CVE-2026-76949: 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 ashauthentication 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.signinusingrememberme/3 skips re-authenticating an already-signed-in visitor by checking the session for "<subjectname>token", but storeinsession/2 writes that key only when requiretokenpresenceforauthentication? 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 authenticateresourcefromsession/4 selects the key correctly, so the guard and the reader disagree about which key holds the session.

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