CVE-2025-4086 Overview
CVE-2025-4086 is a user interface spoofing vulnerability affecting Thunderbird for Android. A specially crafted filename containing a large number of encoded newline characters can obscure the file extension when displayed in the download dialog. Attackers can leverage this behavior to disguise executable or malicious attachments as benign file types. The issue is tracked under [CWE-451: User Interface (UI) Misrepresentation of Critical Information]. Mozilla addressed the flaw in Firefox 138 and Thunderbird 138. Desktop versions of Thunderbird are not affected by this specific issue.
Critical Impact
Attackers can trick mobile users into opening malicious files by hiding the true extension in the download dialog, enabling phishing and malware delivery scenarios.
Affected Products
- Mozilla Thunderbird for Android (versions prior to 138)
- Mozilla Firefox (versions prior to 138)
- Mozilla Thunderbird (versions prior to 138)
Discovery Timeline
- 2025-04-29 - CVE-2025-4086 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-4086
Vulnerability Analysis
The vulnerability resides in how the Thunderbird for Android download dialog renders filenames supplied by remote content. When a filename contains a large number of encoded newline characters, the rendering logic wraps the visible text such that the true file extension is pushed out of view. Users see a truncated or misleading name and cannot verify the actual file type before choosing to open or save the attachment.
This class of issue falls under user interface misrepresentation. The underlying file is delivered unchanged, but the security-critical metadata shown to the user is unreliable. Attackers can pair this with social engineering to distribute Android application packages or executable content disguised as documents, images, or media files.
Mozilla fixed the defect in Thunderbird 138 and the parallel Firefox 138 release. Details are tracked in Mozilla Bug Report #1945705 and documented in Mozilla Security Advisory MFSA-2025-28 and MFSA-2025-31.
Root Cause
The download dialog does not sanitize or normalize newline sequences within filenames before rendering. Encoded newline characters expand the visible name across multiple lines, hiding the extension segment beyond the visible dialog area on mobile screens.
Attack Vector
An attacker hosts a file or sends an email attachment whose Content-Disposition header specifies a filename padded with many encoded newline characters followed by an executable or Android-specific extension. When the target opens the download prompt on Thunderbird for Android, the dialog obscures the real extension. No authentication or user privilege is required to deliver the crafted filename.
The vulnerability is triggered client-side without exploitation code. See the referenced Mozilla advisory for further technical context.
Detection Methods for CVE-2025-4086
Indicators of Compromise
- Downloaded files on Android devices with filenames containing repeated %0A, %0D, or literal newline byte sequences.
- Email attachments delivered to Thunderbird for Android with abnormally long filenames or filenames containing control characters.
- Executable or APK files saved through Thunderbird with names that visually resemble documents or images.
Detection Strategies
- Inspect mail gateway logs and MIME headers for Content-Disposition filenames containing encoded newline sequences.
- Alert on attachments where the declared filename length substantially exceeds typical values or contains non-printable characters.
- Correlate mobile device management telemetry with downloads originating from Thunderbird for Android across the enterprise fleet.
Monitoring Recommendations
- Monitor for Thunderbird for Android client versions below 138 across managed devices.
- Track outbound and inbound email flows for attachments with suspicious filename patterns and quarantine for review.
- Review file execution telemetry on Android endpoints for launches originating from Thunderbird's download directory.
How to Mitigate CVE-2025-4086
Immediate Actions Required
- Update Thunderbird for Android to version 138 or later on all managed and personal devices.
- Update Mozilla Firefox to version 138 or later where the fix is bundled.
- Enforce mobile device management policies that block execution of untrusted APKs and executables downloaded from email clients.
Patch Information
Mozilla published fixes in Thunderbird 138 and Firefox 138. Reference Mozilla Security Advisory MFSA-2025-28 and Mozilla Security Advisory MFSA-2025-31 for full release notes and version details.
Workarounds
- Instruct Android users to preview downloaded files in a file manager that shows the full name and MIME type before opening.
- Configure mail security gateways to strip or rewrite filenames containing control characters and encoded newline sequences.
- Restrict installation of applications from unknown sources on managed Android devices through mobile device management controls.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
