Flask is a lightweight Python web framework used to build web apps, APIs, and internal tools. It is popular because it is flexible and simple, but that also means security decisions are often left to the developer. In common deployments, Flask stores session data in signed cookies, so the browser holds client-visible state while the server uses a secret key to verify that the cookie was not altered.
This matters in security because a leaked or reused secret key can let an attacker forge session cookies and impersonate users or change application state. Flask also becomes risky when session data, OAuth callbacks, or other trusted inputs are passed into unsafe features such as Python pickle deserialization. Defenders should keep secrets isolated, rotate them carefully, avoid putting sensitive data in cookies, and never deserialize untrusted data even if it appears valid.