CVE-2025-3033: Mozilla Firefox File Upload Vulnerability

CVE-2025-3033 Overview

CVE-2025-3033 is a file upload vulnerability affecting Mozilla Firefox and Thunderbird on Windows systems. After selecting a malicious Windows .url shortcut from the local filesystem, an unexpected file could be uploaded. This vulnerability exploits external file reference handling (CWE-73), potentially allowing attackers to trick users into uploading sensitive files they did not intend to share.

This bug only affects Firefox and Thunderbird on Windows. Other operating systems are unaffected.

Critical Impact

Attackers can leverage malicious .url shortcut files to cause unintended file uploads, potentially exposing sensitive local files with high confidentiality and integrity impact.

Affected Products

• Mozilla Firefox versions prior to 137
• Mozilla Thunderbird versions prior to 137
• Windows operating systems only (Linux, macOS unaffected)

Discovery Timeline

• 2025-04-01 - CVE-2025-3033 published to NVD
• 2025-04-07 - Last updated in NVD database

Technical Details for CVE-2025-3033

Vulnerability Analysis

This vulnerability involves improper handling of Windows .url shortcut files during file selection operations. The External Control of File Name or Path weakness (CWE-73) allows a malicious .url file to reference a different file than what the user intended to select. When the user believes they are selecting one file, the browser may actually upload a completely different file from the local filesystem.

The attack requires local access, meaning an attacker would need to place a malicious .url file on the victim's system or convince the user to download one. No privileges are required to exploit this vulnerability, and while no user interaction is needed beyond normal file selection behavior, the attack targets confidentiality and integrity without affecting availability.

Root Cause

The root cause lies in how Firefox and Thunderbird process Windows .url shortcut files during file upload operations. These shortcut files contain references to other resources, and the browser fails to properly validate that the actual file being uploaded matches what the user selected in the file picker dialog. This external control of file name or path allows the .url file to redirect the upload operation to an unintended target file.

Attack Vector

The attack vector is local, requiring the malicious .url file to be present on the victim's Windows filesystem. An attacker could distribute these malicious shortcut files through various means:

1. Email attachments delivered via Thunderbird or other email clients
2. Files downloaded from compromised or malicious websites
3. Files placed on shared network drives
4. Files distributed through messaging applications or file-sharing services

When a victim attempts to upload a file through Firefox or Thunderbird and selects what appears to be a legitimate .url shortcut, the application may instead upload a different file referenced within the malicious shortcut. This could result in unintended disclosure of sensitive files such as configuration files, documents, or other private data.

For technical details on the vulnerability mechanism, see the Mozilla Bug Report #1950056 and the associated security advisories.

Detection Methods for CVE-2025-3033

Indicators of Compromise

• Unexpected .url shortcut files appearing in download directories or commonly accessed folders
• File upload events where the uploaded file differs from what was selected in the file picker
• Presence of .url files with URLs or references pointing to sensitive local file paths
• Network traffic showing file uploads that don't match user-initiated actions

Detection Strategies

• Monitor for the creation of .url files in user directories, especially those with suspicious URL targets
• Implement endpoint detection rules to alert on file upload operations that follow .url file access
• Review browser logs for anomalous file selection and upload patterns
• Deploy SentinelOne Singularity to detect and correlate suspicious file access patterns with network upload events

Monitoring Recommendations

• Enable verbose logging for Firefox and Thunderbird file operations on Windows endpoints
• Monitor outbound HTTP/HTTPS traffic for unexpected file upload payloads
• Implement Data Loss Prevention (DLP) solutions to detect sensitive file exfiltration
• Review endpoint telemetry for .url file creation and subsequent file read operations

How to Mitigate CVE-2025-3033

Immediate Actions Required

• Update Mozilla Firefox to version 137 or later immediately
• Update Mozilla Thunderbird to version 137 or later immediately
• Review recent file uploads for any unexpected content
• Scan Windows systems for suspicious .url shortcut files in common directories
• Educate users about the risks of interacting with .url files from untrusted sources

Patch Information

Mozilla has released patches addressing this vulnerability in Firefox 137 and Thunderbird 137. Organizations should prioritize updating all Windows installations of these applications. Detailed patch information is available in the Mozilla Security Advisory MFSA-2025-20 for Firefox and Mozilla Security Advisory MFSA-2025-23 for Thunderbird.

Workarounds

• Block or quarantine .url files at the email gateway and web proxy level until patching is complete
• Implement application control policies to prevent execution or interaction with .url files from untrusted locations
• Use enterprise browser policies to restrict file upload capabilities to trusted domains only
• Consider temporarily using Firefox or Thunderbird on non-Windows platforms for sensitive operations
• Deploy endpoint protection solutions like SentinelOne to monitor and block suspicious file manipulation activities

# Example: Block .url file downloads via browser policy (Firefox)
# Create or edit policies.json in Firefox installation directory
# Location: C:\Program Files\Mozilla Firefox\distribution\policies.json

# Add download restriction policy:
# {
#   "policies": {
#     "Extensions": {
#       "Install": ["https://addons.mozilla.org/*/"]
#     },
#     "DisableBuiltinPDFViewer": false,
#     "DownloadDirectory": "C:\\Downloads",
#     "PromptForDownloadLocation": true
#   }
# }