Our email assistant can read the inbox, browse the web and send mail. How do you stop a malicious web page from making it forward the user's inbox to an attacker?
Threat model
The agent holds all three legs of the lethal trifecta: private data (the inbox), untrusted content (web pages and incoming mail) and an outbound channel (send mail, plus any URL it can fetch). Any text the model reads can be read as an instruction.
Controls, strongest first
- Remove a leg. Once untrusted content is in context, sending is limited to recipients the user named in this session, or the session loses send entirely.
- Separate planning from data. A privileged planner never sees untrusted text; a quarantined model processes it and returns values the planner cannot read as instructions.
- Confirm at the boundary. A human approves each send with the recipient and content shown, not a summary written by the model.
- Close side channels. No automatic URL fetches or image rendering from model output; egress allowlists.
- Detect and log. Injection classifiers and anomaly alerts, treated as signals, not as the defence.