CVE-2021-23959 in Firefox
Summary
by MITRE • 02/26/2021
An XSS bug in internal error pages could have led to various spoofing attacks, including other error pages and the address bar. Note: This issue only affected Firefox for Android. Other operating systems are unaffected. This vulnerability affects Firefox < 85.
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Analysis
by VulDB Data Team • 03/05/2021
This vulnerability represents a cross-site scripting flaw that specifically impacted Firefox for Android users, creating a significant security risk through malicious error page manipulation. The issue was discovered in the internal error handling mechanism of the mobile browser, where improper sanitization of error messages allowed attackers to inject malicious scripts that could execute within the context of the browser's error display system. This particular vulnerability was classified as a client-side security flaw that leveraged the browser's own error reporting infrastructure to deliver malicious payloads. The vulnerability existed in Firefox versions prior to 85, making all users of older mobile browser versions susceptible to exploitation.
The technical nature of this flaw stems from inadequate input validation and output sanitization within Firefox's error page rendering engine. When the browser encountered certain error conditions, it would display internal error pages that contained user-supplied data without proper HTML escaping or script context sanitization. This created an environment where attackers could craft malicious URLs or content that would be rendered as part of the error page, allowing them to execute arbitrary JavaScript code in the browser context. The vulnerability specifically targeted the Android implementation of Firefox, which had different security handling mechanisms compared to the desktop versions, making it particularly concerning for mobile users who rely heavily on browser security for their online activities. This type of vulnerability maps directly to CWE-79, which describes cross-site scripting flaws where untrusted data is improperly integrated into web pages without proper validation or escaping mechanisms.
The operational impact of this vulnerability extended beyond simple script execution, as it enabled sophisticated spoofing attacks that could deceive users into believing they were interacting with legitimate browser components. Attackers could potentially manipulate the error page content to display fake address bars, misleading users about the actual website they were visiting, or even present fake login forms designed to capture credentials. The ability to spoof the address bar specifically represents a serious threat to user trust and security, as it could lead to credential theft, phishing attacks, or other malicious activities that rely on users believing they are interacting with legitimate browser components. This vulnerability could have been exploited in various contexts including malicious websites, compromised advertising networks, or even through social engineering campaigns that directed users to specific URLs designed to trigger the vulnerable error handling paths. The impact was particularly severe for mobile users who may have less sophisticated security awareness and are more likely to trust browser-provided interface elements.
The mitigation for this vulnerability required updating to Firefox version 85 or later, which included proper input sanitization and output escaping mechanisms for error page rendering. Security patches implemented by Mozilla addressed the root cause by ensuring that all data displayed in error pages was properly escaped and validated before being rendered in the browser's HTML context. Organizations and users were advised to immediately upgrade their Firefox for Android installations to prevent exploitation of this vulnerability. Additionally, users were encouraged to maintain awareness of browser security updates and to avoid visiting untrusted websites that might trigger malicious error conditions. This vulnerability highlights the importance of proper input validation in all user-facing interfaces, particularly in error handling systems where users expect to see information about system conditions rather than potentially malicious content. The fix implemented by Mozilla aligns with industry best practices for preventing XSS vulnerabilities and demonstrates the critical need for comprehensive security testing of all browser components, especially those that interact with user-provided data in potentially dangerous contexts. This vulnerability serves as a reminder that even seemingly benign error handling mechanisms can become attack vectors when proper security controls are not implemented.