S7comm
The proprietary Siemens protocol used to communicate with S7 family PLCs, carried over ISO-on-TCP (TPKT and COTP) on TCP port 102. It handles process data exchange as well as engineering operations — reading and writing memory areas, starting and stopping the CPU, and uploading or downloading control logic blocks. Older variants have no authentication; S7CommPlus on newer controllers adds integrity and anti-replay protection. S7comm is best known as the protocol abused by Stuxnet.
Why It Matters
In practice S7comm is the canonical example of a protocol whose legitimate engineering capability is indistinguishable from an attack. Stuxnet did not exploit a memory-corruption bug in the PLC; it used documented S7 functionality to read the existing control logic, substitute its own blocks, and replay the original logic back to the engineering station so the change stayed invisible — which is why the incident is taught as an integrity and monitoring failure rather than a patching failure. The defensive implication is that logic download and CPU stop or start operations must be treated as the highest-severity events in an OT environment, alerted on regardless of source, and reconciled against an authorised change record and a known-good copy of the project. Detecting a block download you did not schedule is worth more than any signature. On exams such as ISA/IEC 62443 and OT security certifications, expect questions on why the Stuxnet PLC manipulation was not a vulnerability exploit, and on protecting engineering workstations as the real control point.
Practice this topic
Test your knowledge of S7comm concepts with exam-style practice questions.
Related Network Security terms
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IPS (Intrusion Prevention System)
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