Remember-me sign-in guard reads a session key that is never written in ash_authentication, allowing session replacement

Authentication Bypass by Spoofing vulnerability in team-alembic ash_authentication allows an attacker who can plant a remember-me cookie in a victim's browser to replace that victim's authenticated session with one for the attacker's own account.

AshAuthentication.Plug.Helpers.sign_in_using_remember_me/3 skips re-authenticating an already-signed-in visitor by checking the session for "<subject_name>_token", but store_in_session/2 writes that key only when require_token_presence_for_authentication? is enabled and otherwise writes the bare subject name. At the default setting the guard therefore reads a key that is never written, its already-signed-in branch is unreachable, and the remember-me sign-in runs on every request through the per-request browser pipeline plug. A planted remember-me cookie is consequently honoured even for a visitor holding a live authenticated session, so whatever the victim enters afterwards lands in data the attacker controls. The read path in authenticate_resource_from_session/4 selects the key correctly, so the guard and the reader disagree about which key holds the session.

This issue affects ash_authentication: from 4.10.0 before 4.15.0 and from 5.0.0-rc.0 before 5.0.0-rc.14.

Set require_token_presence_for_authentication? true on the authenticated resource's token configuration. store_in_session/2 then writes the key the guard reads, so the already-signed-in branch fires and the remember-me sign-in no longer runs against a live session. Note this changes session storage semantics for the whole application and requires a token resource.

Otherwise remove the remember-me plug from the browser pipeline, which disables remember-me auto-login entirely.

The application must enable the remember_me strategy and run its per-request plug in the browser pipeline, with require_token_presence_for_authentication? at its default of false. Enabling that setting makes the guard read the key the session actually holds, which closes the issue.

The attacker also needs a position from which to set a cookie in the victim's browser, such as cookie tossing from a sibling subdomain, an HTTP host without HSTS, or a shared or kiosk browser.