CVE-2024-3863 – This vulnerability allows attackers to download... (How to Fix)

Q: How do I fix CVE-2024-3863?

1. Open the browser/application. 2. Go to Settings > Help > About Firefox/Thunderbird. 3. Allow the update to download and install. 4. Restart the application when prompted.

Q: What is the severity of CVE-2024-3863?

CVE-2024-3863 has a CVSS score of 9.8 (CRITICAL). This vulnerability allows attackers to download malicious .xrm-ms files without the usual executable file warning in Firefox, Thunderbird, and Firefox ESR on Windows. Attackers could trick users into executing harmful code by disguising it as a legitimate download. Only Windows users of affected browser versions are impacted.

📋 TL;DR

This vulnerability allows attackers to download malicious .xrm-ms files without the usual executable file warning in Firefox, Thunderbird, and Firefox ESR on Windows. Attackers could trick users into executing harmful code by disguising it as a legitimate download. Only Windows users of affected browser versions are impacted.

💻 Affected Systems

Products:

Firefox
Firefox ESR
Thunderbird

Versions: Firefox < 125, Firefox ESR < 115.10, Thunderbird < 115.10

Operating Systems: Windows

Default Config Vulnerable: ⚠️ Yes

Notes: Only affects Windows; Linux, macOS, and other OS are not vulnerable.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Remote code execution leading to full system compromise, data theft, or ransomware deployment.

🟠

Likely Case

Malware installation through social engineering where users download and execute disguised malicious files.

🟢

If Mitigated

Limited impact if users have updated browsers, use alternative OS, or have endpoint protection blocking suspicious downloads.

🌐 Internet-Facing: HIGH - Attackers can host malicious files on websites accessible to any user.

🏢 Internal Only: MEDIUM - Requires user interaction to download and execute, but internal phishing could still succeed.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

Exploitation requires user interaction to download and execute the file, but the technical barrier is low once the malicious file is crafted.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Firefox 125, Firefox ESR 115.10, Thunderbird 115.10

Vendor Advisory: https://www.mozilla.org/security/advisories/mfsa2024-18/

Restart Required: Yes

Instructions:

1. Open the browser/application. 2. Go to Settings > Help > About Firefox/Thunderbird. 3. Allow the update to download and install. 4. Restart the application when prompted.

🔧 Temporary Workarounds

Disable automatic downloads for .xrm-ms files

windows

Configure browser settings to prompt before downloading .xrm-ms files or block them entirely.

Manual configuration in browser settings: Settings > Privacy & Security > Downloads > Always ask you where to save files

Use alternative browser or OS

all

Switch to an unaffected browser (e.g., Chrome, Edge) or use a non-Windows operating system until patched.

🧯 If You Can't Patch

Implement network filtering to block downloads of .xrm-ms files from untrusted sources.
Educate users to avoid downloading .xrm-ms files and to verify file sources before execution.

🔍 How to Verify

Check if Vulnerable:

Check browser version in Settings > Help > About Firefox/Thunderbird and compare to affected versions.

Check Version:

In Firefox/Thunderbird, navigate to about:support and check 'Application Version'.

Verify Fix Applied:

Confirm version is Firefox 125+, Firefox ESR 115.10+, or Thunderbird 115.10+ after update.

📡 Detection & Monitoring

Log Indicators:

Log entries showing downloads of .xrm-ms files from suspicious domains
Endpoint logs indicating execution of .xrm-ms files

Network Indicators:

HTTP traffic downloading .xrm-ms files from unknown IPs
DNS requests for domains hosting .xrm-ms files

SIEM Query:

source="browser_logs" AND file_extension=".xrm-ms" AND user_action="download"

📊 Metadata

CVE ID: CVE-2024-3863

CVSS v3 Score: 9.8 (CRITICAL)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CWE: CWE-434

Published: April 16, 2024

Last Updated: March 28, 2025

🔗 References

https://bugzilla.mozilla.org/show_bug.cgi?id=1885855 Issue Tracking
https://www.mozilla.org/security/advisories/mfsa2024-18/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2024-19/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2024-20/ Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1885855 Issue Tracking
https://www.mozilla.org/security/advisories/mfsa2024-18/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2024-19/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2024-20/ Vendor Advisory

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🔗 Related Vulnerabilities

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