Claude web_fetch Exfiltration Flaw — Nested-Link Attack Leaks User Memory

Summary

Security researcher Ayush Paul disclosed a data-exfiltration vulnerability in Claude's `web_fetch` tool, published on Simon Willison's weblog on July 15, 2026. The tool was designed to resist direct URL manipulation by only navigating to user-entered URLs or links returned by search — but Paul found `web_fetch` would also follow links embedded inside pages it had already fetched. An attacker could stand up a honeypot page presenting a fake "authentication" prompt that instructed Claude to walk a