CVE-2019-20920 in handlebars
Summary
by MITRE • 10/04/2020
Handlebars before 3.0.8 and 4.x before 4.5.3 is vulnerable to Arbitrary Code Execution. The lookup helper fails to properly validate templates, allowing attackers to submit templates that execute arbitrary JavaScript. This can be used to run arbitrary code on a server processing Handlebars templates or in a victim's browser (effectively serving as XSS).
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Analysis
by VulDB Data Team • 11/14/2020
The vulnerability identified as CVE-2019-20920 represents a critical security flaw in the Handlebars templating engine that affects versions prior to 3.0.8 and 4.x prior to 4.5.3. This issue stems from improper template validation within the lookup helper functionality, creating a pathway for malicious actors to inject and execute arbitrary JavaScript code. The vulnerability operates at the intersection of server-side template processing and client-side execution contexts, making it particularly dangerous as it can be exploited in both environments.
The technical flaw manifests through the lookup helper's failure to properly sanitize and validate template inputs before processing. When Handlebars encounters templates containing malicious code, the lookup helper does not adequately filter or escape potentially harmful constructs, allowing attackers to craft templates that execute arbitrary JavaScript during the rendering process. This vulnerability specifically targets the template parsing and execution mechanisms, leveraging the engine's ability to dynamically evaluate template content to execute unauthorized code. The flaw can be categorized under CWE-94, which describes "Improper Control of Generation of Code ('Code Injection')" and aligns with ATT&CK technique T1203 for "Exploitation for Client Execution" and T1059.007 for "Command and Scripting Interpreter: JavaScript."
The operational impact of this vulnerability extends across multiple attack vectors and system components. On server-side implementations, attackers can execute arbitrary code on systems processing Handlebars templates, potentially leading to complete system compromise, data exfiltration, or lateral movement within network environments. When exploited in browser contexts, the vulnerability serves as an effective cross-site scripting (XSS) vector, allowing attackers to execute malicious JavaScript in victims' browsers and potentially steal session cookies, perform unauthorized actions on behalf of users, or redirect them to malicious sites. The vulnerability's broad applicability makes it particularly dangerous as it affects both server-side template rendering engines and client-side web applications that utilize Handlebars for dynamic content generation.
Organizations should implement immediate mitigations including updating to Handlebars versions 3.0.8 or 4.5.3 and higher, which contain proper template validation and sanitization measures. Additional protective measures include implementing strict input validation for all template inputs, utilizing content security policies to prevent script execution, and employing sandboxing techniques for template processing. Security monitoring should focus on detecting unusual template processing patterns and unauthorized code execution attempts. The vulnerability demonstrates the importance of proper input validation and output encoding in templating systems, aligning with security best practices outlined in the OWASP Top Ten and the NIST Cybersecurity Framework for preventing code injection attacks. Organizations should also consider implementing automated vulnerability scanning and dependency monitoring to prevent similar issues in other templating engines and web application frameworks.