XSS in Handlebarsjs Handlebars
CVE-2026-33916
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.
Vulnerability class: XSS (Cross-Site Scripting)
EPSS: 0.003 (19.8th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 4.7 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N.
Affected products
- Handlebarsjs Handlebars
- Handlebars-lang Handlebars.js — versions >= 4.0.0, < 4.7.9
Weakness classification (CWE)
References
- security-advisories@github.com (x_refsource_CONFIRM, Exploit, Vendor Advisory)
- security-advisories@github.com (Patch, x_refsource_MISC)
- security-advisories@github.com (x_refsource_MISC, Release Notes)
Frequently asked questions
- What is CVE-2026-33916?
- CVE-2026-33916 is a medium-severity vulnerability in Handlebarsjs Handlebars, classified under Cross-site Scripting. CVSS score: 4.7/10. Published 2026-03-27.
- How severe is CVE-2026-33916?
- Medium severity. CVSS v3 base score is 4.7 out of 10.