CVE-2025-5270
📋 TL;DR
This vulnerability allows unencrypted transmission of Server Name Indication (SNI) data even when encrypted DNS is enabled, potentially exposing which websites users are visiting. It affects Firefox versions before 139 and Thunderbird versions before 139. Attackers on the network could intercept this information to monitor user browsing activity.
💻 Affected Systems
- Mozilla Firefox
- Mozilla Thunderbird
📦 What is this software?
Firefox by Mozilla
⚠️ Risk & Real-World Impact
Worst Case
Attackers could map all websites visited by users, enabling targeted phishing, surveillance, or censorship based on browsing patterns.
Likely Case
Network observers could see which domains users access, compromising privacy but not directly enabling code execution or data theft.
If Mitigated
With proper network segmentation and monitoring, impact is limited to privacy leakage of domain names only.
🎯 Exploit Status
Exploitation is passive - attackers only need to monitor network traffic, no active attack required.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firefox 139, Thunderbird 139
Vendor Advisory: https://www.mozilla.org/security/advisories/mfsa2025-42/
Restart Required: Yes
Instructions:
1. Open Firefox/Thunderbird. 2. Click menu → Help → About Firefox/Thunderbird. 3. Allow automatic update to version 139 or higher. 4. Restart browser when prompted.
🔧 Temporary Workarounds
Disable Encrypted DNS
allRevert to standard DNS which doesn't have this SNI leakage issue
Firefox: about:preferences → General → Network Settings → Settings → Enable DNS over HTTPS (uncheck)
Thunderbird: Preferences → Privacy & Security → Enable DNS over HTTPS (uncheck)
🧯 If You Can't Patch
- Use VPN to encrypt all network traffic, preventing SNI interception
- Disable encrypted DNS features and use standard DNS resolution
🔍 How to Verify
Check if Vulnerable:
Check browser version: Firefox/Thunderbird → Help → About. If version is below 139 and encrypted DNS is enabled, system is vulnerable.Check Version:
Firefox: about:support → Application Basics → Version. Thunderbird: Help → About Thunderbird.Verify Fix Applied:
Confirm version is 139 or higher and encrypted DNS remains functional without SNI leakage.📡 Detection & Monitoring
Log Indicators:
- No direct application logs for this vulnerability
Network Indicators:
- Network monitoring showing unencrypted SNI in TLS handshakes from Firefox/Thunderbird clients with encrypted DNS enabled
SIEM Query:
Not applicable - this is a privacy leakage not an active attack