---
title: "CVE-2026-33937: Handlebars.js has JavaScript Injection via AST Type Confusion"
url: https://erroneo.us/security/cve/CVE-2026-33937/
description: "CVE-2026-33937 (handlebars (npm)): Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-03-27
date_modified: 2026-09-04
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-33937"
kev: false
cvss: 9.8
epss: 0.01736
license: All rights reserved. Quote with attribution and a link.
---

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

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Not in CISA KEV , but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
- EPSS 1.7% , percentile 77: FIRST’s model estimates a 1.7% chance of exploitation activity in the next 30 days, higher than 77% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes handlebars (npm) >= 4.0.0, <= 4.7.8, fixed in 4.7.9 . Check your lockfile, including transitive dependencies.

## What it is

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 packages

- npm `handlebars` >= 4.0.0, <= 4.7.8 → fixed: 4.7.9

## Affected products (CVE record)

- handlebars-lang handlebars.js: >= 4.0.0, < 4.7.9

## Severity

- GitHub_M CVSS 3.1: 9.8 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`
- redhat-SADP CVSS 3.1: 9.8 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`
- GitHub CVSS 3.1: 9.8 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation poc , automatable yes , technical impact total .
- FIRST EPSS (model of 2026-10-21): 1.74% probability of exploitation activity in the next 30 days; percentile 77.

## Sources

- [access.redhat.com](https://access.redhat.com/errata/RHSA-2026:34342)
- [access.redhat.com](https://access.redhat.com/errata/RHSA-2026:10175)
- [GitHub Advisory GHSA-2w6w-674q-4c4q](https://github.com/advisories/GHSA-2w6w-674q-4c4q)
- [CVE.org record CVE-2026-33937](https://www.cve.org/CVERecord?id=CVE-2026-33937)
- [NVD entry CVE-2026-33937](https://nvd.nist.gov/vuln/detail/CVE-2026-33937)
- [github.com](https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-2w6w-674q-4c4q)
- [github.com](https://github.com/handlebars-lang/handlebars.js/commit/68d8df5a88e0a26fe9e6084c5c6aaebe67b07da2)
- [github.com](https://github.com/handlebars-lang/handlebars.js/releases/tag/v4.7.9)
- [access.redhat.com](https://access.redhat.com/security/cve/CVE-2026-33937)
- [RHBZ#2452523](https://bugzilla.redhat.com/show_bug.cgi?id=2452523)
- [security.access.redhat.com](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-33937.json)

---
Source page: https://erroneo.us/security/cve/CVE-2026-33937/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt