Protecting action-taking agents from prompt injection: move defense out of the model layer
Informal-Dust4499 · reddit · 2026-10-29
A Reddit discussion from the co-founder of Aissist on prompt injection risks for agents that can take real actions via APIs. While most examples focus on leaking instructions, the bigger risk is injections hidden in order notes, retrieved docs, or API responses that trigger dangerous operations during legitimate tasks — like turning a $200 refund limit into $2,000.
His thesis: prompt-level defenses shouldn't be the primary security boundary. More important controls:
- Restrict each agent's tool access
- Validate sensitive actions in the backend
- Enforce transaction limits independently of the model
- Require human approval for high-risk operations
- Test with malicious instructions embedded in external content
The core challenge is balancing workflow flexibility against keeping untrusted content away from privileged actions. He asks production deployers how they handle it and whether they've seen attacks disguised as ordinary customer data.
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