CVE-2025-4086 in Thunderbird
Summary
by MITRE • 04/29/2025
A specially crafted filename containing a large number of encoded newline characters could obscure the file's extension when displayed in the download dialog. *This bug only affects Firefox for Android. Other versions of Firefox are unaffected.* This vulnerability affects Firefox < 138 and Thunderbird < 138.
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Analysis
by VulDB Data Team • 01/30/2026
This vulnerability represents a sophisticated display manipulation attack that exploits the way graphical user interfaces handle filename rendering in mobile browser environments. The flaw specifically targets Firefox for Android's download dialog implementation where maliciously crafted filenames containing excessive encoded newline characters can manipulate the visual presentation of file extensions. This creates a deceptive user interface element where the actual file type becomes obscured or hidden from view, potentially enabling social engineering attacks or malicious file execution scenarios. The vulnerability demonstrates a critical weakness in input validation and display sanitization within mobile browser environments where character encoding and line break handling can be manipulated to alter visual presentation.
The technical implementation of this vulnerability relies on the manipulation of newline character sequences within filename strings to exploit rendering behaviors in Firefox's Android download dialog. When the browser processes these specially crafted filenames, the encoded newline characters interfere with the normal display formatting logic, causing the file extension portion to be visually truncated, overlapped, or otherwise obscured in the user interface. This creates a situation where users cannot properly identify the actual file type they are about to download, as the extension that typically indicates file type and security implications becomes visually hidden or misrepresented. The vulnerability operates at the application layer interface rendering level, specifically targeting the visual presentation logic rather than the underlying file processing or security mechanisms.
The operational impact of this vulnerability extends beyond simple visual deception to potentially enable more serious security consequences. Attackers could craft filenames that appear to be benign extensions like .txt or .pdf while actually containing malicious executables with hidden extensions. This creates a significant risk for mobile users who rely on visual cues to make security decisions about file downloads. The vulnerability affects both Firefox and Thunderbird applications, though the specific implementation impacts only Firefox for Android, indicating that the core issue lies in mobile-specific rendering code paths. This type of attack aligns with attack patterns categorized under CWE-155 and CWE-74, which address improper neutralization of special elements and improper handling of line terminators in input processing.
The security implications of this vulnerability are particularly concerning in mobile environments where users may have limited ability to verify file types through alternative means. Mobile users often rely heavily on visual interface cues for security decisions, making this form of UI manipulation particularly dangerous. The vulnerability affects versions prior to 138, indicating that proper input sanitization and display validation mechanisms were not adequately implemented in earlier releases. Organizations should consider this vulnerability as part of broader mobile browser security assessments and implement proper input validation at the application layer. The remediation requires updating to affected versions where proper filename sanitization and display handling has been implemented, along with potential security awareness training for users about the risks of downloading files with suspicious or unexpected filenames.
This vulnerability highlights the importance of proper input validation and display sanitization in mobile applications, particularly those that handle user interface elements with security implications. The attack vector demonstrates how seemingly minor implementation details in UI rendering can create significant security risks. Security practitioners should monitor for similar issues in other mobile applications and browser implementations, as this type of vulnerability often indicates broader architectural weaknesses in how applications handle special character sequences and display formatting. The issue also underscores the need for comprehensive security testing of user interface elements, particularly in mobile environments where display constraints and character handling can create unexpected attack surfaces. Organizations should implement proper character encoding validation and ensure that user interface elements properly sanitize and validate all input before display to prevent similar manipulation attacks.