CVE-2026-33916: Handlebarsjs Handlebars

Medium severity, CVSS 4.7. EPSS: 0.4% chance of exploitation in the next 30 days.

Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, `resolvePartial()` in the Handlebars runtime resolves partial names via a plain property lookup on `options.partials` without guarding against prototype-chain traversal. When `Object.prototype` has been polluted with a string value whose key matches a partial reference in a template, the polluted string is used as the partial body and rendered without HTML escaping, resulting in reflected or stored XSS. Version 4.7.9 fixes the issue. Some workarounds are available. Apply `Object.freeze(Object.prototype)` early in application startup to prevent prototype pollution. Note: this may break other libraries, and/or use the Handlebars runtime-only build (`handlebars/runtime`), which does not compile templates and reduces the attack surface.

Affected products

• Handlebarsjs Handlebars: from 4.0.0, before 4.7.9 (fixed in 4.7.9)

Published 2026-03-27. Last modified 2026-06-17.