Session id is not renewed on authentication in ash_authentication, allowing session fixation
Published Sep 17, 2026 · Updated Sep 17, 2026
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.
Summary
What happened
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.
The record
- CVE
- CVE-2026-86688
- Published
- Sep 17, 2026
- Updated
- Sep 17, 2026
- Vendor
- Alembic
- Product
- Ash Authentication
- Classifications
- Unavailable
- Attack vector
- Unavailable
- Privileges
- Unavailable
Timeline
How it unfolded
- Sep 17, 2026CVE publishedPublication date reported by the CVE source.
- Sep 17, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=0.2.0 <4.15.0
- Affected versionversion=5.0.0-rc.0 <5.0.0-rc.14
- Affected versionversion=a939dde9b917c072cdf10c4b0913a9886a4b0231 <* changes=[{"at":"872db454405ecad4fcdabd9ff3d8755d1d6a69ae","status":"unaffected"},{"at":"3e1d452cbf1564e87f5f97be882b66fe25af7cfa","status":"unaffected"}]
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What is affected?
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Attacks
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Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Activity data is delayed. The latest report is Sep 28, 2026.
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