CVE-2026-33937: Handlebars.js JavaScript Injection via AST Type Confusion
Summary
CVE-2026-33937: Handlebars.js has JavaScript Injection via AST Type Confusion. Affects handlebars versions >= 4.0.0, < 4.7.9. Fixed in 4.7.9. How to fix ↓
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Background
Handlebars.js is a popular templating engine for JavaScript that allows developers to create dynamic HTML content by embedding expressions within templates. It is widely used in web applications to separate HTML generation from the underlying logic. The vulnerability in question arises from the way Handlebars handles Abstract Syntax Tree (AST) objects, specifically when it processes NumberLiteral nodes, which can lead to severe security issues if exploited.
Vulnerability Detail
The vulnerability exists in the Handlebars.compile() function, which can accept either a template string or a pre-parsed AST object. When an AST is provided, the value of a NumberLiteral node is emitted directly into the generated JavaScript code without any sanitization or quoting. This allows an attacker to inject arbitrary JavaScript code by crafting a malicious AST. If an endpoint deserializes user-controlled JSON and passes it directly to Handlebars.compile(), it becomes susceptible to Remote Code Execution (RCE) on the server, potentially allowing attackers to execute arbitrary commands.
How To Fix
Latest published version: 4.7.10 as of 22 Oct 2026
npm install handlebars@4.7.10Upgrading to it may include breaking changes; the version below is the minimum fixed release for this vulnerability.
If you need the minimum change
4.7.9
This is the first release that contains the fix.
npm install handlebars@4.7.9Tip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Workaround
As a temporary workaround, validate the input type before calling Handlebars.compile() to ensure that the argument is always a string and not a plain object or JSON-deserialized value. Here is a code snippet to illustrate this:
if (typeof templateInput !== 'string') {
throw new TypeError('Template must be a string');
}
Conclusion
The vulnerability identified as CVE-2026-33937 in Handlebars.js poses a critical risk due to its potential for Remote Code Execution via JavaScript injection. It is essential for developers using this library to update to the patched version or implement input validation to mitigate the risk. Tools like Vulert can assist in monitoring for such vulnerabilities in real-time.
See every vulnerability recorded for handlebars →
FAQ
The vulnerability has a CVSS score of 9.8, indicating it is critical and poses a significant risk to affected systems.
You can use Vulert, which monitors and alerts for open-source vulnerabilities in real-time. You can check your application for this vulnerability using the Vulert playground at Vulert Scanner without any signup.
If exploited, this vulnerability can lead to Remote Code Execution (RCE), allowing attackers to execute arbitrary commands on the server, which can compromise the entire application.
Yes, you can implement input validation to ensure that only strings are passed to Handlebars.compile(), thus preventing the execution of malicious code.
Handlebars.js is a templating engine that allows developers to create dynamic HTML content by embedding expressions within templates, facilitating the separation of HTML generation from application logic.
References
Related vulnerabilities
Recent vulnerabilities recorded for handlebars.
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