A session boundary is the security line that keeps one authenticated user’s state, data, and permissions separate from another user’s context. It helps ensure that cookies, tokens, cached results, temporary files, tool outputs, and backend work stay tied to the correct identity. In modern SaaS and AI systems, the boundary may span the browser, API gateway, sandbox, and connected apps, so isolation must be enforced at every layer.
Session boundaries matter because many attacks aim to cross them. If an application mixes contexts, an attacker may read another user’s data, reuse a privileged token, or influence a sandbox that should be isolated. Defenders test for confused-deputy bugs, cross-tenant access, and container leakage, and they limit token scope, rotate credentials, and verify every request against the current session. Strong session boundaries are a core control for preventing account-to-account data exposure.