Vulnerability in handlebars-lang handlebars.js

[ Why it matters ]Priority signals

• Public proof-of-concept exploit exists (CISA SSVC)

[ Description ]Official 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.

[ Fixes ]Fixes and patches

No fix or interim patch is linked from our sources yet. Check the vendor references below and limit exposure of affected systems until a fix is available.

[ Advisories ]Government CERT advisories

• BSI CERT-Bund29 Sep 2026WID-SEC-2026-3655

  IBM InfoSphere Data Replication: Multiple vulnerabilities Translated from German

  CVE-2026-33937CVE-2026-33941CVE-2026-33939+3 more CVEs

[ References ]References

• github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-2qvq-rjwj-gvw9x_refsource_CONFIRM
• github.com/handlebars-lang/handlebars.js/commit/68d8df5a88e0a26fe9e6084c5c6aaebe67b07da2x_refsource_MISC
• github.com/handlebars-lang/handlebars.js/releases/tag/v4.7.9x_refsource_MISC