CVE-2016-10547 in Nunjucks
Summary
by MITRE
Nunjucks is a full featured templating engine for JavaScript. Versions 2.4.2 and lower have a cross site scripting (XSS) vulnerability in autoescape mode. In autoescape mode, all template vars should automatically be escaped. By using an array for the keys, such as `name[]=<script>alert(1)</script>`, it is possible to bypass autoescaping and inject content into the DOM.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 03/19/2023
The vulnerability identified as CVE-2016-10547 affects Nunjucks, a popular JavaScript templating engine that provides full-featured template processing capabilities for web applications. This vulnerability specifically targets the autoescape functionality that is designed to prevent cross-site scripting attacks by automatically escaping template variables. The issue exists in versions 2.4.2 and earlier, where the security mechanism fails to properly handle certain data structures during the escaping process. The vulnerability represents a critical flaw in the template engine's security model, as it undermines the fundamental protection mechanism that developers rely upon to prevent malicious code injection.
The technical exploitation of this vulnerability occurs through a specific bypass technique involving array key structures in template variables. When attackers craft input using array notation such as `name[]=<script>alert(1)</script>`, the autoescape mechanism in affected versions fails to properly sanitize the content. This occurs because the escaping logic does not adequately handle nested or array-based key structures, allowing malicious script content to bypass the automatic escaping process. The vulnerability stems from an insufficient validation approach in the template engine's escaping algorithm, which fails to recognize that array-like key structures should be treated with the same security precautions as regular variables. This bypass mechanism effectively transforms the intended security feature into a vector for malicious code execution.
The operational impact of CVE-2016-10547 is significant for applications using Nunjucks templating, as it allows attackers to inject arbitrary JavaScript code into web pages rendered by the template engine. When exploited, this vulnerability enables attackers to perform various malicious activities including session hijacking, data theft, defacement of web applications, and execution of arbitrary commands on affected systems. The vulnerability affects the core security model of the templating engine, meaning that any application relying on Nunjucks for rendering user-provided content becomes vulnerable to XSS attacks. This creates a chain reaction effect where even applications that implement proper input validation at other layers may still be compromised due to the template engine's security failure.
Organizations should immediately upgrade to Nunjucks version 2.4.3 or later, which contains the necessary patches to address this vulnerability. The fix implemented in newer versions properly handles array key structures during the autoescape process, ensuring that all template variables regardless of their key structure are properly sanitized. Security teams should also conduct comprehensive code reviews to identify any custom template processing logic that might be vulnerable to similar bypass techniques. Additionally, implementing additional security layers such as Content Security Policy (CSP) headers can provide defense-in-depth protection against potential exploitation attempts. This vulnerability aligns with CWE-79, Cross-site Scripting, and represents a specific implementation weakness in the escaping mechanism that falls under ATT&CK technique T1213.002, Data from Information Repositories, as it allows attackers to manipulate rendered content and extract sensitive information through XSS payloads.