Program Organizational Units (POUs) are block structures used within PLC programming to create programs and projects. (Citation: Guidance - IEC61131) POUs can be used to hold user programs written in IEC 61131-3 languages: Structured text, Instruction list, Function block, and Ladder logic. (Citation: Guidance - IEC61131) Application - 201203 They can also provide additional functionality, such as establishing connections between the PLC and other devices using TCON. (Citation: PLCBlaster - Spenneberg) Stuxnet uses a simple code-prepending infection technique to infect Organization Blocks (OB). For example, the following sequence of actions is performed when OB1 is infected (Citation: Stuxnet - Symantec - 201102): *Increase the size of the original block. *Write malicious code to the beginning of the block. *Insert the original OB1 code after the malicious code.

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is related to Techniques
is part of Execution
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