CVE-2026-33916: Handlebars.js has Prototype Pollution Leading to XSS through Partial Template Injection

Severity: medium (CVSS 4.7 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-03-27 · updated 2026-10-26
Weakness: CWE-79

Description

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

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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