CVE-2026-33940

HighPublic PoCRCE

Published: 27 March 2026

Published

27 March 2026

Modified

31 March 2026

KEV Added

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Patch

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CVSS Score 8.1 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS Score 0.0003 8.9th percentile

Risk Priority 16 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-33940 is a high-severity Code Injection (CWE-94) vulnerability in Handlebarsjs Handlebars. Its CVSS base score is 8.1 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 8.9th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.

The strongest mitigations our analysis identified are NIST 800-53 SI-10 (Information Input Validation) and SI-2 (Flaw Remediation).

Threat & Defense at a Glance

What attackers do: exploitation maps to Exploit Public-Facing Application (T1190) and 1 other technique. What defenders deploy: see the NIST 800-53 controls recommended below.

Threat & Defense Details

Mitigating Controls (NIST 800-53 r5)AI

SI-2 Flaw Remediation good match

prevent

Directly mitigates the vulnerability by requiring timely application of the vendor patch (Handlebars 4.7.9) to remediate the code injection flaw.

SI-10 Information Input Validation good match

prevent

Requires validation and sanitization of template context data to block crafted non-primitive objects that bypass resolvePartial guards and enable compilation of injected AST code.

CM-6 Configuration Settings partial match

prevent

Enforces secure configuration settings such as using the runtime-only Handlebars build to eliminate the compile fallback path exploited in dynamic partial lookups.

MITRE ATT&CK Enterprise TechniquesAI

T1190 Exploit Public-Facing Application Initial Access

Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.

attack.mitre.org →

T1059.007 JavaScript Execution

Adversaries may abuse various implementations of JavaScript for execution.

attack.mitre.org →

Why these techniques?

CVE enables remote exploitation of a public-facing templating engine (Handlebars) for server-side arbitrary JavaScript execution via crafted AST injection into dynamic partials/compile path.

Confidence: HIGH · MITRE ATT&CK Enterprise v18.1

NVD Description

Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, a crafted object placed in the template context can bypass all conditional guards in `resolvePartial()` and cause `invokePartial()` to return `undefined`. The Handlebars runtime…

then treats the unresolved partial as a source that needs to be compiled, passing the crafted object to `env.compile()`. Because the object is a valid Handlebars AST containing injected code, the generated JavaScript executes arbitrary commands on the server. The attack requires the adversary to control a value that can be returned by a dynamic partial lookup. Version 4.7.9 fixes the issue. Some workarounds are available. First, use the runtime-only build (`require('handlebars/runtime')`). Without `compile()`, the fallback compilation path in `invokePartial` is unreachable. Second, sanitize context data before rendering: Ensure no value in the context is a non-primitive object that could be passed to a dynamic partial. Third, avoid dynamic partial lookups (`{{> (lookup ...)}}`) when context data is user-controlled.

Deeper analysisAI

CVE-2026-33940 is a high-severity vulnerability (CVSS 3.1 score of 8.1) affecting Handlebars.js, a templating engine used for building semantic templates, specifically in versions 4.0.0 through 4.7.8. The issue stems from a crafted object placed in the template context that bypasses all conditional guards in the `resolvePartial()` function. This causes `invokePartial()` to return `undefined`, prompting the Handlebars runtime to treat the unresolved partial as a compilable source and pass the object to `env.compile()`. As the object is a valid Handlebars Abstract Syntax Tree (AST) with injected code, it results in the generation and execution of arbitrary JavaScript on the server, tied to CWE-94 (code injection) and CWE-843 (type confusion).

A remote attacker (AV:N, PR:N) can exploit this vulnerability over the network with high attack complexity (AC:H) by controlling a value returned by a dynamic partial lookup, such as in expressions like `{{> (lookup ...)}}` where the context data is user-controlled. Successful exploitation grants high confidentiality, integrity, and availability impacts (C:H/I:H/A:H), enabling arbitrary command execution on the server without user interaction (UI:N) in a single-instance context (S:U).

The vulnerability is fixed in Handlebars version 4.7.9, as detailed in the project's security advisory (GHSA-xhpv-hc6g-r9c6), release notes, and the fixing commit. Advisories recommend upgrading to the patched version and provide workarounds including: using the runtime-only build (`require('handlebars/runtime')`) to avoid the `compile()` fallback path; sanitizing context data to ensure no non-primitive objects are passed to dynamic partials; and avoiding dynamic partial lookups with user-controlled context data.