CVE-2021-3773
📋 TL;DR
A netfilter flaw allows network-connected attackers to infer OpenVPN connection endpoint information by analyzing network traffic patterns. This affects Linux systems running OpenVPN with netfilter enabled. Attackers can use this information to target VPN endpoints with traditional network attacks.
💻 Affected Systems
- Linux kernel with netfilter
- OpenVPN
📦 What is this software?
Communications Cloud Native Core Binding Support Function by Oracle
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Communications Cloud Native Core Network Exposure Function by Oracle
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Communications Cloud Native Core Policy by Oracle
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Fedora by Fedoraproject
Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →⚠️ Risk & Real-World Impact
Worst Case
Attackers identify and directly target VPN endpoints with DDoS, port scanning, or other network attacks, potentially compromising VPN infrastructure and exposing internal networks.
Likely Case
Attackers map VPN infrastructure and perform reconnaissance to plan targeted attacks against exposed endpoints.
If Mitigated
Limited information leakage with minimal impact if VPN endpoints are already hardened and monitored.
🎯 Exploit Status
Exploitation requires network access to monitor traffic patterns but no authentication. Public research and presentations demonstrate the technique.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Varies by Linux distribution - check vendor advisories for specific kernel versions
Vendor Advisory: https://bugzilla.redhat.com/show_bug.cgi?id=2004949
Restart Required: Yes
Instructions:
1. Check your Linux distribution's security advisories. 2. Update kernel packages to patched versions. 3. Reboot system to load new kernel. 4. Verify OpenVPN functionality post-update.
🔧 Temporary Workarounds
Disable affected netfilter modules
linuxRemove or disable netfilter modules that leak timing information
modprobe -r nf_conntrack_ftp
modprobe -r nf_nat_ftp
Network segmentation
allIsolate VPN traffic from potential attackers
🧯 If You Can't Patch
- Implement network monitoring and intrusion detection for unusual traffic patterns to VPN endpoints
- Harden VPN endpoints with additional firewall rules and rate limiting
🔍 How to Verify
Check if Vulnerable:
Check kernel version against vendor advisories and verify netfilter modules are loaded: lsmod | grep nf_conntrack_ftpCheck Version:
uname -rVerify Fix Applied:
Verify updated kernel version and test that affected netfilter modules are not loaded or patched📡 Detection & Monitoring
Log Indicators:
- Unusual connection patterns to VPN endpoints
- Increased scanning activity against VPN infrastructure
Network Indicators:
- Abnormal traffic analysis patterns targeting VPN ports
- Reconnaissance activity preceding attacks on VPN endpoints
SIEM Query:
source_ip OUTSIDE internal_net AND dest_port IN (1194, 443) AND protocol=tcp AND bytes_transferred < threshold GROUP BY hour🔗 References
- https://bugzilla.redhat.com/show_bug.cgi?id=2004949
- https://citizenlab.ca/2024/07/vulnerabilities-in-vpns-paper-presented-at-the-privacy-enhancing-technologies-symposium-2024/
- https://www.oracle.com/security-alerts/cpujul2022.html
- https://bugzilla.redhat.com/show_bug.cgi?id=2004949
- https://citizenlab.ca/2024/07/vulnerabilities-in-vpns-paper-presented-at-the-privacy-enhancing-technologies-symposium-2024/
- https://security.netapp.com/advisory/ntap-20250328-0004/
- https://www.oracle.com/security-alerts/cpujul2022.html
