A computer-use agent perceives a screen and acts by clicking and typing — and the moment the display it reads is attacker-influenced, the pixels on screen become an untrusted input that drives real actions. This threat lab maps the perceive-ground-decide-act-observe loop as a trust boundary, showing where hostile display content enters and what it can make an agent do. Each exposure — UI-grounding decoys, action hijack and irreversibility, the environment as adversary — is taught by the assumption it breaks, the mechanism that makes it work, and the containment control that limits the blast radius. Product-agnostic, grounded in the GUI-agent and computer-use attack literature, and tied back to the AI-agent stack every time.
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