Adversaries may abuse SQL stored procedures to establish persistent access to systems. SQL Stored Procedures are code that can be saved and reused so that database users do not waste time rewriting frequently used SQL queries. Stored procedures can be invoked via SQL statements to the database using the procedure name or via defined events (e.g. when a SQL server application is started/restarted).

Adversaries may craft malicious stored procedures that can provide a persistence mechanism in SQL database servers.(Citation: NetSPI Startup Stored Procedures)(Citation: Kaspersky MSSQL Aug 2019) To execute operating system commands through SQL syntax the adversary may have to enable additional functionality, such as xp_cmdshell for MSSQL Server.(Citation: NetSPI Startup Stored Procedures)(Citation: Kaspersky MSSQL Aug 2019)(Citation: Microsoft xp_cmdshell 2017) 

Microsoft SQL Server can enable common language runtime (CLR) integration. With CLR integration enabled, application developers can write stored procedures using any .NET framework language (e.g. VB .NET, C#, etc.).(Citation: Microsoft CLR Integration 2017) Adversaries may craft or modify CLR assemblies that are linked to stored procedures since these CLR assemblies can be made to execute arbitrary commands.(Citation: NetSPI SQL Server CLR) 
SQL Stored Procedures
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1505/001  external_id: T1505.001     source_name: NetSPI Startup Stored Procedures  description: Sutherland, S. (2016, March 7). Maintaining Persistence via SQL Server – Part 1: Startup Stored Procedures. Retrieved July 8, 2019.  url: https://blog.netspi.com/sql-server-persistence-part-1-startup-stored-procedures/     source_name: Kaspersky MSSQL Aug 2019  description: Plakhov, A., Sitchikhin, D. (2019, August 22). Agent 1433: remote attack on Microsoft SQL Server. Retrieved September 4, 2019.  url: https://securelist.com/malicious-tasks-in-ms-sql-server/92167/     source_name: Microsoft xp_cmdshell 2017  description: Microsoft. (2017, March 15). xp_cmdshell (Transact-SQL). Retrieved September 9, 2019.  url: https://docs.microsoft.com/en-us/sql/relational-databases/system-stored-procedures/xp-cmdshell-transact-sql?view=sql-server-2017     source_name: Microsoft CLR Integration 2017  description: Microsoft. (2017, June 19). Common Language Runtime Integration. Retrieved July 8, 2019.  url: https://docs.microsoft.com/en-us/sql/relational-databases/clr-integration/common-language-runtime-integration-overview?view=sql-server-2017     source_name: NetSPI SQL Server CLR  description: Sutherland, S. (2017, July 13). Attacking SQL Server CLR Assemblies. Retrieved July 8, 2019.  url: https://blog.netspi.com/attacking-sql-server-clr-assemblies/     