CVE-2026-86688

Session Fixation vulnerability in team-alembic ash_authentication allows an attacker who can plant a session identifier in a victim's browser to hold an authenticated session once that victim signs in. AshAuthentication.Plug.Helpers.store_in_session/2 writes the authenticated subject into the existing session with Plug.Conn.put_session/3 and never calls Plug.Conn.configure_session(renew: true), so the identifier the visitor arrived with carries into their authenticated session. Every authentication event reaches this one function: the default success/4 injected by AshAuthentication.Phoenix.Controller.__using__/1, the AuthController emitted by mix ash_authentication_phoenix.install, and remember-me auto-login. AshAuthentication.Phoenix.Plug.store_in_session/2 is a defdelegate to it. Logout does not close the window either, because clear_session/2 ends with Plug.Conn.clear_session/1, which clears session contents but leaves the identifier intact, so a planted identifier survives a logout-then-login cycle. This issue affects ash_authentication: from 0.2.0 before 4.15.0 and from 5.0.0-rc.0 before 5.0.0-rc.14.

• https://cna.erlef.org/cves/CVE-2026-86688.html
• https://github.com/team-alembic/ash_authentication/commit/3e1d452cbf1564e87f5f97be882b66fe25af7cfa
• https://github.com/team-alembic/ash_authentication/commit/872db454405ecad4fcdabd9ff3d8755d1d6a69ae
• https://github.com/team-alembic/ash_authentication/commit/a939dde9b917c072cdf10c4b0913a9886a4b0231
• https://github.com/team-alembic/ash_authentication/security/advisories/GHSA-v577-944g-7h3x
• https://osv.dev/vulnerability/EEF-CVE-2026-86688