CVE-2026-33939

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 function

Because 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:

  try {
    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.

CVE-2026-33939 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.7.9); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-33939? CVE-2026-33939 is a high-severity security vulnerability in handlebars (npm), affecting versions >= 4.0.0, <= 4.7.8. It is fixed in 4.7.9.
2. How severe is CVE-2026-33939? CVE-2026-33939 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of handlebars are affected by CVE-2026-33939? handlebars (npm) versions >= 4.0.0, <= 4.7.8 is affected.
4. Is there a fix for CVE-2026-33939? Yes. CVE-2026-33939 is fixed in 4.7.9. Upgrade to this version or later.
5. Is CVE-2026-33939 exploitable, and should I be worried? Whether CVE-2026-33939 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-33939 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-33939? Upgrade handlebars to 4.7.9 or later.