{"id":"CVE-2026-86688","aliases":["GHSA-v577-944g-7h3x"],"url":"https://o3.security/vulnerability/CVE-2026-86688","summary":"Session id is not renewed on authentication in ash_authentication, allowing session fixation","details":"## Summary\n\nSession 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.\n\n`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.\n\nThis 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.\n\n## Impact\n\nAn attacker holding a session identifier they planted before the victim signed in gains that victim's authenticated session, and with it full account takeover, in deployments using a server-side session store.\n\n## Workarounds\n\nCall `Plug.Conn.configure_session(conn, renew: true)` in your own `success/4` before `store_in_session/2`. This is a one-line change in application code and closes the controller sign-in path, though not remember-me auto-login, which does not pass through application code.\n\nAdd `Plug.Conn.configure_session(conn, drop: true)` at sign-out so a planted identifier does not survive a logout-then-login cycle.\n\n## Configurations\n\nExploitation for account takeover requires a server-side session store (ETS, Mnesia, Redis or a database), where the cookie carries a stable session identifier, and a position from which the attacker can plant that cookie in the victim's browser: cookie tossing from a sibling subdomain, an HTTP host without HSTS, or a shared or kiosk browser.\n\nPhoenix defaults to the signed cookie store, and neither package's installer changes it. Under that store the authenticated session lives in the re-signed cookie delivered only to the victim, so a planted pre-authentication copy does not become an authenticated session. Attacker-planted session contents, such as a `return_to` value, still survive into the authenticated session there.","published":"2026-09-17T21:58:00.853Z","modified":"2026-09-17T22:11:00.019764599Z","cvss":null,"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"Hex","name":"ash_authentication","fixedVersion":"4.15.0"}],"fix":{"url":"https://github.com/team-alembic/ash_authentication/commit/872db454405ecad4fcdabd9ff3d8755d1d6a69ae","label":"team-alembic/ash_authentication@872db45"},"references":[{"type":"ADVISORY","url":"https://github.com/team-alembic/ash_authentication/security/advisories/GHSA-v577-944g-7h3x"},{"type":"WEB","url":"https://cna.erlef.org/cves/CVE-2026-86688.html"},{"type":"WEB","url":"https://github.com/team-alembic/ash_authentication/commit/a939dde9b917c072cdf10c4b0913a9886a4b0231"},{"type":"FIX","url":"https://github.com/team-alembic/ash_authentication/commit/872db454405ecad4fcdabd9ff3d8755d1d6a69ae"},{"type":"FIX","url":"https://github.com/team-alembic/ash_authentication/commit/3e1d452cbf1564e87f5f97be882b66fe25af7cfa"},{"type":"PACKAGE","url":"https://hex.pm/packages/ash_authentication"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-09-17T22:11:00.019764599Z"}}