CVE-2021-3403
📋 TL;DR
CVE-2021-3403 is a double-free vulnerability in ytnef's TNEFSubjectHandler function that allows remote attackers to cause denial-of-service or potentially execute arbitrary code via a crafted TNEF file. This affects systems processing TNEF attachments (commonly from Microsoft Outlook) with vulnerable ytnef versions. Email servers, mail clients, and applications using ytnef for TNEF parsing are at risk.
💻 Affected Systems
- ytnef
- applications using ytnef library for TNEF parsing
📦 What is this software?
Fedora by Fedoraproject
Ytnef by Ytnef Project
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to full system compromise, data theft, or lateral movement within the network.
Likely Case
Denial-of-service causing application crashes when processing malicious TNEF attachments, disrupting email processing.
If Mitigated
Contained application crash with no privilege escalation if proper sandboxing and memory protections are enabled.
🎯 Exploit Status
Exploitation requires user interaction to open a crafted TNEF file. Proof-of-concept code exists in public bug reports demonstrating the double-free condition.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: ytnef 1.9.4 and later
Vendor Advisory: https://github.com/Yeraze/ytnef/issues/85
Restart Required: Yes
Instructions:
1. Update ytnef to version 1.9.4 or later using your package manager. 2. For source installations: download latest release from GitHub, compile, and install. 3. Restart any services or applications using ytnef.
🔧 Temporary Workarounds
Disable TNEF processing
allConfigure email systems to reject or quarantine TNEF attachments instead of processing them with ytnef.
# Configure mail server to block .dat (TNEF) attachments
# Example for Postfix: body_checks = regexp:/etc/postfix/body_checks
# Add line to body_checks: /^.*Content-Type: application\/ms-tnef.*$/ REJECT
Use alternative TNEF parser
linuxReplace ytnef with a different TNEF parsing library that isn't vulnerable.
# Install alternative like libytnef alternatives
# apt-get install tnef (if available for your distribution)
🧯 If You Can't Patch
- Implement strict email filtering to block TNEF attachments at the perimeter
- Run ytnef in a sandboxed environment with limited privileges and memory protections
🔍 How to Verify
Check if Vulnerable:
Check ytnef version: ytnef --version or dpkg -l | grep ytnef or rpm -q ytnefCheck Version:
ytnef --version 2>/dev/null || dpkg -l | grep ytnef || rpm -q ytnef || find /usr -name '*ytnef*' -type f -executable 2>/dev/null | head -1 | xargs {} --version 2>/dev/nullVerify Fix Applied:
Confirm version is 1.9.4 or higher and test with known malicious TNEF file to ensure no crash occurs📡 Detection & Monitoring
Log Indicators:
- Application crashes with segmentation faults in ytnef processes
- Error messages mentioning TNEFSubjectHandler or double-free
- Unexpected termination of email processing services
Network Indicators:
- Inbound emails with TNEF attachments (.dat files) from untrusted sources
- Unusual spikes in email attachment processing failures
SIEM Query:
source="*ytnef*" AND ("segmentation fault" OR "double free" OR "SIGSEGV")