CVE-2021-4221 in Firefox
Summary
by MITRE • 12/22/2022
If a domain name contained a RTL character, it would cause the domain to be rendered to the right of the path. This could lead to user confusion and spoofing attacks. <br>*This bug only affects Firefox for Android. Other operating systems are unaffected.*<br>*Note*: Due to a clerical error this advisory was not included in the original announcement, and was added in Feburary 2022. This vulnerability affects Firefox < 92.
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Analysis
by VulDB Data Team • 04/16/2025
This vulnerability represents a sophisticated internationalization issue that exploits Unicode bidirectional algorithm behavior in Firefox for Android. The flaw occurs when domain names contain right-to-left Unicode characters such as Arabic or Hebrew script characters, which cause the browser to render the domain name in a manner that appears to be positioned to the right of the URL path. This creates a deceptive visual presentation where malicious actors could manipulate domain names to appear legitimate while actually directing users to different targets. The vulnerability specifically impacts Firefox for Android users, making it a mobile-specific security concern that demonstrates the complexities of international character set handling in web browsers.
The technical root cause of this vulnerability lies in how Firefox for Android processes Unicode bidirectional text rendering within URL display mechanisms. When a domain name contains right-to-left characters, the browser's rendering engine applies Unicode bidirectional algorithm rules that can cause the domain portion to be visually displaced from its expected position in the URL bar. This creates a situation where users might be misled into believing they are visiting a trusted domain when they are actually navigating to a different location. The flaw essentially undermines the fundamental security principle of URL authenticity that users rely upon for safe navigation.
The operational impact of this vulnerability extends beyond simple user confusion to enable sophisticated phishing attacks and domain spoofing techniques. Attackers could craft domain names that appear legitimate due to the visual displacement caused by RTL characters, potentially leading users to inadvertently submit sensitive information to malicious sites. This vulnerability particularly affects mobile users who may be less vigilant about URL verification due to smaller screen sizes and different browsing habits compared to desktop users. The issue represents a significant risk to user security and trust in the browser's navigation interface, as it creates an avenue for deception that traditional security measures cannot easily detect or prevent.
Mitigation strategies for this vulnerability focus on both immediate browser updates and user education approaches. The primary solution involves updating to Firefox version 92 or later where the rendering behavior has been corrected to properly handle Unicode bidirectional text without causing visual displacement of domain names. Organizations should ensure their mobile device management policies include mandatory browser updates for all Firefox for Android installations. Users should be educated about the importance of carefully verifying URLs even when they appear to be from trusted sources, particularly when browsing on mobile devices. Security teams should also consider implementing additional URL monitoring systems that can detect suspicious character sequences in domain names, as this vulnerability could potentially be exploited in combination with other internationalization-based attacks. This issue aligns with CWE-177 weakness category for insecure handling of Unicode characters and could be mapped to ATT&CK technique T1566 for credential harvesting through deceptive web navigation.
The vulnerability demonstrates the critical importance of comprehensive internationalization testing in security-critical applications, particularly mobile browsers that must handle diverse character sets from global users. It highlights the need for security teams to consider localization and internationalization factors when assessing potential attack vectors, as these seemingly benign character rendering issues can create significant security implications. The delayed disclosure and correction of this vulnerability also underscores the importance of thorough testing processes and the need for security researchers to maintain awareness of all aspects of browser behavior, including edge cases related to Unicode handling and text rendering algorithms.