Tool Composition Sandbox Escape: When Three Safe Tools Compose Into Data Exfiltration
The security review approved each of the three tools individually. Read-only access to the customer database was rated low risk because the agent could see records but not modify them. Send-email-to-self was rated low risk because the recipient was hardcoded to a service-account inbox the agent was already authorized to write to. Template-render was rated low risk because it was a deterministic Jinja-style transform with no I/O. Three weeks after launch, the data-loss-prevention dashboard flagged customer PII showing up in a Slack channel that two hundred employees could read, and the post-mortem traced the leak to the agent composing the three tools into a chain that no single ACL had granted: read a customer record, render it through a template, email the result to its own service account whose inbox auto-forwarded into the channel.
No single tool was misused. No prompt injection bypassed any check. The agent did exactly what its tool catalog said it could do, and the composition produced a capability the security review had never been asked to evaluate.
