Identify whether the app uses Flask’s default sessions or an extension with server-side storage. Obtain the existing production key through the authorised secret-management process.

1. Keep the production secret key stable across ordinary releases and consistent across all instances serving the same application.

2. Store only appropriate small session values. A signed cookie is not encrypted storage for confidential data, and large values can exceed practical cookie limits.

3. Configure secure, HTTP-only and suitable SameSite cookie behavior for the real HTTPS and cross-site flows. Test any external identity or payment return that depends on the session.

4. For deliberate rotation, use the supported fallback-key mechanism where available in the installed Flask version, or plan the expected session invalidation. Review extensions separately because they may handle keys differently.

5. After the transition window, remove old keys according to the recovery and security plan. Restart all relevant processes with the intended configuration.

Test login, logout and the selected external return flow across multiple requests. Confirm a normal deployment does not unexpectedly log out every user.

Intermittent invalid sessions can come from different keys on different workers, cookies scoped to the wrong host or an incompatible extension configuration.

• Configuring trusted proxy headers for Flask
• Rotating an application secret without breaking active traffic
• Moving runtime secrets out of a public document root
• Deploying a Flask application factory on WSGI hosting

• Flask configuration
• Flask sessions