CVE-2026-33938: Handlebars.js has JavaScript Injection via AST Type Confusion by tampering @partial-block

Severity: high (CVSS 8.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.7% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-03-27 · updated 2026-10-26
Weakness: CWE-94

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.

Affected products

handlebars-lang handlebars.js: from 4.0.0 before 4.7.9 — fixed in 4.7.9
Npm Handlebars: from 4.0.0 before 4.7.9 — fixed in 4.7.9

How to fix

Upgrade to a fixed version: handlebars-lang handlebars.js 4.7.9; Npm Handlebars 4.7.9. Check the vendor advisory for mitigations.

References

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