CVE-2026-33939: Handlebars.js has Denial of Service via Malformed Decorator Syntax in Template Compilation

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-03-27 · updated 2026-10-26
Weakness: CWE-754

Description

Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, 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. Version 4.7.9 fixes the issue. Some workarounds are available. Wrap compilation and rendering in `try/catch`. 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 products

handlebars-lang handlebars.js: from 4.0.0 before 4.7.9 — fixed in 4.7.9
Npm Handlebars: from 4.0.0 before 4.7.9 — fixed in 4.7.9

How to fix

Upgrade to a fixed version: handlebars-lang handlebars.js 4.7.9; Npm Handlebars 4.7.9. Check the vendor advisory for mitigations.

References

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