CVE-2026-33916
Handlebars.js has Prototype Pollution Leading to XSS through Partial Template Injection
Impact
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 packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| npm | handlebars | >= 4.0.0, < 4.7.9 | 4.7.9 |
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Handlebarsjs | Handlebars | from 4.0.0 before 4.7.9 |
Weakness classification
References
- NVD record
- CVE.org record
- GitHub advisory
- https://github.com/handlebars-lang/handlebars.js/commit/68d8df5a88e0a26fe9e6084c5c6aaebe67b07da2
- https://github.com/handlebars-lang/handlebars.js/releases/tag/v4.7.9
- https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-2qvq-rjwj-gvw9
- https://github.com/advisories/GHSA-2qvq-rjwj-gvw9
- https://nvd.nist.gov/vuln/detail/CVE-2021-23369
- https://nvd.nist.gov/vuln/detail/CVE-2021-23383
- https://nvd.nist.gov/vuln/detail/CVE-2026-33916