CVE-2026-33937: Handlebars.js has JavaScript Injection via AST Type Confusion

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.9% 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, `Handlebars.compile()` accepts a pre-parsed AST object in addition to a template string. The `value` field of a `NumberLiteral` AST node is emitted directly into the generated JavaScript without quoting or sanitization. An attacker who can supply a crafted AST to `compile()` can therefore inject and execute arbitrary JavaScript, leading to Remote Code Execution on the server. Version 4.7.9 fixes the issue. Some workarounds are available. Validate input type before calling `Handlebars.compile()`; ensure the argument is always a `string`, never a plain object or JSON-deserialized value. Use the Handlebars runtime-only build (`handlebars/runtime`) on the server if templates are pre-compiled at build time; `compile()` will be unavailable.

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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