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« Common Web Application Vulnerabilities

Session Management Vulnerabilities

For background information on the security requirements for web application session management, check out the OWASP Session Management Cheat Sheet.

No server-side session revocation

The default session management for a new Plug/Phoenix application uses the cookie session store. With this session store the session state is maintained entirely in the client’s browser: there is no server-side session state, meaning the server cannot revoke a session. Signing out merely clears the session cookie in the client’s browser, but does not invalidate the session cookie value.

In practice this means that a session cookie, once captured by an attacker, can be injected into HTTP requests to resume the session at any time, even after the user has signed out. The only way to invalidate a session on the server would be to rotate the signing key used to validate the session (the secret_key_base and/or the signing_salt), but this would invalidate all sessions of all clients.

Proper session management combines the client-side cookie with server-side state. A common way of achieving this is to store the session contents in the server’s database, identified by a random session identifier to be stored in the cookie. The session can then be invalidated by deleting the record from the database or marking it as invalid, for instance when the user signs out.

The mix phx.gen.auth task, included in recent Phoenix versions, uses a slightly different approach, where the session contents is stored in the cookie rather than the database, along with the session identifier (here called the session_token, stored in the user_tokens table). The task generates implementations for session lifecycle events, such as invalidating sessions on sign-out and on password change.

No server-side session timeout

A session may not always be explicitly invalidated: the user might forget to sign out, or the browser might crash. It is important that such ‘orphaned’ sessions do not remain valid indefinitely. The server should therefore invalidate old or idle sessions. Note that setting an expiry time on the session cookie is not sufficient, as this only takes care of clearing the cookie in the browser: it does not invalidate the session in the server.

This requires a server-side session store, as described above, with additional fields for tracking the session creation timestamp and/or the timestamp the session was last used. Invalidation can then happen periodically through a background task, or as a filter on these fields in the query that looks up a session in the database.

The aforementioned mix phx.gen.auth task by default invalidates sessions after 60 days. It may be wise to reduce this interval and/or to add a mechanism to invalidate sessions much sooner if they are found to be idle.

Session leakage (session hijacking)

If a session cookie falls into the wrong hands, the attacker can take over the user’s session, at least until the user or the server invalidates that session (see No server-side session revocation). Browsers support several mechanisms for minimizing the risk of cookie leakage, but these only really work when they are configured correctly on the server, in particular:

The HttpOnly attribute is set by default by Plug.Session or when setting cookies directly using Plug.Conn.put_resp_cookie/4. The Secure attribute is set based when Plug believes the request was made over HTTPS. This means care must be taken to ensure the conn struct correctly reflects the transport protocol (see Misconfigured TLS offload).

Session fixation

In a way, a session fixation attack is the opposite of session hijacking: the attacker tricks the user into resuming a session that was started by the attacker. Intuitively this may not seem like a big issue: why would the attacker volunteer to have their session hijacked? However, session fixation can lead to account compromise or information leakage.

The actual fixation attack usually involves tricking users into clicking a link to a vulnerable site, which seeds the session with a session ID provided by the attacker. But depending on the exact vulnerability, even “drive-by” session fixation triggered by e.g. an image embedded in a public forum might be possible.

The most dangerous session fixation vulnerabilities are those that start with an “anonymous” session: some servers assign a session identifier even before a user signs in, for instance for analytics purposes. An attacker might start such an anonymous session, take the session identifier and trick a victim into taking over that session. The user might then sign into the application with their user credentials, upgrading the anonymous session to an authenticated session. If the session identifier does not change as a result of the sign-in action, the attacker can now use the original session cookie to take over the user’s authenticated session and access the user’s data.

This attack can be mitigated by ensuring session identifiers are always rotated when the session’s privilege level changes. The Plug.Session API provides the configure_session/2 function to request session identifier renewal. This functionality needs to be provided by the session store implementation. The session management implemented by mix phx.gen.auth provides the renew_session/1 function to rotate the session identifier, and the default implementations for signing in/out call this function to prevent fixation.

Another fixation attack uses an already authenticated session, prepared by the attacker, and aims to leak information by tricking the user into entering (or uploading) the information into an account under the attacker’s control without realizing it. This attack cannot be mitigated by session identifier rotation.

To fully protect against all session fixation attacks it is necessary to block such attacks at the source.

The most common source is an endpoint that allows a session to be created from request parameters, either in the request URL or in the request body. This is typically done to enable external authentication servers (OAuth2, SAML, OIDC) to redirect back to the relying application. These protocols all support a server-generated nonce or state that must match the value returned in the redirection, thus blocking session creation attempts that were not triggered by the user. Make sure to study the mechanisms provided by the authentication server and apply them correctly.

Another, less common source is HTTP header splitting: if a server sets an HTTP response header with a value that contains user input, a malicious user may attempt to inject a value that includes a newline (CR+LF characters), followed by another HTTP header name/value pair. This would allow the attacker to send a Set-Cookie header that appears to originate from the server itself. The Plug.Conn APIs for setting response headers protect against this by disallowing newline characters in HTTP response header values.

Session information leakage

Storing data in the session cookie, or cookies in general, can reveal sensitive information to the user. For instance, imagine a website that performs a credit check on its users and stores the result in the session for later use: this might reveal information that is not intended to be shared with the user.

The default cookie session store in Plug/Phoenix protects the integrity of the session using a signature, meaning that the session contents is protected from tampering, but it does not encrypt the session. The following code snippet can be used by the user to inspect the contents of the session:

|> String.split(".")
|> Base.decode64!(padding: false)
|> :erlang.binary_to_term()

To protect the session contents from prying eyes, set the encryption_salt parameter when invoking the Plug.Session plug, e.g. in the project’s Endpoint. For other (non-session) cookies, pass the encrypt: true option when calling Plug.Conn.put_resp_cookie/4.

Session lifecycle and WebSocket connections

Whenever a user’s session is revoked, WebSocket connections that were established as part of that session should be disconnected. The connection is authenticated only once, at the start of the connection, and event handlers do not (typically) check if the session is still active.

The mix phx.gen.auth task uses a per-user users_sessions#{id} channel for this purpose, which each authenticated socket implicitly subscribes to. On user sign-out a disconnect message is broadcast to the user session’s channel, which causes the socket to be closed. A similar mechanism should be used when building a custom authentication solution.

Remember to also disconnect sockets when the session is revoked by the server, due to an overall session time expiry or due to inactivity. You may also have to consider whether to reset the session inactivity timer as a result of socket activity, perhaps only in response to explicitly user-triggered events.

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