CVE-2021-3738
📋 TL;DR
CVE-2021-3738 is a use-after-free vulnerability in Samba's DCE/RPC implementation where shared database handles can reference invalid user credential state after connection termination. This could lead to crashes or potentially allow privilege escalation by pointing to different user state. Organizations running vulnerable Samba versions are affected.
💻 Affected Systems
- Samba
📦 What is this software?
Samba by Samba
Samba by Samba
Samba by Samba
⚠️ Risk & Real-World Impact
Worst Case
Privilege escalation allowing attackers to gain administrative access to Samba servers and potentially compromise domain controllers or sensitive file shares.
Likely Case
Service crashes leading to denial of service and disruption of file sharing, authentication, and directory services.
If Mitigated
Limited impact with proper network segmentation, access controls, and monitoring in place.
🎯 Exploit Status
Exploitation requires authenticated access and specific conditions with association groups. No public exploits have been documented.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Samba 4.13.18, 4.14.13, 4.15.6 and later
Vendor Advisory: https://www.samba.org/samba/security/CVE-2021-3738.html
Restart Required: Yes
Instructions:
1. Identify Samba version with 'smbd -V'. 2. Update using package manager: 'apt update && apt upgrade samba' (Debian/Ubuntu) or 'yum update samba' (RHEL/CentOS). 3. Restart Samba services: 'systemctl restart smbd nmbd winbind' or 'service samba restart'.
🔧 Temporary Workarounds
Disable AD DC role if not needed
linuxIf Samba is only used as file server without Active Directory Domain Controller functionality, disable AD DC role to remove vulnerable component.
# Check if AD DC is enabled
samba-tool domain level show
# To disable AD DC, reconfigure Samba as standalone server
Network segmentation
linuxRestrict access to Samba DCE/RPC ports (135, 139, 445) to trusted networks only.
# Example iptables rule to restrict port 445
iptables -A INPUT -p tcp --dport 445 -s trusted_network -j ACCEPT
iptables -A INPUT -p tcp --dport 445 -j DROP
🧯 If You Can't Patch
- Implement strict access controls to limit which users can authenticate to Samba services
- Monitor for unusual authentication patterns or service crashes that might indicate exploitation attempts
🔍 How to Verify
Check if Vulnerable:
Run 'smbd -V' and check if version is between 4.13.0-4.13.17, 4.14.0-4.14.12, or 4.15.0-4.15.5. Also verify AD DC role is enabled with 'samba-tool domain level show'.Check Version:
smbd -VVerify Fix Applied:
After patching, verify version is 4.13.18+, 4.14.13+, or 4.15.6+ with 'smbd -V'. Test Samba services are functioning normally.📡 Detection & Monitoring
Log Indicators:
- Samba service crashes in system logs
- Unusual authentication patterns in Samba logs
- Errors related to 'struct session_info' or association groups
Network Indicators:
- Multiple DCE/RPC connections from single source with different credentials
- Unusual traffic patterns on ports 135, 139, 445
SIEM Query:
source="samba.log" AND ("crash" OR "segfault" OR "use-after-free" OR "association group")🔗 References
- https://bugzilla.redhat.com/show_bug.cgi?id=2021726
- https://bugzilla.samba.org/show_bug.cgi?id=14468
- https://security.gentoo.org/glsa/202309-06
- https://www.samba.org/samba/security/CVE-2021-3738.html
- https://bugzilla.redhat.com/show_bug.cgi?id=2021726
- https://bugzilla.samba.org/show_bug.cgi?id=14468
- https://security.gentoo.org/glsa/202309-06
- https://www.samba.org/samba/security/CVE-2021-3738.html
