TypeError-Induced Denial of Service in Handlebars Template Compiler

Executive Summary

This vulnerability exists in Handlebars versions 4.0.0 through 4.7.8. When a Handlebars template contains decorator syntax referencing an unregistered decorator (for example, {{*n}}), the compiled template calls a function that returns undefined. The runtime then tries to invoke this undefined value as a function, which causes an unhandled TypeError and crashes the Node.js process.

Any application that compiles user-supplied templates without wrapping the compilation call in a try/catch block is vulnerable to a single-request Denial of Service (DoS) attack.

Version 4.7.9 of Handlebars fixes this issue. Workarounds include wrapping compilation and rendering in try/catch blocks, validating template input to reject templates containing decorator syntax if decorators are not used, and using a pre-compilation workflow to compile templates at build time instead of at request time.

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Impact Analysis

This vulnerability can cause a Denial of Service (DoS) by crashing the Node.js process when an unregistered decorator is referenced in a user-supplied Handlebars template. This means that an attacker can send a specially crafted template that triggers the crash, causing the application to become unavailable.

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Detection Guidance

This vulnerability occurs when a Handlebars template contains decorator syntax referencing an unregistered decorator, causing a Node.js process crash due to an unhandled TypeError.

To detect this vulnerability, you should check if your application compiles user-supplied templates without wrapping the compilation and rendering calls in try/catch blocks.

Additionally, inspect templates for the presence of decorator syntax (e.g., {{*n}}) that might be unregistered.

There are no specific commands provided in the available information to detect this vulnerability on your network or system.

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Mitigation Strategies

Immediate mitigation steps include:

• Wrap the compilation and rendering of Handlebars templates in try/catch blocks to handle potential errors gracefully.
• Validate template input before passing it to the compile() function; specifically, reject templates containing decorator syntax ({{*...}}) if decorators are not used in your application.
• Use the pre-compilation workflow by compiling templates at build time and serving only pre-compiled templates, avoiding calls to compile() at request time.
• Upgrade Handlebars to version 4.7.9 or later, where this issue is fixed.

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