CVE-2025-6426: Mozilla Firefox File Warning Vulnerability

CVE-2025-6426 Overview

CVE-2025-6426 is an Insufficient Verification of Data Authenticity vulnerability (CWE-345) affecting Mozilla Firefox and Thunderbird on macOS. The executable file warning mechanism failed to alert users before opening files with the .terminal extension, allowing potentially malicious terminal scripts to execute without the expected security warning that would normally prompt users to confirm execution of executable content.

This vulnerability specifically impacts macOS users, as .terminal files are executable by macOS Terminal.app and can contain arbitrary shell commands. When a user downloads or opens such a file through Firefox or Thunderbird on macOS, the browser's normal executable warning dialog is bypassed, removing a critical security barrier that protects users from inadvertent execution of malicious scripts.

Critical Impact

Attackers can deliver malicious .terminal files to macOS users via web downloads or email attachments, bypassing Firefox/Thunderbird's executable file warning and potentially achieving code execution with user privileges.

Affected Products

• Mozilla Firefox versions prior to 140 (macOS only)
• Mozilla Firefox ESR versions prior to 128.12 (macOS only)
• Mozilla Thunderbird versions prior to 140 (macOS only)
• Mozilla Thunderbird ESR versions prior to 128.12 (macOS only)

Discovery Timeline

• 2025-06-24 - CVE-2025-6426 published to NVD
• 2025-07-14 - Last updated in NVD database

Technical Details for CVE-2025-6426

Vulnerability Analysis

The vulnerability stems from an incomplete implementation of Firefox's executable file type detection mechanism on macOS. Firefox maintains a list of file extensions that should trigger a warning dialog before opening, alerting users that a file may be executable and potentially dangerous. The .terminal extension, which is recognized by macOS as an executable format that launches Terminal.app, was not included in this protective list.

When users encounter files with extensions like .exe, .app, or .dmg, Firefox's security layer intercepts the open request and displays a warning. However, .terminal files were processed without this intervention, allowing them to be opened directly by the operating system's default handler—Terminal.app—which immediately executes any commands contained within the file.

This represents a significant gap in the defense-in-depth model that browsers employ to protect users from social engineering attacks. The oversight is particularly concerning because .terminal files can contain arbitrary shell commands with full access to user-level system resources.

Root Cause

The root cause is an Insufficient Verification of Data Authenticity issue (CWE-345) in Firefox's file handling subsystem. The browser's executable file type whitelist for macOS did not include the .terminal extension, despite this file type being capable of arbitrary code execution when opened. This oversight in the file type classification allowed these files to bypass the security warning mechanism entirely.

Attack Vector

The attack vector is network-based and requires user interaction. An attacker could craft a malicious .terminal file containing shell commands and distribute it via:

1. Web Downloads: Hosting the file on a compromised or malicious website where victims are socially engineered to download it
2. Email Attachments: Sending the file as an attachment through Thunderbird, where the same warning bypass exists
3. Drive-by Downloads: Automatically triggering a download through JavaScript on a malicious webpage

When the victim opens the downloaded file—expecting Firefox's warning dialog—the file instead opens directly in Terminal.app and executes the embedded commands with the user's privileges. This could lead to data theft, malware installation, or further system compromise.

The attack is particularly effective because macOS users may not recognize .terminal as an executable file type, and the absence of Firefox's warning removes the last line of defense before execution.

Detection Methods for CVE-2025-6426

Indicators of Compromise

• Presence of unexpected .terminal files in browser download directories or email attachment folders
• Terminal.app process launches that correlate with browser download activity or email attachment opens
• Unusual shell command execution originating from Terminal.app shortly after file downloads
• Network connections initiated by Terminal.app processes following browser file interactions

Detection Strategies

• Monitor file system events for .terminal file creation in user download directories and temporary folders
• Implement endpoint detection rules that correlate Firefox/Thunderbird download events with subsequent Terminal.app execution
• Configure browser extension or endpoint security policies to block or quarantine .terminal file downloads
• Review macOS unified logs for Terminal.app launches that occur within seconds of browser file handling operations

Monitoring Recommendations

• Deploy SentinelOne agents configured to monitor and alert on suspicious Terminal.app execution patterns
• Enable enhanced file type monitoring for macOS-specific executable extensions including .terminal, .command, and .workflow
• Implement download monitoring solutions that inspect file content regardless of extension
• Configure security information and event management (SIEM) rules to correlate browser downloads with shell execution events

How to Mitigate CVE-2025-6426

Immediate Actions Required

• Update Mozilla Firefox to version 140 or later on all macOS systems
• Update Mozilla Firefox ESR to version 128.12 or later on all macOS systems
• Update Mozilla Thunderbird to version 140 or later on all macOS systems
• Update Mozilla Thunderbird ESR to version 128.12 or later on all macOS systems
• Audit systems for any recently downloaded .terminal files and quarantine suspicious files

Patch Information

Mozilla has released patches addressing this vulnerability in Firefox 140, Firefox ESR 128.12, Thunderbird 140, and Thunderbird ESR 128.12. The fix adds the .terminal extension to the list of file types that trigger the executable file warning dialog on macOS. Organizations should prioritize updating all affected browsers and email clients to these versions or later.

For detailed patch information, refer to Mozilla Security Advisory MFSA-2025-51 and Mozilla Security Advisory MFSA-2025-53. The underlying bug details are tracked in Mozilla Bug Report #1964385.

Workarounds

• Configure macOS Launch Services to prevent .terminal files from automatically opening in Terminal.app
• Implement endpoint security policies that quarantine or block .terminal file downloads
• Use browser extensions or enterprise policies to restrict downloads of potentially dangerous file types
• Educate users to avoid opening unexpected .terminal files and to manually verify downloads before execution