CVE-2026-106446: Handlebars: JavaScript Injection via AST Type Confusion in compile (Program.blockParams)

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.8% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-06 · updated 2026-10-07
Weakness: CWE-94

Description

Handlebars provides the power necessary to let users build semantic templates. From 4.0.0 until 4.7.10, Handlebars.compile() and Handlebars.precompile() accept pre-parsed AST objects while validating only selected PathExpression, NumberLiteral, and BooleanLiteral values. This issue bypasses the AST validation introduced in version 4.7.9 for CVE-2026-33937. An attacker who can supply an object instead of a template string can place JavaScript expressions in unchecked values such as Program.blockParams.length, a non-PathExpression parameter depth, a non-string StringLiteral.value, or a non-string PathExpression.original. The compiler emits those values into generated JavaScript, causing code execution in the server process when compile output renders or wherever precompile output is loaded. Applications that pass only template strings are not affected. This issue is fixed in version 4.7.10.

Affected products

Npm Handlebars: from 4.0.0 before 4.7.10 — fixed in 4.7.10
handlebars-lang handlebars.js: from 4.0.0, < 4.7.10

How to fix

Upgrade to a fixed version: Npm Handlebars 4.7.10. Check the vendor advisory for mitigations.

References

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