CVE-2026-33938

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.

We have discovered 27,902 live websites that are affected by CVE-2026-33938.

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Affected Software

Product	Handlebars
Category	JavaScript Frameworks
Vulnerable Domains	27,902 live websites (90% of Handlebars install base)
Vulnerable Versions	from 4 through 4.7.9
Vulnerable Versions Count	34 versions ( 68% of all versions)