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

> Attackers can inject arbitrary JavaScript into Handlebars templates via crafted AST objects, leading to remote code execution on the server.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 27 Mar 2026
- **Updated:** 4 Sep 2026
- **Also known as:** CLEANSTART-2026-CI51547, CLEANSTART-2026-EC90376, DEBIAN-CVE-2026-33937, GHSA-2w6w-674q-4c4q, MINI-37rx-w7wh-w67x, MINI-53pg-78mf-c5ff, MINI-5fww-rj9q-fjcq, MINI-84jv-48jx-3qm8, MINI-8558-8pvv-9fpm, MINI-94f5-7v9q-fvf2, MINI-998w-x4qq-4w8r, MINI-9qv2-gpmp-359w, MINI-mj85-qqw3-hqwf, MINI-pg4v-xpqq-gvqj, MINI-pmjw-7gvq-vjqc, MINI-q54m-2xqh-fr2j, MINI-rm23-vwm5-8498, MINI-vgc4-8v2f-9jpj, ROOT-APP-NPM-CVE-2026-33937, UBUNTU-CVE-2026-33937

## What is CVE-2026-33937?

CVE-2026-33937 is a critical code injection vulnerability in handlebars-lang handlebars.js, published on 27 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-94 (Improper Control of Generation of Code), CWE-843 (Access of Resource Using Incompatible Type)

## Which versions are affected by CVE-2026-33937?

2 open source packages ship an affected version, including handlebars. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `handlebars` | npm | From 4.0.0 | 4.7.9 |
| `handlebars` | npm | From 4.0.0 | 4.7.9 |

Products: Handlebarsjs Handlebars.

## How do I fix CVE-2026-33937?

Upgrade handlebars to 4.7.9 or later.

## Is CVE-2026-33937 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33937 exists.

Public exploit code:

- Handlebars.js AST Injection Remote Code Execution Vulnerability (github_poc): https://github.com/EQSTLab/CVE-2026-33937
- Python POC, Exploit for Handlebars.js AST Injection RCE, Handlebars.js versions 4.0.0 through 4.7.8 are affected. CVSS score: 9.8 Critical. (github_poc): https://github.com/c0gnit00/CVE-2026-33937
- CVE-2026-33937 Handlebars RCE exploit PoC (AST Injection) (github_poc): https://github.com/dinhvaren/cve-2026-33937
- this is a poc for CVE-2026-33937  (github_poc): https://github.com/garlic-wizard/CVE-2026-33937-for-DarkZeroReturns

## Other vulnerabilities in handlebars-lang handlebars.js

- [CVE-2026-33941](https://insights.sbomify.com/CVE-2026-33941/): Handlebars.js has JavaScript Injection in CLI Precompiler via Unescaped Names and Options (high 8.3)
- [CVE-2026-33938](https://insights.sbomify.com/CVE-2026-33938/): Handlebars.js has JavaScript Injection via AST Type Confusion by tampering @partial-block (high 8.1)
- [CVE-2026-33940](https://insights.sbomify.com/CVE-2026-33940/): Handlebars.js has JavaScript Injection via AST Type Confusion when passing an object as dynamic partial (high 8.1)
- [CVE-2019-20920](https://insights.sbomify.com/CVE-2019-20920/): Arbitrary Code Execution in Handlebars (high 8.1)
- [CVE-2026-33939](https://insights.sbomify.com/CVE-2026-33939/): Handlebars.js has Denial of Service via Malformed Decorator Syntax in Template Compilation (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-33937 and fix it if it is affected.

What it is: CVE-2026-33937 is a critical code injection vulnerability in handlebars-lang handlebars.js, published on 27 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33937 exists.

Affected packages and fixed versions:
- handlebars (npm): affected from 4.0.0; fixed in 4.7.9
- handlebars (npm): affected from 4.0.0; fixed in 4.7.9

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-33937/ (Markdown: https://insights.sbomify.com/CVE-2026-33937/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33937
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33937
- OSV: https://osv.dev/vulnerability/CVE-2026-33937

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-33937/
Summaries are generated and may contain mistakes; the official description above is the record.