CVE-2021-23974 in Firefox
Summary
by MITRE • 02/26/2021
The DOMParser API did not properly process '' elements for escaping. This could be used as an mXSS vector to bypass an HTML Sanitizer. This vulnerability affects Firefox < 86.
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Analysis
by VulDB Data Team • 03/05/2021
The vulnerability identified as CVE-2021-23974 represents a critical flaw in Firefox's DOMParser API implementation that enables malicious actors to bypass HTML sanitization mechanisms through improper handling of specific elements. This issue specifically affects Firefox versions prior to 86, where the DOMParser API fails to correctly process certain elements for proper escaping, creating an mXSS (multi-vector Cross-Site Scripting) attack vector. The flaw resides in how the browser processes and sanitizes HTML content when using the DOMParser API, which is commonly employed by web applications to parse and manipulate HTML documents programmatically.
The technical root cause of this vulnerability stems from inadequate input validation and sanitization within the DOMParser implementation. When processing HTML content containing specific element structures, the API does not properly escape or sanitize certain characters that could be interpreted as executable code. This misconfiguration allows attackers to craft malicious payloads that appear harmless to standard sanitization routines but can execute unintended JavaScript when processed through the vulnerable DOMParser API. The vulnerability operates at the intersection of HTML parsing, DOM manipulation, and security sanitization, making it particularly dangerous as it can bypass security controls that are designed to prevent XSS attacks.
The operational impact of this vulnerability extends beyond simple script execution, as it enables sophisticated attack scenarios where malicious code can be injected into web applications that rely on DOMParser for content processing. Attackers can exploit this weakness to perform session hijacking, steal sensitive user data, redirect users to malicious sites, or execute arbitrary code within the browser context. The mXSS nature of this vulnerability means that the attack can be delivered through multiple vectors, including user-generated content, third-party services, or even legitimate web applications that process user input through DOMParser. This makes the vulnerability particularly challenging to defend against as it can be triggered through various legitimate application pathways.
Security professionals should implement immediate mitigations including upgrading to Firefox version 86 or later where the vulnerability has been patched, implementing additional input validation layers, and monitoring for suspicious DOMParser usage patterns in web applications. Organizations should also consider employing Content Security Policy (CSP) headers to limit script execution, implementing proper HTML sanitization libraries, and conducting regular security assessments of applications that utilize DOMParser functionality. This vulnerability aligns with CWE-116 for improper encoding or escaping and relates to ATT&CK technique T1211 for exploitation for defense evasion, highlighting the need for comprehensive security measures. The patch for this vulnerability specifically addresses the DOMParser's handling of element escaping, ensuring that all potentially malicious content is properly sanitized before processing, thereby preventing the mXSS attack vector from being exploited.