CVE-2023-6534
📋 TL;DR
A vulnerability in FreeBSD's pf(4) packet filter incorrectly validates TCP sequence numbers, allowing attackers to execute denial-of-service attacks against hosts behind affected firewalls. This affects FreeBSD 12.4, 13.2, and 14.0 systems running vulnerable versions. The vulnerability enables disruption of network services protected by the firewall.
💻 Affected Systems
- FreeBSD
📦 What is this software?
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Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
⚠️ Risk & Real-World Impact
Worst Case
Complete denial of service for all hosts behind the firewall, disrupting critical network services and business operations.
Likely Case
Targeted DoS attacks against specific services or hosts, causing service interruptions and degraded performance.
If Mitigated
Limited impact with proper network segmentation and monitoring, though vulnerable systems remain at risk.
🎯 Exploit Status
The advisory provides technical details but no public exploit code. Attackers could craft malicious TCP packets to trigger the vulnerability.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: FreeBSD 12.4-RELEASE-p9, 13.2-RELEASE-p7, 14-RELEASE-p2
Vendor Advisory: https://security.freebsd.org/advisories/FreeBSD-SA-23:17.pf.asc
Restart Required: Yes
Instructions:
1. Update system: 'freebsd-update fetch' then 'freebsd-update install' 2. Rebuild kernel if using custom kernel: 'cd /usr/src && make buildkernel && make installkernel' 3. Reboot system: 'shutdown -r now'
🔧 Temporary Workarounds
Disable pf packet filter
allTemporarily disable the vulnerable pf(4) packet filter if patching is not immediately possible.
pfctl -d
Implement rate limiting
allAdd rate limiting rules to pf configuration to mitigate DoS impact.
Add 'scrub in all max-mss 1440' and appropriate 'limit' rules to pf.conf
🧯 If You Can't Patch
- Implement network segmentation to isolate critical systems from potentially compromised firewalls
- Deploy additional DoS protection mechanisms like upstream DDoS mitigation services
🔍 How to Verify
Check if Vulnerable:
Check FreeBSD version: 'uname -a' and verify if using vulnerable version range. Check pf status: 'pfctl -s info'Check Version:
uname -aVerify Fix Applied:
Verify updated version: 'uname -a' should show patched version. Test pf functionality with legitimate traffic.📡 Detection & Monitoring
Log Indicators:
- Unusual pf state table entries
- Excessive TCP connection resets
- pf error messages in /var/log/messages
Network Indicators:
- Abnormal TCP sequence number patterns
- Sudden increase in TCP RST packets
- Service unavailability behind firewall
SIEM Query:
source="pf" AND ("state table" OR "TCP" OR "sequence") | stats count by src_ip, dest_ip