### Impact
It may be possible for a deactivated user to login when using uncommon configurations.

This only applies if any of the following are true:

* [JSON Web Tokens are enabled for login](https://matrix-org.github.io/synapse/latest/jwt.html) via the `jwt_config.enabled` configuration setting
* The [local password database is enabled](https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html#password_config) via the `password_config.enabled` and `password_config.localdb_enabled` configuration settings *and* a user’s password is [updated via an admin API](https://matrix-org.github.io/synapse/latest/admin_api/user_admin_api.html#create-or-modify-account) after a user is deactivated.

**Note that the local password database is enabled by default**, but it is uncommon to set a user’s password after they’ve been deactivated.

Installations that are configured to only allow login via Single Sign-On (SSO) via CAS, SAML or OpenID Connect (OIDC); or via an external password provider (e.g. LDAP) are not affected.

### Patches

* If using JSON Web Token logins: #15624
* For other users: #15634

### Workarounds

If not using JSON Web Tokens, ensure that deactivated users do not have a password set. This list of users can be queried from PostgreSQL:

“`sql
SELECT name FROM users WHERE password_hash IS NOT NULL AND deactivated = 1;
“`Read More