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, the `@partial-block` special variable is stored in the template data context and is reachable and mutable from within a template via helpers that accept arbitrary objects. When a helper overwrites `@partial-block` with a crafted Handlebars AST, a subsequent invocation of `{{> @partial-block}}` compiles and executes that AST, enabling arbitrary JavaScript execution on the server. Version 4.7.9 fixes the issue. Some workarounds are available. First, use the runtime-only build (`require('handlebars/runtime')`). The `compile()` method is absent, eliminating the vulnerable fallback path. Second, audit registered helpers for any that write arbitrary values to context objects. Helpers should treat context data as read-only. Third, avoid registering helpers from third-party packages (such as `handlebars-helpers`) in contexts where templates or context data can be influenced by untrusted input.

[ 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-3mfm-83xf-c92rx_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