CVE-2026-81637: Replayable OAuth2 CSRF state retained after a failed callback in AshAuthentication

Severity: low (CVSS 2.3 v4.0)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-17 · updated 2026-09-18
Weakness: CWE-613

Description

Insufficient Session Expiration vulnerability in team-alembic AshAuthentication allows an attacker who obtains a victim's OAuth2 state value to replay the callback and sign that victim into an attacker-controlled account. AshAuthentication.Strategy.OAuth2.Plug.callback/2 clears the stored session_params through a rebinding step inside its with chain, conn <- delete_session(conn, session_key). Elixir evaluates the else block in the scope enclosing the with, so every failure path (the provider returning ?error=access_denied, an invalid code, a token-exchange error, or a registration or sign-in failure) reaches store_authentication_result/2 holding the original connection and the session entry is never removed. The value the module's own comment describes as protection against a CSRF-related attack is therefore consumed only when authentication succeeds, and survives a cancelled or failed attempt until the next request phase or session expiry. This issue affects ash_authentication: from 0.6.0 before 4.15.0 and from 5.0.0-rc.0 before 5.0.0-rc.14.

Affected products

team-alembic ash_authentication: from 0.6.0 before 4.15.0; from 5.0.0-rc.0 before 5.0.0-rc.14 — fixed in 4.15.0, 5.0.0-rc.14

How to fix

Upgrade to a fixed version: team-alembic ash_authentication 4.15.0, 5.0.0-rc.14. Check the vendor advisory for mitigations.

References

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