Vulnerability · Published Sep 17, 2026 · Assigned by EEF
CVE-2026-86688Session id is not renewed on authentication in ash_authentication, allowing session fixation
High 7.4
In plain English
A high-severity session fixation in ash_authentication from team-alembic. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.7% (higher than 55% of all known vulnerabilities).
Official description
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.
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| team-alembic | ash_authentication | 0.2.0 – before 4.15.05.0.0-rc.0 – before 5.0.0-rc.14 |
| team-alembic | ash_authentication | a939dde9b917c072cdf10c4b0913a9886a4b0231 – before * |
Weakness: CWE-384 · Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
References
- GitHub Advisory related, vendor-advisory
- EEF CNA record for CVE-2026-86688 related
- OSV record EEF-CVE-2026-86688 related
- Introducing commit a939dde in team-alembic/ash_authentication related
- Fix commit 872db45 in team-alembic/ash_authentication patch
- Fix commit 3e1d452 in team-alembic/ash_authentication patch