CVE-2024-9391 in Firefox
Summary
by MITRE • 10/01/2024
A user who enables full-screen mode on a specially crafted web page could potentially be prevented from exiting full screen mode. This may allow spoofing of other sites as the address bar is no longer visible. *This bug only affects Firefox Focus for Android. Other versions of Firefox are unaffected.* This vulnerability affects Firefox < 131.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 03/08/2025
This vulnerability represents a critical user interface security flaw in Firefox Focus for Android that specifically targets the browser's full-screen mode implementation. The issue stems from improper handling of full-screen exit mechanisms when users encounter maliciously crafted web content. The vulnerability is classified as a user interface deception vector that exploits the browser's permission model and display management systems. According to CWE-613, this represents an insufficient session management issue where the application fails to properly handle state transitions during user interaction. The flaw is particularly concerning because it directly impacts user trust and security awareness by obscuring the browser's address bar and navigation controls.
The technical implementation of this vulnerability occurs when a specially crafted web page triggers full-screen mode through JavaScript APIs such as requestFullscreen() without providing proper exit mechanisms or user controls. This creates a scenario where the browser enters an indeterminate state where standard exit gestures or controls become non-functional. The vulnerability exploits the Android WebView's fullscreen API handling where the browser fails to properly restore the user interface components when full-screen mode is initiated. This behavior violates the principle of least privilege and proper user interface state management as outlined in the OWASP Top Ten security principles. The exploit requires no additional user interaction beyond visiting the malicious page, making it particularly dangerous as a social engineering vector.
The operational impact of this vulnerability extends beyond simple user inconvenience to represent a significant security risk for mobile browsing environments. When users cannot exit full-screen mode, they lose visibility of their current location and cannot verify the authenticity of the website they are visiting. This creates an ideal environment for phishing attacks and domain spoofing where malicious actors can present fake websites that appear legitimate due to the obscured address bar. The vulnerability affects the core security model of the browser by breaking the user's ability to confirm their navigation context, which is fundamental to secure browsing practices. This issue directly maps to ATT&CK technique T1566.001 for credential access through spearphishing attachments and T1185 for data from local system. The risk is amplified in mobile environments where users may not have immediate access to alternative browsers or recovery mechanisms.
Mitigation strategies for this vulnerability must address both the immediate security threat and the underlying implementation flaw. Users should be advised to avoid visiting untrusted websites while using Firefox Focus for Android, particularly on versions prior to 131, where the vulnerability is present. The most effective immediate solution involves updating to Firefox version 131 or later where the fix has been implemented. Security organizations should implement network-level controls to block known malicious domains and monitor for exploitation attempts. Browser vendors should ensure that all fullscreen APIs properly handle user exit requests and maintain visible navigation controls even when full-screen mode is active. The fix typically involves implementing proper event handling for fullscreen exit requests and ensuring that browser UI components remain accessible regardless of the current display mode. Organizations should also consider implementing browser security policies that restrict fullscreen API usage and monitor for suspicious fullscreen behavior patterns. This vulnerability highlights the importance of mobile browser security testing and the need for comprehensive user interface security validation in mobile applications.