Writer AI Flaw Could Let Agent Previews Leak Session Tokens Across Tenants

2026-07-08

A WriteOut session isolation bug in Writer let attackers steal session cookies via live agent preview links and hijack other companies' accounts, including admin access. The exploit used sandboxed code to read and exfiltrate cookies attached by logged-in users' browsers. Writer fixed it by blocking cookie forwarding into sandbox previews and isolating them on a separate origin.