REDHAT-BUG-2547120: High severity npm/handlebars vulnerability

Q: Which applications are exposed to this issue?

Applications using npm/handlebars versions from 4.0.0 through 4.7.10 are exposed only if they pass attacker-controlled pre-parsed AST objects to Handlebars.compile() or Handlebars.precompile(). Applications that pass only template strings are not affected.

Q: What does an attacker need to exploit the vulnerability?

An attacker must be able to supply an object in place of a template string to compile() or precompile(). They can then place JavaScript expressions in unchecked AST fields, which are emitted into generated JavaScript.

Q: When does the injected code execute?

Code execution occurs in the server process when output from compile() is rendered. For precompile(), execution can occur wherever the generated precompiled output is loaded.

Q: What mitigation is available if upgrading cannot happen immediately?

Ensure that compile() and precompile() receive only template strings and do not accept pre-parsed AST objects from untrusted sources. This removes the affected input path described for this issue.

Published Oct 6, 2026

Updated

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 Software

1 affected component

npm/handlebars>=4.0.0<4.7.10

Remediation

Recommended actions to resolve this vulnerability, in priority order.

Upgrade
Upgrade Handlebars to a version that resolves this vulnerability.
Fixed in 4.7.10

Event History

Oct 6, 2026

Data Sourced

via Red Hat·07:55 PM

DescriptionSeverityAffected Software

Frequently Asked Questions

Which applications are exposed to this issue?

Applications using npm/handlebars versions from 4.0.0 through 4.7.10 are exposed only if they pass attacker-controlled pre-parsed AST objects to Handlebars.compile() or Handlebars.precompile(). Applications that pass only template strings are not affected.

What does an attacker need to exploit the vulnerability?

An attacker must be able to supply an object in place of a template string to compile() or precompile(). They can then place JavaScript expressions in unchecked AST fields, which are emitted into generated JavaScript.

When does the injected code execute?

Code execution occurs in the server process when output from compile() is rendered. For precompile(), execution can occur wherever the generated precompiled output is loaded.

What mitigation is available if upgrading cannot happen immediately?

Ensure that compile() and precompile() receive only template strings and do not accept pre-parsed AST objects from untrusted sources. This removes the affected input path described for this issue.

REDHAT-BUG-2547120: High severity npm/handlebars vulnerability

Affected Software

Remediation

Event History

Frequently Asked Questions

Which applications are exposed to this issue?

What does an attacker need to exploit the vulnerability?

When does the injected code execute?

What mitigation is available if upgrading cannot happen immediately?

Company

Resources

Contact

Frequently Asked Questions

Which applications are exposed to this issue?

What does an attacker need to exploit the vulnerability?

When does the injected code execute?

What mitigation is available if upgrading cannot happen immediately?