Server Software Component

Adversaries may abuse legitimate extensible development features of server applications to establish persistent access to systems. Enterprise server applications may include features that allow application developers to write and install software to extend the functionality of the main application. Adversaries may install malicious software components to maliciously extend and abuse server applications.

Transport Agent

Microsoft Exchange transport agents can operate on email messages passing through the transport pipeline to perform various tasks such as filtering spam, filtering malicious attachments, journaling, or adding a corporate signature to the end of all outgoing emails.[1][2] Transport agents can be written by application developers and then compiled to .NET assemblies that are subsequently registered with the Exchange server. Transport agents will be invoked during a specified stage of email processing and carry out developer defined tasks.

Adversaries may register a malicious transport agent to provide a persistence mechanism in Exchange Server that can be triggered by adversary-specified email events.[2] Though a malicious transport agent may be invoked for all emails passing through the Exchange transport pipeline, the agent can be configured to only carry out specific tasks in response to adversary defined criteria. For example, the transport agent may only carry out an action like copying in-transit attachments and saving them for later exfiltration if the recipient email address matches an entry on a list provided by the adversary.

SQL Stored Procedures

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.[3][4] To execute operating system commands through SQL syntax the adversary may have to enable additional functionality, such as xp_cmdshell for MSSQL Server.[3][4][5]

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.).[6] Adversaries may craft or modify CLR assemblies that are linked to stored procedures, these CLR assemblies can be made to execute arbitrary commands.[7]

ID: T1505
Tactic: Persistence
Platform: Windows, Linux
Permissions Required: Administrator, SYSTEM, root
Data Sources: File monitoring, Application logs
Contributors: Christoffer Strömblad; Kaspersky; Lucas da Silva Pereira, @vulcanunsec, CIP; Carlos Borges, @huntingneo, CIP; ESET
Version: 1.0

Procedure Examples

Name Description
LightNeuron LightNeuron uses a malicious Microsoft Exchange transport agent for persistence. [2]


Mitigation Description
Audit Regularly check component software on critical services that adversaries may target for persistence to verify the integrity of the systems and identify if unexpected changes have been made.
Code Signing Ensure all application component binaries are signed by the correct application developers.
Privileged Account Management Do not allow administrator accounts that have permissions to add component software on these services to be used for day-to-day operations that may expose them to potential adversaries on unprivileged systems.


Consider monitoring application logs for abnormal behavior that may indicate suspicious installation of application software components. Consider monitoring file locations associated with the installation of new application software components such as paths from which applications typically load such extensible components. On MSSQL Server, consider monitoring for xp_cmdshell usage.[3]