Sessionid

Sessionid refers to a unique token used to identify a user session across multiple HTTP requests. It enables a server to associate requests from a client with stored state, such as authentication status, preferences, and other data, without requiring the client to resend credentials on every request.

Typically, the session identifier is transmitted as a cookie named sessionid, and browsers automatically include it in requests to the same origin. Cookies should be marked HttpOnly to reduce access by client-side scripts and Secure to require a protected transport channel. The SameSite attribute helps mitigate cross-site request forgery. Some systems support embedding the identifier in URLs, but this practice is generally discouraged due to security risks.

A session is usually created by the server when needed, and its data is stored on the server side, mapped to the sessionid issued to the client. With each request, the server reads the sessionid, retrieves the associated data, and updates the session as appropriate. Sessions may expire after a period of inactivity or be invalidated on logout. Institutions may rotate session identifiers on privilege changes to reduce risk of fixation.

Storage and scope vary: session data can reside in memory, on disk, in a database, or in a distributed cache. In scalable architectures, a central or distributed session store is common. Some approaches use stateless tokens (for example, JWTs) instead of traditional server-side sessions, in which case the term sessionid may refer to a token rather than a stored session object.

Security considerations include using sufficiently random identifiers, rotating IDs on login, enforcing TLS, invalidating sessions on logout, and applying appropriate CSRF protections and access controls.