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High

Handlebars.js has Denial of Service via Malformed Decorator Syntax in Template Compilation

  • Disclosed: 27.03.2026
  • Updated: 11.10.2026 10:22

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:

js
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

js
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:
    js
    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.

Affected software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
npm package handlebars >= 4.0.0, < 4.7.9 4.7.9

References

CVSS score 7.5/10
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness (CWE)
CWE-754
Platform
npm
Package
handlebars
Disclosed
27.03.2026
Updated
11.10.2026 10:22

Sources and licenses

OSV / GitHub Advisory Database - GHSA-9cx6-37pm-9jff - 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 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
  2. Handlebars: JavaScript Injection via Own Property Check Bypass
    Critical CVE-2026-106445 >= 3.0.0, < 4.7.10 4.7.10
  3. 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
  4. 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
  5. Handlebars.js has Denial of Service via Malformed Decorator Syntax in Template Compilation this entry
    High · 7.5 CVE-2026-33939 >= 4.0.0, < 4.7.9 4.7.9
  6. 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
  7. Handlebars.js has JavaScript Injection via AST Type Confusion
    Critical · 9.8 CVE-2026-33937 >= 4.0.0, < 4.7.9 4.7.9
  8. Prototype Pollution in handlebars
    Critical · 9.8 CVE-2021-23383 < 4.7.7 4.7.7
  9. Regular Expression Denial of Service in Handlebars
    High · 7.5 CVE-2019-20922 >= 4.0.0, < 4.4.5 4.4.5
  10. 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
  11. Remote code execution in handlebars when compiling templates
    Critical · 9.8 CVE-2021-23369 < 4.7.7 4.7.7
  12. 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
  13. Prototype Pollution in handlebars
    High ghsa-g9r4-xpmj-mj65 < 3.0.8, >= 4.0.0, < 4.5.3 3.0.8, 4.5.3
  14. 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
  15. 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
  16. 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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