CVE-2026-33916
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.
How to fix
- handlebarsjs handlebars4.7.9 or later
- handlebars4.7.9 or later
Affected products
| Product | Affected versions | Fixed in |
|---|---|---|
| handlebarsjs handlebars | 4.0.0 ≤ v < 4.7.9 | 4.7.9 |
| handlebarsnpm | 4.0.0 ≤ v < 4.7.9 | 4.7.9 |
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Attack Surface Management
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