash output:
TypeError: lookupProperty(...) is not a function
at Function.eval [as decorator] (eval at compile (...javascript-compiler.js:134:36))Workarounds
- Wrap compilation and rendering in
try/catch:jstry { const result = Handlebars.compile(userInput)(context); res.send(result); } catch (err) { res.status(400).send('Invalid template'); } - Validate template input before passing it to
compile(). Reject templates containing decorator syntax ({{*...}}) if decorators are not used in your application. - Use the pre-compilation workflow: compile templates at build time and serve only pre-compiled templates; do not call
compile()at request time.
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-9cx6-37pm-9jff
- https://nvd.nist.gov/vuln/detail/CVE-2026-33939
- 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:L/PR:N/UI:N/S:U/C:N/I:N/A:H- Słabość (CWE)
CWE-754- Platforma
- npm
- Paczka
handlebars- Ujawniono
- 27.03.2026
- Aktualizacja
- 11.10.2026 10:22
Źródła i licencje
OSV / GitHub Advisory Database - GHSA-9cx6-37pm-9jff - CC BY 4.0
Historia podatności: handlebars
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